Level 2: Application/Analysis -...

186 Unit tWo The Cell

controls whether the sugar will continue on in the glycolytic pathway. Considering this graph, under which condition is phosphofructokinase more active? Given what you know about glycolysis and regulation of metabolism by this enzyme, explain the mechanism by which phosphofructokinase activity differs depending on ATP concentration. Explain why it makes sense that regulation of this enzyme has evolved so that it works this way.

12. DRAW It The graph here shows the pH difference across the inner mitochondrial membrane over time in an actively respiring cell. At the time indicated by the vertical arrow, a metabolic poison is added that specifically and completely inhibits all function of mitochondrial ATP synthase. Draw what you would expect to see for the rest of the graphed line, and explain your graph.

13. eVoLUtIon ConneCtIon ATP synthases are found in the prokaryotic plasma membrane and in mitochondria and chloroplasts. (a) Propose a hypothesis to account for an evolutionary relationship of these eukaryotic organelles and prokaryotes. (b) Explain how the amino acid sequences of the ATP synthases from the different sources could be used to support or refute your hypothesis.

14. sCIentIFIC InQUIRy In the 1930s, some physicians prescribed low doses of a compound called dinitrophenol (DNP) to help patients lose weight. This unsafe method was abandoned after some patients died. DNP uncouples the chemiosmotic machinery by making the lipid bilayer of the inner mitochondrial membrane leaky to H+. Explain how this could cause weight loss and death.

15. WRIte ABoUt A tHeMe: oRGAnIZAtIon In a short essay (100–150 words), explain how oxidative phosphorylation—production of ATP using energy from the redox reactions of a spatially organized electron transport chain followed by chemiosmosis—is an example of how new properties emerge at each level of the biological hierarchy.

16. syntHesIZe yoUR KnoWLeDGe

Coenzyme Q (CoQ) is sold as a nutritional supplement. One company uses this market-ing slogan for CoQ: “Give your heart the fuel it craves most.” Considering the role of coenzyme Q, critique this claim. How do you think this product might function to benefit the heart? Is CoQ used as a “fuel” during cel-lular respiration?

3. The final electron acceptor of the electron transport chain that functions in aerobic oxidative phosphorylation is(A) oxygen.(B) water.(C) NAD+.(D) pyruvate.

4. In mitochondria, exergonic redox reactions(A) are the source of energy driving prokaryotic ATP synthesis.(B) provide the energy that establishes the proton gradient.(C) reduce carbon atoms to carbon dioxide.(D) are coupled via phosphorylated intermediates

to endergonic processes.

Level 2: Application/Analysis

5. What is the oxidizing agent in the following reaction?

Pyruvate + NADH + H + S Lactate + NAD +

(A) oxygen(B) NADH(C) lactate(D) pyruvate

6. When electrons flow along the electron transport chains of mitochondria, which of the following changes occurs?(A) The pH of the matrix increases.(B) ATP synthase pumps protons by active transport.(C) The electrons gain free energy.(D) NAD+ is oxidized.

7. Most CO2 from catabolism is released during(A) glycolysis.(B) the citric acid cycle.(C) lactate fermentation.(D) electron transport.

8. MAKe ConneCtIons Step 3 in Figure 9.9 is a major point of regulation of glycolysis. The enzyme phosphofructokinase is allosterically regulated by ATP and related molecules (see Concept 8.5). Considering the overall result of glycolysis, would you expect ATP to inhibit or stimulate activity of this enzyme? Explain. (Hint: Make sure you consider the role of ATP as an allosteric regulator, not as a substrate of the enzyme.)

9. MAKe ConneCtIons The proton pump shown in Figures 7.17 and 7.18 is a type of ATP synthase (see Figure 9.14). Compare the processes shown in the two figures, and say whether they are involved in active or passive transport (see Concepts 7.3 and 7.4).

10. VIsUAL sKILLs This computer model shows the four parts of ATP synthase, each part consisting of a number of polypeptide subunits (the structure in gray is still an area of active research). Using Figure 9.14 as a guide, label the rotor, stator, internal rod, and catalytic knob of this molecular motor.

Level 3: synthesis/evaluation

11. InteRpRet tHe DAtA Phosphofructokinase is an enzyme that acts on fructose 6-phosphate at an early step in glucose breakdown. Regulation of this enzyme

Fructose 6-phosphateconcentration

Low ATPconcentration

High ATPconcentrationPh

osph

ofru

ctok

inas

eac

tivity

Time

pH d

iffer

ence

acro

ss m

embr

ane

For selected answers, see Appendix A.

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187

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Figure 10.1 How does sunlight help build the trunk, branches, and leaves of this broadleaf tree?

Key ConCepts10.1 Photosynthesis converts light

energy to the chemical energy of food

10.2 The light reactions convert solar energy to the chemical energy of ATP and NADPH

10.3 The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar

10.4 Alternative mechanisms of carbon fixation have evolved in hot, arid climates

10.5 Life depends on photosynthesis

the process that Feeds the BiosphereLife on Earth is solar powered. Plants and other photosynthetic organisms contain cellular organelles called chloroplasts. Specialized molecular complexes in chloro-plasts capture light energy that has traveled 150 million km from the sun and con-vert it to chemical energy that is stored in sugar and other organic molecules. This conversion process is called photosynthesis. Let’s begin by placing photosynthe-sis in its ecological context.

Photosynthesis nourishes almost the entire living world directly or indirectly. An organism acquires the organic compounds it uses for energy and carbon skel-etons by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Autotrophs are “self-feeders” (auto- means “self,” and trophos means “feeder”); they sustain themselves without eating anything derived from other living beings. Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment. They are the ultimate sources of organic compounds for all nonautotrophic organisms, and for this reason, biologists refer to autotrophs as the producers of the biosphere.

Almost all plants are autotrophs; the only nutrients they require are water and minerals from the soil and carbon dioxide from the air. Specifically, plants are photoautotrophs, organisms that use light as a source of energy to synthesize organic substances (Figure 10.1). Photosynthesis also occurs in algae, certain

photosynthesis 10

other organisms also benefit from photosynthesis.


Heterotrophs obtain organic material by the second major mode of nutrition. Unable to make their own food, they live on compounds produced by other organisms (hetero- means “other”). Heterotrophs are the biosphere’s consumers. The most obvious “other-feeding” occurs when an animal eats plants or other organisms. But heterotrophic nutrition may be more subtle. Some heterotrophs consume the remains of other organisms by decomposing and feeding on organic litter such as dead organisms, feces, and fallen leaves; these types of heterotrophs are known as decomposers. Most fungi and many types of prokaryotes get their nourishment this way. Almost all heterotrophs, including humans, are completely depen-dent, either directly or indirectly, on photoautotrophs for food—and also for oxygen, a by-product of photosynthesis.

unicellular eukaryotes, and some prokaryotes (Figure 10.2). In this chapter, we will touch on these other groups in pass-ing, but our emphasis will be on plants. Variations in auto-trophic nutrition that occur in prokaryotes and algae will be described in Concept 27.3.

(e) Purple sulfur bacteria

(a) Plants

(b) Multicellular alga

(c) Unicellular eukaryotes

(d) Cyanobacteria

1 μm

10 μ

m

40 μm

Figure 10.2 photoautotrophs. These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed themselves and the entire living world. On land, (a) plants are the predominant producers of food. In aquatic environments, photoautotrophs include unicellular and (b) multicellular algae, such as this kelp; (c) some non-algal unicellular eukaryotes, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (the yellow globules within the cells) (c–e, LMs).

BioFlix® Animation: The Flow of Carbon Atoms in Producers, Consumers, and Decomposers

Figure 10.3 Alternative fuels from algae. The power of sunlight can be tapped to generate a sustainable alternative to fossil fuels. Species of unicellular algae that are prolific producers of plant oils can be cultured in long, transparent tanks called photobioreactors, such as the one shown here at Arizona State University. A simple chemical process can yield “biodiesel,” which can be mixed with gasoline or used alone to power vehicles.

ABC News Video: Turning Algae into Biofuel

WHAt IF? The main product of fossil fuel combustion is CO2 , and this is the source of the increase in atmospheric CO2 concentration. Scientists have proposed strategically situating containers of these algae near industrial plants or near highly congested city streets. Considering the process of photosynthesis, how does this arrangement make sense?

The Earth’s supply of fossil fuels was formed from remains of organisms that died hundreds of millions of years ago. In a sense, then, fossil fuels represent stores of the sun’s energy from the distant past. Because these resources are being used at a much higher rate than they are replenished, researchers are exploring methods of capitalizing on the photosynthetic process to provide alternative fuels (Figure 10.3).

In this chapter, you’ll learn how photosynthesis works. After discussing general principles of photosynthesis, we’ll consider the two stages of photosynthesis: the light reactions, which capture solar energy and transform it into chemi-cal energy; and the Calvin cycle, which uses that chemical

chApter 10 Photosynthesis 189

Stomata

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

Chloroplast

Granum

Stroma

Leaf cross section

Chloroplast

VeinChloroplasts

Mesophyll

1 μm

Mesophyll cell

20 μm

O2CO2

Figure 10.4 Zooming in on the location of photosynthesis in a plant. Leaves are the major organs of photosynthesis in plants. These images take you into a leaf, then into a cell, and finally into a chloroplast, the organelle where photosynthesis occurs (middle, LM; bottom, TEM).

Video: Chloroplasts in Motion

energy to make the organic molecules of food. Finally, we will consider some aspects of photo-synthesis from an evolutionary perspective.

ConCept 10.1Photosynthesis converts light energy to the chemical energy of foodThe remarkable ability of an organism to harness light energy and use it to drive the synthesis of organic com-pounds emerges from structural organization in the cell: Photosynthetic enzymes and other molecules are grouped together in a biological membrane, enabling the necessary series of chemical reactions to be carried out efficiently. The process of photosynthesis most likely originated in a group of bacteria that had infolded regions of the plasma membrane containing clusters of such molecules. In exist-ing photosynthetic bacteria, infolded photosynthetic mem-branes function similarly to the internal membranes of the chloroplast, a eukaryotic organelle. According to what has come to be known as the endosymbiont theory, the origi-nal chloroplast was a photosynthetic prokaryote that lived inside an ancestor of eukaryotic cells. (You learned about this theory in Concept 6.5, and it will be described more fully in Concept 25.3.) Chloroplasts are present in a variety of photo-synthesizing organisms (see some examples in Figure 10.2), but here we focus on chloroplasts in plants.

Chloroplasts: The Sites of Photosynthesis in PlantsAll green parts of a plant, including green stems and unrip-ened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants (Figure 10.4). There are about half a million chloroplasts in a chunk of leaf with a top surface area of 1 mm2. Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf. Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata (singular, stoma; from the Greek, meaning “mouth”). Water absorbed by the roots is delivered to the leaves in veins. Leaves also use veins to export sugar to roots and other nonphotosynthetic parts of the plant.

A typical mesophyll cell has about 30–40 chloroplasts, each measuring about 2–4 µm by 4–7 µm. A chloroplast has two membranes surrounding a dense fluid called the stroma. Suspended within the stroma is a third membrane system, made up of sacs called thylakoids, which segregates the stroma from the thylakoid space inside these sacs. In some places, thylakoid sacs are stacked in columns called grana (singular, granum). Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast. (The internal photosynthetic membranes


the prevailing hypothesis was that photosynthesis split car-bon dioxide (CO2 S C + O2) and then added water to the carbon (C + H2O S [CH2O]). This hypothesis predicted that the O2 released during photosynthesis came from CO2. This idea was challenged in the 1930s by C. B. van Niel, of Stanford University. Van Niel was investigating photosyn-thesis in bacteria that make their carbohydrate from CO2 but do not release O2. He concluded that, at least in these bacteria, CO2 is not split into carbon and oxygen. One group of bacteria used hydrogen sulfide (H2S) rather than water for photosynthesis, forming yellow globules of sulfur as a waste product (these globules are visible in Figure 10.2e). Here is the chemical equation for photosynthesis in these sulfur bacteria:

CO2 + 2 H2S S [CH2O] + H2O + 2 S

Van Niel reasoned that the bacteria split H2S and used the hydrogen atoms to make sugar. He then generalized that idea, proposing that all photosynthetic organisms require a hydro-gen source but that the source varies:

Sulfur bacteria: CO2 + 2 H2S S [CH2O] + H2O + 2 S Plants: CO2 + 2 H2O S [CH2O] + H2O + O2

General: CO2 + 2 H2X S [CH2O] + H2O + 2 X

Thus, van Niel hypothesized that plants split H2O as a source of electrons from hydrogen atoms, releasing O2 as a by-product.

Nearly 20 years later, scientists confirmed van Niel’s hypothesis by using oxygen-18 (18O), a heavy isotope, as a tracer to follow the path of oxygen atoms during photosyn-thesis. The experiments showed that the O2 from plants was labeled with 18O only if water was the source of the tracer (experiment 1). If the 18O was introduced to the plant in the form of CO2, the label did not turn up in the released O2 (experiment 2). In the following summary, magenta denotes labeled atoms of oxygen (18O):

Experiment 1: CO2 + 2 H2O S [CH2O] + H2O + O2

Experiment 2: CO2 + 2 H2O S [CH2O] + H2O + O2

A significant result of the shuffling of atoms during pho-tosynthesis is the extraction of hydrogen from water and its incorporation into sugar. The waste product of photosynthesis, O2, is released to the atmosphere. Figure 10.5 shows the fates of all atoms in photosynthesis.

of some prokaryotes are also called thylakoid membranes; see Figure 27.8b.) It is the light energy absorbed by chlorophyll that drives the synthesis of organic molecules in the chloro-plast. Now that we have looked at the sites of photosynthesis in plants, we are ready to look more closely at the process of photosynthesis.

Tracking Atoms Through Photosynthesis: Scientific InquiryScientists have tried for centuries to piece together the process by which plants make food. Although some of the steps are still not completely understood, the overall photosynthetic equation has been known since the 1800s: In the presence of light, the green parts of plants produce organic compounds and oxygen from carbon dioxide and water. Using molecular formulas, we can summarize the complex series of chemical reactions in photosynthesis with this chemical equation:

6 CO2 + 12 H2O + Light energy S C6H12O6 + 6 O2 + 6 H2O

We use glucose (C6H12O6) here to simplify the relationship between photosynthesis and respiration, but the direct prod-uct of photosynthesis is actually a three-carbon sugar that can be used to make glucose. Water appears on both sides of the equation because 12 molecules are consumed and 6 molecules are newly formed during photosynthesis. We can simplify the equation by indicating only the net consumption of water:

6 CO2 + 6 H2O + Light energy S C6H12O6 + 6 O2

Writing the equation in this form, we can see that the overall chemical change during photosynthesis is the reverse of the one that occurs during cellular respiration (see Concept 9.1). Both of these metabolic processes occur in plant cells. However, as you will soon learn, chloroplasts do not synthesize sugars by simply reversing the steps of respiration.

Now let’s divide the photosynthetic equation by 6 to put it in its simplest possible form:

CO2 + H2O S [CH2O] + O2

Here, the brackets indicate that CH2O is not an actual sugar but represents the general formula for a carbohydrate (see Concept 5.2). In other words, we are imagining the synthesis of a sugar molecule one carbon at a time (with six repetitions theoretically adding up to a glucose molecule—C6H12O6). Let’s now see how researchers tracked the elements C, H, and O from the reactants of photosynthesis to the products.

The Splitting of WaterOne of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants is derived from H2O and not from CO2. The chloroplast splits water into hydrogen and oxygen. Before this discovery,

Reactants:

Products:

6 CO2

C6H12O6 6 O26 H2O

12 H2O

Figure 10.5 tracking atoms through photosynthesis. The atoms from CO2 are shown in magenta, and the atoms from H2O are shown in blue.


a source of electrons and protons (hydrogen ions, H+) and giving off O2 as a by-product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP1 (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. (The electron acceptor NADP+ is first cousin to NAD+, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP+ molecule.) The light reactions use solar energy to reduce NADP+ to NADPH by adding a pair of elec-trons along with an H+. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron acceptor, reducing it, while ATP is the versatile energy currency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle.

The Calvin cycle is named for Melvin Calvin, who, along with his colleagues James Bassham and Andrew Benson, began to elucidate its steps in the late 1940s. The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO2 to carbohydrate, the Calvin cycle also requires chemical energy

Photosynthesis as a Redox ProcessLet’s briefly compare photosynthesis with cellular respiration. Both processes involve redox reactions. During cellular respira-tion, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product (see Concept 9.1). The electrons lose potential energy as they “fall” down the electron transport chain toward electronegative oxygen, and the mitochondrion harnesses that energy to synthesize ATP (see Figure 9.15). Photosynthesis reverses the direction of electron flow. Water is split, and its electrons are transferred along with hydrogen ions (H+) from the water to carbon dioxide, reducing it to sugar.

becomes reduced

becomes oxidized

C6H12O66 CO2 6 O26 H2OEnergy + + +

Because the electrons increase in potential energy as they move from water to sugar, this process requires energy—in other words, it is endergonic. This energy boost that occurs during photosynthesis is provided by light.

The Two Stages of Photosynthesis: A PreviewThe equation for photosynthesis is a deceptively simple sum-mary of a very complex process. Actually, photosynthesis is not a single process, but two processes, each with multiple steps. These two stages of photosynthesis are known as the light reactions (the photo part of photosynthesis) and the Calvin cycle (the synthesis part) (Figure 10.6).

The light reactions are the steps of photosynthesis that con-vert solar energy to chemical energy. Water is split, providing

Light

Chloroplast

Thylakoid Stroma

O2

CALVINCYCLE

LIGHTREACTIONS

[CH2O](sugar)

NADPH

NADP+

ADP+

P i

H2O CO2

ATP

Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes (green) are the sites of the light reactions, whereas the Calvin cycle occurs in the stroma (gray). The light reactions use solar energy to make ATP and NADPH, which supply chemical energy and reducing power, respectively, to the Calvin cycle. The Calvin cycle incorporates CO2 into organic molecules, which are converted to sugar. (Recall that most simple sugars have formulas that are some multiple of CH2O.) To visualize these processes in their cellular context, see Figure 6.32.

BioFlix® Animation: Photosynthesis


dropping a pebble into a pond. Electromagnetic waves, how-ever, are disturbances of electric and magnetic fields rather than disturbances of a material medium such as water.

The distance between the crests of electromagnetic waves is called the wavelength. Wavelengths range from less than a nanometer (for gamma rays) to more than a kilome-ter (for radio waves). This entire range of radiation is known as the electromagnetic spectrum (Figure 10.7). The seg-ment most important to life is the narrow band from about 380 nm to 750 nm in wavelength. This radiation is known as visible light because it can be detected as various colors by the human eye.

The model of light as waves explains many of light’s properties, but in certain respects light behaves as though it consists of discrete particles, called photons. Photons are not tangible objects, but they act like objects in that each of them has a fixed quantity of energy. The amount of energy is inversely related to the wavelength of the light: The shorter the wavelength, the greater the energy of each photon of that light. Thus, a photon of violet light packs nearly twice as much energy as a photon of red light (see Figure 10.7).

Although the sun radiates the full spectrum of electromag-netic energy, the atmosphere acts like a selective window, allowing visible light to pass through while screening out a substantial fraction of other radiation. The part of the spec-trum we can see—visible light—is also the radiation that drives photosynthesis.

Photosynthetic Pigments: The Light ReceptorsWhen light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as

in the form of ATP, which is also generated by the light reac-tions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the light reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or light-independent reactions, because none of the steps requires light directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the light reactions provide the NADPH and ATP that the Calvin cycle requires. In essence, the chloroplast uses light energy to make sugar by coordinating the two stages of photosynthesis.

As Figure 10.6 indicates, the thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the stroma. On the outside of the thylakoids, mol-ecules of NADP+ and ADP pick up electrons and phosphate, respectively, and NADPH and ATP are then released to the stroma, where they play crucial roles in the Calvin cycle. The two stages of photosynthesis are treated in this figure as meta-bolic modules that take in ingredients and crank out products. In the next two sections, we’ll look more closely at how the two stages work, beginning with the light reactions.

ConCept CHeCK 10.11. MAKe ConneCtIons how do the co2 molecules

used in photosynthesis reach and enter the chloroplasts inside leaf cells? (See concept 7.2.)

2. explain how the use of an oxygen isotope helped eluci-date the chemistry of photosynthesis.

3. WHAt IF? the calvin cycle requires Atp and nADph, products of the light reactions. if a classmate asserted that the light reactions don’t depend on the calvin cycle and, with continual light, could just keep on producing Atp and nADph, how would you respond?

For suggested answers, see Appendix A.

ConCept 10.2The light reactions convert solar energy to the chemical energy of ATP and NADPHChloroplasts are chemical factories powered by the sun. Their thylakoids transform light energy into the chemical energy of ATP and NADPH, which will be used to synthe-size glucose and other molecules that can be used as energy sources. To better understand the conversion of light to chemical energy, we need to know about some important properties of light.

The Nature of SunlightLight is a form of energy known as electromagnetic energy, also called electromagnetic radiation. Electromagnetic energy travels in rhythmic waves analogous to those created by

380 450 500 550 600 650 700 750 nm

Visible light

Gamma rays

X-rays UV InfraredMicro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

(109 nm) 103 m

Longer wavelengthShorter wavelengthLower energyHigher energy

Figure 10.7 the electromagnetic spectrum. White light is a mixture of all wavelengths of visible light. A prism can sort white light into its component colors by bending light of different wavelengths at different angles. (Droplets of water in the atmosphere can act as prisms, causing a rainbow to form.) Visible light drives photosynthesis.


photosynthesis (Figure 10.10b), which profiles the relative effectiveness of different wavelengths of radiation in driving the process. An action spectrum is prepared by illuminating chloroplasts with light of different colors and then plotting wavelength against some measure of photosynthetic rate, such as CO2 consumption or O2 release. The action spectrum for photosynthesis was first demonstrated by Theodor W. Engelmann, a German botanist, in 1883. Before equipment for measuring O2 levels had even been invented, Engelmann

pigments. Different pigments absorb light of different wave-lengths, and the wavelengths that are absorbed disappear. If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black.) We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light (Figure 10.8). The ability of a pigment to absorb various wavelengths of light can be measured with an instru-ment called a spectrophotometer. This machine directs beams of light of different wavelengths through a solution of the pigment and measures the fraction of the light transmit-ted at each wavelength. A graph plotting a pigment’s light absorption versus wavelength is called an absorption spectrum (Figure 10.9).

The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis, since light can perform work in chloroplasts only if it is absorbed. Figure 10.10a shows the absorption spectra of three types of pigments in chlo-roplasts: chlorophyll a, the key light-capturing pigment that participates directly in the light reactions; the accessory pigment chlorophyll b; and a separate group of accessory pigments called carotenoids. The spectrum of chlorophyll a suggests that violet-blue and red light work best for photo-synthesis, since they are absorbed, while green is the least effective color. This is confirmed by an action spectrum for

Figure 10.9

Research Method Determining an Absorption Spectrum

Application An absorption spectrum is a visual representation of how well a particular pigment absorbs different wavelengths of visible light. Absorption spectra of various chloroplast pigments help scien-tists decipher the role of each pigment in a plant.

technique A spectrophotometer measures the relative amounts of light of different wavelengths absorbed and transmitted by a pigment solution.

1 White light is separated into colors (wavelengths) by a prism.

2 One by one, the different colors of light are passed through the sample (chlorophyll in this example). Green light and blue light are shown here.

3 The transmitted light strikes a photoelectric tube, which converts the light energy to electricity.

4 The electric current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed.

White light

Refracting prism

Slit moves to pass light of selected wavelength.

Greenlight

Chlorophyllsolution

Photoelectrictube

Galvanometer

The low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.

The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.

Bluelight

1

2 3

4 0

0

100

100

Results See Figure 10.10a for absorption spectra of three types of chloroplast pigments.

Chloroplast

LightReflectedlight

Transmittedlight

Absorbedlight

Granum

Figure 10.8 Why leaves are green: interaction of light with chloroplasts. The chlorophyll molecules of chloroplasts absorb violet-blue and red light (the colors most effective in driving photosynthesis) and reflect or transmit green light. This is why leaves appear green.

Animation: Light Energy and Pigments


performed a clever experiment in which he used bacteria to measure rates of photosynthesis in filamentous algae (Figure 10.10c). His results are a striking match to the mod-ern action spectrum shown in Figure 10.10b.

Notice by comparing Figures 10.10a and 10.10b that the action spectrum for photosynthesis is much broader than the absorption spectrum of chlorophyll a. The absorption spectrum of chlorophyll a alone underestimates the effective-ness of certain wavelengths in driving photosynthesis. This is partly because accessory pigments with different absorption spectra also present in chloroplasts—including chlorophyll b and carotenoids—broaden the spectrum of colors that can be used for photosynthesis. Figure 10.11 shows the structure of chlorophyll a compared with that of chlorophyll b. A slight structural difference between them is enough to cause the two pigments to absorb at slightly different wavelengths in the red and blue parts of the spectrum (see Figure 10.10a). As a result, chlorophyll a appears blue green and chlorophyll b olive green under visible light.

Other accessory pigments include carotenoids, hydro-carbons that are various shades of yellow and orange because they absorb violet and blue-green light (see Figure 10.10a). Carotenoids may broaden the spectrum of colors that can

Figure 10.10

Inquiry Which wavelengths of light are most effective in driving photosynthesis?

experiment Absorption and action spectra, along with a classic experiment by Theodor W. Engelmann, reveal which wavelengths of light are photosynthetically important.

Results

400 500 600 700

Wavelength of light (nm)

Abs

orpt

ion

of li

ght

bych

loro

plas

t pi

gmen

ts

Chlorophyll bChloro-phyll a

Carotenoids

(a) Absorption spectra. The three curves show the wavelengths of lightbest absorbed by three types of chloroplast pigments.

Rate

of

phot

osyn

thes

is(m

easu

red

by O

2 re

leas

e)

(b)

400 500 600 700

Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.

Filamentof alga

Aerobic bacteria

(c)

400 500 600 700

Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which congregate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light.

N

C

C

CC

N

C

C

C

C N

C

N C

C CC

C

C

C

C

C

C

C

CH2

CH H

CHO in chlorophyll bCH3 in chlorophyll a

H3C

HH

CH2 CH3

H

CH3

H3C

H

CH2

CH2

C

O

CH3

OC

O

CH2

OC

CH

O

H

Porphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshown

Mg

CH3

Figure 10.11 structure of chlorophyll molecules in chloroplasts of plants. Chlorophyll a and chlorophyll b differ in only one of the functional groups bonded to the porphyrin ring. (Also see the space-filling model of chlorophyll in Figure 1.3.)

Instructors: A related experimental inquiry tutorial can be assigned in MasteringBiology.

InteRpRet tHe DAtA According to the graph, which wavelengths of light drive the highest rates of photosynthesis?

Data from T. W. Engelmann, Bacterium photometricum. Ein Beitrag zur vergleichenden Physiologie des Licht-und Farbensinnes, Archiv. für Physiologie 30:95–124 (1883).

Conclusion The action spectra, confirmed by Engelmann’s experiment, show which portions of the spectrum are most effective in driving photosynthesis.

Animation: Space-Filling Model of Chlorophyll


wavelengths, which is why each pigment has a unique absorption spectrum.

Once absorption of a photon raises an electron to an excited state, the electron cannot stay there long. The excited state, like all high-energy states, is unstable. Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground-state orbital in a billionth of a second, releasing their excess energy as heat. This conversion of light energy to heat is what makes the top of an automobile so hot on a sunny day. (White cars are coolest because their paint reflects all wave-lengths of visible light.) In isolation, some pigments, includ-ing chlorophyll, emit light as well as heat after absorbing photons. As excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence. An illuminated solution of chlorophyll isolated from chloro-plasts will fluoresce in the red part of the spectrum and also give off heat (Figure 10.12). This is best seen by illuminating with ultraviolet light, which chlorophyll can also absorb (see Figures 10.7 and 10.10a). Viewed under visible light, the fluorescence would be difficult to see against the green of the solution.

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting ComplexesChlorophyll molecules excited by the absorption of light energy produce very different results in an intact chloro-plast than they do in isolation (see Figure 10.12). In their native environment of the thylakoid membrane, chlo-rophyll molecules are organized along with other small organic molecules and proteins into complexes called photosystems.

drive photosynthesis. However, a more important function of at least some carotenoids seems to be photoprotection: These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxy-gen, forming reactive oxidative molecules that are dangerous to the cell. Interestingly, carotenoids similar to the photopro-tective ones in chloroplasts have a photoprotective role in the human eye. (Carrots, known for aiding night vision, are rich in carotenoids.) These and related molecules are, of course, found naturally in many vegetables and fruits. They are also often advertised in health food products as “phytochemicals” (from the Greek phyton, plant), some of which have antioxi-dant properties. Plants can synthesize all the antioxidants they require, but humans and other animals must obtain some of them from their diets.

Excitation of Chlorophyll by LightWhat exactly happens when chlorophyll and other pig-ments absorb light? The colors corresponding to the absorbed wavelengths disappear from the spectrum of the transmitted and reflected light, but energy cannot disap-pear. When a molecule absorbs a photon of light, one of the molecule’s electrons is elevated to an orbital where it has more potential energy (see Figure 2.6b). When the electron is in its normal orbital, the pigment molecule is said to be in its ground state. Absorption of a photon boosts an electron to an orbital of higher energy, and the pig-ment molecule is then said to be in an excited state. The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and an excited state, and this energy difference varies from one kind of molecule to another. Thus, a particular com-pound absorbs only photons corresponding to specific

Photon

Photon(fluorescence)

Excitedstate

Heat

Chlorophyllmolecule

e–

(a)

Ener

gy o

f el

ectr

on

Excitation of isolated chlorophyll molecule (b) Fluorescence

Groundstate

Figure 10.12 excitation of isolated chlorophyll by light. (a) Absorption of a photon causes a transition of the chlorophyll molecule from its ground state to its excited state. The photon boosts an electron to an orbital where it has more potential energy. If the illuminated molecule exists in isolation, its excited electron immediately drops back down to the ground-state orbital, and its excess energy is given off as heat and fluorescence (light). (b) A chlorophyll solution excited with ultraviolet light fluoresces with a red-orange glow.

WHAt IF? If a leaf containing the same concentration of chlorophyll as in the solution was exposed to the same ultraviolet light, no fluorescence would be seen. Propose an explanation for the difference in fluorescence emission between the solution and the leaf.


organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor. Each light-harvesting complex consists of various pig-ment molecules (which may include chlorophyll a, chloro-phyll b, and multiple carotenoids) bound to proteins. The number and variety of pigment molecules enable a photo-system to harvest light over a larger surface area and a larger portion of the spectrum than could any single pigment mol-ecule alone. Together, these light-harvesting complexes act as an antenna for the reaction-center complex. When a pig-ment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting complex, like a human “wave” at a sports arena, until it is passed to the pair of chlorophyll a molecules in the reaction-center complex. The pair of chlorophyll a mol-ecules in the reaction-center complex are special because their molecular environment—their location and the other mol-ecules with which they are associated—enables them to use the energy from light not only to boost one of their electrons to a higher energy level, but also to transfer it to a different molecule—the primary electron acceptor, which is a mol-ecule capable of accepting electrons and becoming reduced.

The solar-powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step of the light reactions. As soon as the chlorophyll elec-tron is excited to a higher energy level, the primary electron acceptor captures it; this is a redox reaction. In the flask shown in Figure 10.12b, isolated chlorophyll fluoresces because there is no electron acceptor, so electrons of photoexcited chloro-phyll drop right back to the ground state. In the structured environment of a chloroplast, however, an electron acceptor is readily available, and the potential energy represented by the excited electron is not dissipated as light and heat. Thus, each photosystem—a reaction-center complex surrounded by light-harvesting complexes—functions in the chloroplast as a unit. It converts light energy to chemical energy, which will ultimately be used for the synthesis of sugar.

The thylakoid membrane is populated by two types of pho-tosystems that cooperate in the light reactions of photosyn-thesis: photosystem II (PS II) and photosystem I (PS I). (They were named in order of their discovery, but photosystem II functions first in the light reactions.) Each has a characteristic reaction-center complex—a particular kind of primary electron acceptor next to a special pair of chlorophyll a molecules associ-ated with specific proteins. The reaction-center chlorophyll a of photosystem II is known as P680 because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum). The chlorophyll a at the reaction-center complex of photosystem I is called P700 because it most effec-tively absorbs light of wavelength 700 nm (in the far-red part of the spectrum). These two pigments, P680 and P700, are nearly identical chlorophyll a molecules. However, their association with different proteins in the thylakoid membrane affects the

(a) How a photosystem harvests light. When a photon strikes a pig-ment molecule in a light-harvesting complex, the energy is passed from molecule to molecule until it reaches the reaction-center com-plex. Here, an excited electron from the special pair of chlorophyll amolecules is transferred to the primary electron acceptor.

Thyl

akoi

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embr

ane

Transferof energy

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Pigmentmolecules

Primaryelectronacceptor

Photon

Thylakoid

STROMAPhotosystem

Reaction-center complex

Light-harvestingcomplexes

Special pair ofchlorophyll amolecules

e–

Figure 10.13 the structure and function of a photosystem.

Chlorophyll (green)

Thyl

akoi

d m

embr

ane

Proteinsubunits(purple)

STROMA

THYLAKOIDSPACE

(b) Structure of a photosystem. This computer model, based on X-ray crystallography, shows two photosystem complexes side by side. Chlorophyll molecules (bright green ball-and-stick models within the membrane; the tails are not shown) are interspersed with protein subunits (purple ribbons; notice the many α helices spanning the membrane). For simplicity, a photosystem will be shown as a single complex in the rest of the chapter.

A photosystem is composed of a reaction-center com-plex surrounded by several light-harvesting complexes (Figure 10.13). The reaction-center complex is an


back to its ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state. The process continues, with the energy being relayed to other pigment molecules until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state.

2 This electron is transferred from the excited P680 to the primary electron acceptor. We can refer to the resulting form of P680, missing the negative charge of an electron, as P680+.

3 An enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions (H+), and an oxygen atom. The electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the primary electron acceptor. (P680+ is the strongest biological oxidiz-ing agent known; its electron “hole” must be filled. This greatly facilitates the transfer of electrons from the split water molecule.) The H+ are released into the thyla-koid space (interior of the thylakoid). The oxygen atom

electron distribution in the two pigments and accounts for the slight differences in their light-absorbing properties. Now let’s see how the two types of photosystems work together in using light energy to generate ATP and NADPH, the two main prod-ucts of the light reactions.

Linear Electron FlowLight drives the synthesis of ATP and NADPH by energizing the two types of photosystems embedded in the thylakoid membranes of chloroplasts. The key to this energy transfor-mation is a flow of electrons through the photosystems and other molecular components built into the thylakoid mem-brane. This is called linear electron flow, and it occurs during the light reactions of photosynthesis, as shown in Figure 10.14. The numbered steps in the text correspond to the numbered steps in the figure.

1 A photon of light strikes one of the pigment molecules in a light-harvesting complex of PS II, boosting one of its electrons to a higher energy level. As this electron falls

H2O

NADP+

reductase

FdPrimaryelectronacceptorPrimary

electronacceptor

Electron transport chain

Electrontransportchain

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Photosystem I(PS I)

Light Light

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+ 1/2

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complex

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O2

Pigmentmolecules

NADP+

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[CH2O] (sugar)

LIGHTREACTIONS

CALVINCYCLE

ATP

NADPH

O2

CO2

Light

Figure 10.14 How linear electron flow during the light reactions generates Atp and nADpH. The gold arrows trace the flow of light-driven electrons from water to NADPH. The black arrows trace the transfer of energy from pigment molecule to pigment molecule.

Figure Walkthrough


reactions of the Calvin cycle. This process also removes an H+ from the stroma.

The energy changes of electrons during their linear flow through the light reactions are shown in a mechanical analogy in Figure 10.15. Although the scheme shown in Figures 10.14 and 10.15 may seem complicated, do not lose track of the big picture: The light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, respectively, to the carbohydrate-synthesizing reac-tions of the Calvin cycle.

Cyclic Electron FlowIn certain cases, photoexcited electrons can take an alterna-tive path called cyclic electron flow, which uses photo-system I but not photosystem II. You can see in Figure 10.16 that cyclic flow is a short circuit: The electrons cycle back from ferredoxin (Fd) to the cytochrome complex, then

immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O2.

4 Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain, the components of which are similar to those of the electron transport chain that functions in cellular respiration. The electron transport chain between PS II and PS I is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plasto-cyanin (Pc). Each component carries out redox reactions as electrons flow down the electron transport chain, re-leasing free energy that is used to pump protons (H+) into the thylakoid space, contributing to a proton gradient across the thylakoid membrane.

5 The potential energy stored in the proton gradient is used to make ATP in a process called chemiosmosis, to be discussed shortly.

6 Meanwhile, light energy has been transferred via light-harvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chloro-phyll a molecules located there. The photoexcited electron is then transferred to PS I’s primary electron acceptor, creating an electron “hole” in the P700—which we now can call P700+. In other words, P700+ can now act as an electron acceptor, accepting an electron that reaches the bottom of the electron transport chain from PS II.

7 Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin (Fd). (This chain does not create a proton gradient and thus does not produce ATP.)

8 The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+. Two electrons are required for its reduction to NADPH. Electrons in NADPH are at a higher energy level than they are in water (where they started), so they are more readily available for the

Phot

on

Phot

on

e–

e–

e–

e–

e–

e–

e–

Millmakes

ATPNADPH

Photosystem IPhotosystem II

ATP

Figure 10.15 A mechanical analogy for linear electron flow during the light reactions.

Photosystem II

Photosystem I

ATP

Pq

FdPrimaryacceptor

Pc

Cytochromecomplex

Fd

NADP+

reductase

NADP+

+ H+

NADPH

Primaryacceptor

Figure 10.16 Cyclic electron flow. Photoexcited electrons from PS I are occasionally shunted back from ferredoxin (Fd) to chlorophyll via the cytochrome complex and plastocyanin (Pc). This electron shunt supplements the supply of ATP (via chemiosmosis) but produces no NADPH. The “shadow” of linear electron flow is included in the diagram for comparison with the cyclic route. The two Fd molecules in this diagram are actually one and the same—the final electron carrier in the electron transport chain of PS I—although it is depicted twice to clearly show its role in two parts of the process.

VIsUAL sKILLs Look at Figure 10.15 and explain how you would alter it to show a mechanical analogy for cyclic electron flow.


A Comparison of Chemiosmosis in Chloroplasts and MitochondriaChloroplasts and mitochondria generate ATP by the same basic mechanism: chemiosmosis (see Figure 9.15). An elec-tron transport chain pumps protons (H+) across a membrane as electrons are passed through a series of carriers that are progressively more electronegative. Thus, electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H+ gradient across a membrane. An ATP synthase complex in the same membrane couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP, forming ATP.

Some of the electron carriers, including the iron-containing proteins called cytochromes, are very similar in chloroplasts and mitochondria. The ATP synthase complexes of the two organelles are also quite similar. But there are noteworthy differences between photophosphorylation in chloroplasts and oxidative phosphorylation in mitochondria. Both work by way of chemiosmosis, but in chloroplasts, the high-energy electrons dropped down the transport chain come from water, while in mitochondria, they are extracted from organic mol-ecules (which are thus oxidized). Chloroplasts do not need molecules from food to make ATP; their photosystems capture light energy and use it to drive the electrons from water to the top of the transport chain. In other words, mitochondria use chemiosmosis to transfer chemical energy from food mol-ecules to ATP, whereas chloroplasts use it to transform light energy into chemical energy in ATP.

Although the spatial organization of chemiosmosis differs slightly between chloroplasts and mitochondria, it is easy to see similarities in the two (Figure 10.17). Electron transport chain

via a plastocyanin molecule (Pc) to a P700 chlorophyll in the PS I reaction-center complex. There is no production of NADPH and no release of oxygen that results from this pro-cess. On the other hand, cyclic flow does generate ATP.

Rather than having both PS II and PS I, several of the cur-rently existing groups of photosynthetic bacteria are known to have a single photosystem related to either PS II or PS I. For these species, which include the purple sulfur bacteria (see Figure 10.2e) and the green sulfur bacteria, cyclic electron flow is the one and only means of generating ATP during the process of photosynthesis. Evolutionary biologists hypoth-esize that these bacterial groups are descendants of ancestral bacteria in which photosynthesis first evolved, in a form similar to cyclic electron flow.

Cyclic electron flow can also occur in photosynthetic species that possess both photosystems; this includes some prokaryotes, such as the cyanobacteria shown in Figure 10.2d, as well as the eukaryotic photosynthetic species that have been tested thus far. Although the process is probably in part an “evolutionary leftover,” research suggests it plays at least one beneficial role for these organisms. Plants with mutations that render them unable to carry out cyclic electron flow are capable of growing well in low light, but do not grow well where light is intense. This is evidence for the idea that cyclic electron flow may be photoprotective. Later you’ll learn more about cyclic electron flow as it relates to a particular adapta-tion of photosynthesis (C4 plants; see Concept 10.4).

Whether ATP synthesis is driven by linear or cyclic elec-tron flow, the actual mechanism is the same. Before we move on to consider the Calvin cycle, let’s review chemiosmosis, the process that uses membranes to couple redox reactions to ATP production.

ATPADP +

H+

H+

H+

Diffusion of H+ through ATP synthase

Mitochondrion Chloroplast

Thylakoidspace

Thylakoidmembrane

Stroma

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Innermembrane

Inter-membrane

space

P i

Electrontransport

chainMITOCHONDRIONSTRUCTURE

CHLOROPLASTSTRUCTURE

Higher [H+]Lower [H+]

Key

Figure 10.17 Comparison of chemiosmosis in mitochondria and chloroplasts. In both kinds of organelles, electron transport chains pump protons (H+) across a membrane from a region of low H+ concentration (light gray in this diagram) to one of high H+ concentration (dark gray). The protons then diffuse back across the membrane through ATP synthase, driving the synthesis of ATP.

MAKe ConneCtIons Describe how you would change the pH in order to artificially cause ATP synthesis (a) outside an isolated mitochondrion (assume H + can freely cross the outer membrane; see Figure 9.15) and (b) in the stroma of a chloroplast. Explain.


comparable spaces in the two organelles, while the mitochon-drial matrix is analogous to the stroma of the chloroplast.

In the mitochondrion, protons diffuse down their con-centration gradient from the intermembrane space through ATP synthase to the matrix, driving ATP synthesis. In the chloroplast, ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase complexes (Figure 10.18), whose catalytic knobs are on the stroma side of the membrane. Thus, ATP forms in the

proteins in the inner membrane of the mitochondrion pump protons from the mitochondrial matrix out to the intermem-brane space, which then serves as a reservoir of hydrogen ions. Similarly, electron transport chain proteins in the thylakoid membrane of the chloroplast pump protons from the stroma into the thylakoid space (interior of the thylakoid), which func-tions as the H+ reservoir. If you imagine the cristae of mitochon-dria pinching off from the inner membrane, this may help you see how the thylakoid space and the intermembrane space are

H2O

LightLight

4 H+

+2 H+ 4 H+

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reductasePhotosystem ICytochromecomplex

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Figure 10.18 the light reactions and chemiosmosis: Current model of the organization of the thylakoid membrane. The gold arrows track the linear electron flow outlined in Figure 10.14. At least three steps in the light reactions contribute to the H+ gradient across the thylakoid membrane: 1 Water is split by photosystem II

on the side of the membrane facing the thylakoid space; 2 as plastoquinone (Pq) transfers electrons to the cytochrome complex, four protons are translocated across the membrane into the thylakoid space; and 3 a hydrogen ion is removed from the

stroma when it is taken up by NADP+. Notice that in step 2, hydrogen ions are being

pumped from the stroma into the thylakoid space, as in Figure 10.17. The diffusion of H+ from the thylakoid space back to the stroma (along the H+ concentration gradient) powers the ATP synthase. These light-driven reactions store chemical energy in NADPH and ATP, which shuttle the energy to the carbohydrate-producing Calvin cycle.

Animation: The Light Reactions


and others exit the cycle. However, the citric acid cycle is catabolic, oxidizing acetyl CoA and using the energy to syn-thesize ATP, while the Calvin cycle is anabolic, building car-bohydrates from smaller molecules and consuming energy. Carbon enters the Calvin cycle in the form of CO2 and leaves in the form of sugar. The cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high-energy electrons to make the sugar.

As we mentioned in Concept 10.1, the carbohydrate produced directly from the Calvin cycle is not glucose. It is actually a three-carbon sugar called glyceraldehyde 3-phosphate (G3P). For the net synthesis of one molecule of G3P, the cycle must take place three times, fixing three molecules of CO2—one per turn of the cycle. (Recall that the term carbon fixation refers to the initial incorporation of CO2 into organic material.) As we trace the steps of the Calvin cycle, keep in mind that we are following three molecules of CO2 through the reactions. Figure 10.19 divides the Calvin cycle into three phases: carbon fixation, reduction, and regeneration of the CO2 acceptor.

Phase 1: Carbon fixation. The Calvin cycle incorporates each CO2 molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate (abbreviated RuBP). The enzyme that catalyzes this first step is RuBP carboxylase-oxygenase, or rubisco. (This is the most abun-dant protein in chloroplasts and is also thought to be the most abundant protein on Earth.) The product of the reac-tion is a six-carbon intermediate that is short-lived because it is so energetically unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate (for each CO2 fixed).

Phase 2: Reduction. Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3-bisphosphoglycerate. Next, a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate, which also loses a phosphate group in the process, becoming glyceral-dehyde 3-phosphate (G3P). Specifically, the electrons from NADPH reduce a carboyxl group on 1,3-bisphosphoglycerate to the aldehyde group of G3P, which stores more potential energy. G3P is a sugar—the same three-carbon sugar formed in glycolysis by the splitting of glucose (see Figure 9.9). Notice in Figure 10.19 that for every three molecules of CO2 that enter the cycle, there are six molecules of G3P formed. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate because the rest are required to complete the cycle. The cycle began with 15 car-bons’ worth of carbohydrate in the form of three molecules of the five-carbon sugar RuBP. Now there are 18 carbons’ worth of carbohydrate in the form of six molecules of G3P. One molecule exits the cycle to be used by the plant cell, but the other five molecules must be recycled to regenerate the three molecules of RuBP.

stroma, where it is used to help drive sugar synthesis during the Calvin cycle.

The proton (H+) gradient, or pH gradient, across the thylakoid membrane is substantial. When chloroplasts in an experimental setting are illuminated, the pH in the thylakoid space drops to about 5 (the H+ concentration increases), and the pH in the stroma increases to about 8 (the H+ concentration decreases). This gradient of three pH units corresponds to a thousandfold difference in H+ con-centration. If the lights are then turned off, the pH gradient is abolished, but it can quickly be restored by turning the lights back on. Experiments such as this provided strong evidence in support of the chemiosmotic model.

The currently accepted model for the organization of the light-reaction “machinery” within the thylakoid membrane is based on several research studies. Each of the molecules and molecular complexes in the figure is present in numerous copies in each thylakoid. Notice that NADPH, like ATP, is pro-duced on the side of the membrane facing the stroma, where the Calvin cycle reactions take place.

Let’s summarize the light reactions. Electron flow pushes electrons from water, where they are at a state of low poten-tial energy, ultimately to NADPH, where they are stored at a state of high potential energy. The light-driven electron flow also generates ATP. Thus, the equipment of the thylakoid membrane converts light energy to chemical energy stored in ATP and NADPH. Oxygen is produced as a by-product. Let’s now see how the Calvin cycle uses the products of the light reactions to synthesize sugar from CO2.

BioFlix® Animation: The Light Reactions

ConCept CHeCK 10.21. What color of light is least effective in driving photosyn-

thesis? explain.

2. in the light reactions, what is the initial electron donor? Where do the electrons finally end up?

3. WHAt IF? in an experiment, isolated chloroplasts placed in an illuminated solution with the appropri-ate chemicals can carry out Atp synthesis. predict what would happen to the rate of synthesis if a compound is added to the solution that makes membranes freely permeable to hydrogen ions.


ConCept 10.3The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugarThe Calvin cycle is similar to the citric acid cycle in that a starting material is regenerated after some molecules enter


ConCept CHeCK 10.31. to synthesize one glucose molecule, the calvin cycle uses

__________ molecules of co2, __________ molecules of Atp, and __________ molecules of nADph.

2. how are the large numbers of Atp and nADph molecules used during the calvin cycle consistent with the high value of glucose as an energy source?

3. WHAt IF? explain why a poison that inhibits an enzyme of the calvin cycle will also inhibit the light reactions.

4. DRAW It redraw the cycle in Figure 10.19 using numerals to indicate the numbers of carbons instead of gray balls, multiplying at each step to ensure that you have accounted for all the carbons. in what forms do the carbon atoms enter and leave the cycle?

5. MAKe ConneCtIons review Figures 9.9 and 10.19. Discuss the roles of intermediate and product played by glyceraldehyde 3-phosphate (G3p) in the two processes shown in these figures.


Phase 3: Regeneration of the CO2 acceptor (RuBP). In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. To accomplish this, the cycle spends three more molecules of ATP. The RuBP is now prepared to receive CO2 again, and the cycle continues.

For the net synthesis of one G3P molecule, the Calvin cycle consumes a total of nine molecules of ATP and six molecules of NADPH. The light reactions regenerate the ATP and NADPH. The G3P spun off from the Calvin cycle becomes the starting material for metabolic pathways that synthesize other organic compounds, including glucose (from two molecules of G3P), the disaccharide sucrose, and other carbohydrates. Neither the light reactions nor the Calvin cycle alone can make sugar from CO2. Photosynthesis is an emergent property of the intact chloroplast, which integrates the two stages of photosynthesis.

Input

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Figure 10.19 the Calvin cycle. This diagram summarizes three turns of the cycle, tracking carbon atoms (gray balls). The three phases of the cycle correspond to the phases discussed in the text. For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P), a three-carbon sugar. The light reactions sustain the Calvin cycle by regenerating the required ATP and NADPH.

Animation: The Calvin Cycle BioFlix® Animation: The Calvin Cycle


How can we explain the existence of a metabolic process that seems to be counterproductive for the plant? According to one hypothesis, photorespiration is evolutionary baggage—a metabolic relic from a much earlier time when the atmo-sphere had less O2 and more CO2 than it does today. In the ancient atmosphere that prevailed when rubisco first evolved, the ability of the enzyme’s active site to bind O2 would have made little difference. The hypothesis suggests that modern rubisco retains some of its chance affinity for O2, which is now so concentrated in the atmosphere that a certain amount of photorespiration is inevitable. There is also some evidence that photorespiration may provide protection against the damaging products of the light reactions, which build up when the Calvin cycle slows due to low CO2.

In many types of plants—including a significant number of crop plants—photorespiration drains away as much as 50% of the carbon fixed by the Calvin cycle. Indeed, if photorespi-ration could be reduced in certain plant species without otherwise affecting photosynthetic productivity, crop yields and food supplies might increase.

In some plant species, alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the Calvin cycle—even in hot, arid climates. The two most important of these photosynthetic adaptations are C4 photo-synthesis and crassulacean acid metabolism (CAM).

C4 PlantsThe C4 plants are so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product. The C4 pathway is believed to have evolved independently at least 45 separate times and is used by several thousand species in at least 19 plant families. Among the C4 plants important to agricul-ture are sugarcane and corn, members of the grass family.

The anatomy of a C4 leaf is correlated with the mechanism of C4 photosynthesis. In C4 plants, there are two distinct types of photosynthetic cells: bundle-sheath cells and mesophyll cells. Bundle-sheath cells are arranged into tightly packed sheaths around the veins of the leaf (Figure 10.20). Between the bundle sheath and the leaf surface are the more loosely arranged mesophyll cells, which, in C4 leaves, are closely asso-ciated and never more than two to three cells away from the bundle-sheath cells. The Calvin cycle is confined to the chloro-plasts of the bundle-sheath cells. However, the Calvin cycle is preceded by incorporation of CO2 into organic compounds in the mesophyll cells. See the numbered steps in Figure 10.20, which are also described here:

1 The first step is carried out by an enzyme present only in mesophyll cells called PEP carboxylase. This enzyme adds CO2 to phosphoenolpyruvate (PEP), forming the four-carbon product oxaloacetate. PEP carboxylase has a much higher affinity for CO2 than does rubisco and

ConCept 10.4Alternative mechanisms of carbon fixation have evolved in hot, arid climates eVoLUtIon Ever since plants first moved onto land about 475 million years ago, they have been adapting to the prob-lems of terrestrial life, particularly the problem of dehydration. In Concept 36.4, we will consider anatomical adaptations that help plants conserve water, while in this chapter we are concerned with metabolic adaptations. The solutions often in-volve trade-offs. An important example is the balance between photosynthesis and the prevention of excessive water loss from the plant. The CO2 required for photosynthesis enters a leaf (and the resulting O2 exits) via stomata, the pores on the leaf surface (see Figure 10.4). However, stomata are also the main avenues of transpiration, the evaporative loss of water from leaves. On a hot, dry day, most plants close their stomata, a response that conserves water but also reduces CO2 levels. With stomata even partially closed, CO2 concentrations begin to decrease in the air spaces within the leaf, and the concentra-tion of O2 released from the light reactions begins to increase. These conditions within the leaf favor an apparently wasteful process called photorespiration.

Photorespiration: An Evolutionary Relic?In most plants, initial fixation of carbon occurs via rubisco, the Calvin cycle enzyme that adds CO2 to ribulose bisphos-phate. Such plants are called C3 plants because the first organic product of carbon fixation is a three-carbon compound, 3-phosphoglycerate (see Figure 10.19). Rice, wheat, and soy-beans are C3 plants that are important in agriculture. When their stomata partially close on hot, dry days, C3 plants produce less sugar because the declining level of CO2 in the leaf starves the Calvin cycle. In addition, rubisco is capable of binding O2 in place of CO2. As CO2 becomes scarce within the air spaces of the leaf and O2 builds up, rubisco adds O2 to the Calvin cycle instead of CO2. The product splits, and a two-carbon com-pound leaves the chloroplast. Peroxisomes and mitochondria within the plant cell rearrange and split this compound, releas-ing CO2. The process is called photorespiration because it occurs in the light (photo) and consumes O2 while producing CO2 (respiration). However, unlike normal cellular respiration, photorespiration uses ATP rather than generating it. And unlike photosynthesis, photorespiration produces no sugar. In fact, photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle and releasing CO2 that would otherwise be fixed. This CO2 can eventually be fixed if it is still in the leaf once the CO2 concentration builds up to a high enough level. In the meantime, though, the process is energeti-cally costly, much like a hamster running on its wheel.


advantageous in hot regions with intense sunlight, where sto-mata partially close during the day, and it is in such environ-ments that C4 plants evolved and thrive today.

The concentration of CO2 in the atmosphere has drasti-cally increased since the Industrial Revolution began in the 1800s, and it continues to rise today due to human activities such as the burning of fossil fuels. The resulting global cli-mate change, including an increase in average temperatures around the planet, may have far-reaching effects on plant species. Scientists are concerned that increasing CO2 concen-tration and temperature may affect C3 and C4 plants differ-ently, thus changing the relative abundance of these species in a given plant community.

Which type of plant would stand to gain more from increas-ing CO2 levels? Recall that in C3 plants, the binding of O2 rather than CO2 by rubisco leads to photorespiration, lowering the efficiency of photosynthesis. C4 plants overcome this problem by concentrating CO2 in the bundle-sheath cells at the cost of ATP. Rising CO2 levels should benefit C3 plants by lowering the amount of photorespiration that occurs. At the same time, rising temperatures have the opposite effect, increasing photo-respiration. (Other factors such as water availability may also come into play.) In contrast, many C4 plants could be largely unaffected by increasing CO2 levels or temperature. Researchers have investigated aspects of this question in several studies; you can work with data from one such experiment in the scientific

skills exercise. In different regions, the particular combina-tion of CO2 concentration and temperature is likely to alter the balance of C3 and C4 plants in varying ways. The effects of such a widespread and variable change in community structure are unpredictable and thus a cause of legitimate concern.

no affinity for O2. Therefore, PEP carboxylase can fix car-bon efficiently when rubisco cannot—that is, when it is hot and dry and stomata are partially closed, causing CO2 concentration in the leaf to be lower and O2 concentra-tion to be relatively higher.

2 After the CO2 is fixed in the mesophyll cells, the four-carbon products (malate in the example shown in Figure 10.20) are exported to bundle-sheath cells through plasmodesmata (see Figure 6.29).

3 Within the bundle-sheath cells, the four-carbon com-pounds release CO2, which is re-fixed into organic mate-rial by rubisco and the Calvin cycle. The same reaction regenerates pyruvate, which is transported to mesophyll cells. There, ATP is used to convert pyruvate to PEP, which can accept addition of another CO2, allowing the reaction cycle to continue. This ATP can be thought of, in a sense, as the “price” of concentrating CO2 in the bundle-sheath cells. To generate this extra ATP, bundle-sheath cells carry out cyclic electron flow, the process described earlier in this chapter (see Figure 10.16). In fact, these cells contain PS I but no PS II, so cyclic electron flow is their only pho-tosynthetic mode of generating ATP.

In effect, the mesophyll cells of a C4 plant pump CO2 into the bundle-sheath cells, keeping the CO2 concentration in those cells high enough for rubisco to bind CO2 rather than O2. The cyclic series of reactions involving PEP carboxylase and the regeneration of PEP can be thought of as an ATP-powered pump that concentrates CO2. In this way, C4 pho-tosynthesis spends ATP energy to minimize photorespiration and enhance sugar production. This adaptation is especially

Mesophyllcell PEP carboxylase

Oxaloacetate (4C)

Stoma

Plasmodesma

PEP (3C)

CO2

Malate (4C)

Pyruvate (3C)

CalvinCycle

Sugar

Vasculartissue

Bundle-sheathcell

The C4 pathway

C4 leaf anatomy

CO2

ADP

Photosyntheticcells of C4 plantleaf

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue) ATP

In mesophyll cells,the enzyme PEPcarboxylase adds CO2 to PEP, forming a four- carbon compound.

1

The four-carboncompound (such as malate) moves into a bundle-sheath cellvia plasmodesmata.

2

In bundle-sheathcells, CO2 isreleased andenters the Calvin cycle.

3

Figure 10.20 C4 leaf anatomy and the C4 pathway. The structure and biochemical functions of the leaves of C4 plants are an evolutionary adaptation to hot, dry climates. This adaptation maintains a CO2 concentration in the bundle sheath that favors photosynthesis over photorespiration.


that the yield of C4 rice might be 30–50% higher than C3 rice with the same input of water and resources.

CAM PlantsA second photosynthetic adaptation to arid conditions has evolved in many succulent (water-storing) plants, numer-ous cacti, pineapples, and representatives of several other plant families. These plants open their stomata during the night and close them during the day, just the reverse of how other plants behave. Closing stomata during the day helps desert plants conserve water, but it also prevents CO2 from

C4 photosynthesis is considered more efficient than C3 photosynthesis because it uses less water and resources. On our planet today, the world population and demand for food are rapidly increasing. At the same time, the amount of land suit-able for growing crops is decreasing due to the effects of global climate change, which include an increase in sea level as well as a hotter, drier climate in many regions. To address issues of food supply, scientists in the Philippines have been working on genetically modifying rice—an important food staple that is a C3 crop—so that it can instead carry out C4 photosynthesis. Results so far seem promising, and these researchers estimate

SCIENTIFIC SkILLS ExERCISE

Making Scatter Plots with Regression LinesDoes Atmospheric CO2 Concentration Affect the Productivity of Agricultural Crops? The atmospheric con-centration of CO2 has been rising globally, and scientists wondered whether this would affect C3 and C4 plants differently. In this exercise, you will make a scatter plot to examine the relationship between CO2 concentration and growth of both corn (maize), a C4 crop plant, and velvetleaf, a C3 weed found in cornfields.

How the Experiment Was Done Researchers grew corn and velvetleaf plants under controlled conditions for 45 days, where all plants received the same amounts of water and light. The plants were divided into three groups, and each was exposed to a different con-centration of CO2 in the air: 350, 600, or 1,000 ppm (parts per million).

Data from the Experiment The table shows the dry mass (in grams) of corn and velvetleaf plants grown at the three concentrations of CO2. The dry mass values are averages of the leaves, stems, and roots of eight plants.

350 ppm CO2

600 ppm CO2

1,000 ppm CO2

Average dry mass of one corn plant (g)

91 89 80

Average dry mass of one velvetleaf plant (g)

35 48 54

Data from D. T. Patterson and E. P. Flint, Potential effects of global atmospheric CO2 enrichment on the growth and competitiveness of C3 and C4 weed and crop plants, Weed Science 28(1):71–75 (1980).

IntErPrEt tHE DAtA

1. To explore the relationship between the two variables, it is useful to graph the data in a scatter plot, and then draw a regression line. (a) First, place labels for the dependent and independent variables on the appropriate axes. Explain your choices. (b) Now plot the data points for corn and velvetleaf using different symbols for each set of data, and add a key for the two symbols. (For additional informa-tion about graphs, see the Scientific Skills Review in Appendix F.)

2. Draw a “best-fit” line for each set of points. A best-fit line does not necessarily pass through all or even most points. Instead, it is a straight line that passes as close as possible to all data points from that set. Draw a best-fit line for each set of data. Because placement of the line is a matter of judgment, two individuals may draw two slightly different lines for a given set of points.

the line that actually fits best, a regres-sion line, can be identified by squar-ing the distances of all points to any candidate line, then selecting the line that minimizes the sum of the squares. (See the graph in the Scientific Skills exercise in chapter 3 for an example of a linear regression line.) excel or other software programs, includ-ing those on a graphing calculator, can plot a regression line once data points are entered. Using either excel or a graphing calculator, enter the data points for each data set and have the program draw the two regression lines. compare them to the lines you drew.

3. Describe the trends shown by the regression lines in your scatter plot. (a) compare the relationship between increasing concen-tration of co2 and the dry mass of corn to that for velvetleaf. (b) considering that velvetleaf is a weed invasive to cornfields, predict how increased co2 concentration may affect interactions between the two species.

4. Based on the data in the scatter plot, estimate the percentage change in dry mass of corn and velvetleaf plants if atmospheric co2 concentration increased from 390 ppm (current levels) to 800 ppm. (a) What is the estimated dry mass of corn and velvetleaf plants at 390 ppm? 800 ppm? (b) to calculate the percentage change in mass for each plant, subtract the mass at 390 ppm from the mass at 800 ppm (change in mass), divide by the mass at 390 ppm (initial mass), and multiply by 100. What is the estimated percentage change in dry mass for corn? For velvetleaf? (c) Do these results support the conclusion from other experiments that c3 plants grow better than c4 plants under increased co2 concentration? Why or why not?

Corn plant

Velvetleafplant

Corn plant surrounded by invasive velvetleaf plants

Instructors: A version of this Scientific Skills exercise can be assigned in MasteringBiology.


ConCept CHeCK 10.41. Describe how photorespiration lowers photosynthetic

output for plants.2. the presence of only pS i, not pS ii, in the bundle-sheath

cells of c4 plants has an effect on o2 concentration. What is that effect, and how might that benefit the plant?

3. MAKe ConneCtIons refer to the discussion of ocean acidification in concept 3.3. ocean acidification and changes in the distribution of c3 and c4 plants may seem to be two very different problems, but what do they have in common? explain.

4. WHAt IF? how would you expect the relative abun-dance of c3 versus c4 and cAM species to change in a geographic region whose climate becomes much hotter and drier, with no change in co2 concentration?


ConCept 10.5Life depends on photosynthesis

The Importance of Photosynthesis: A ReviewIn this chapter, we have followed photosynthesis from pho-tons to food. The light reactions capture solar energy and use it to make ATP and transfer electrons from water to NADP+, forming NADPH. The Calvin cycle uses the ATP and NADPH to produce sugar from carbon dioxide. The energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds. The entire process is reviewed visually in Figure 10.22, where photosynthesis is also put in its natural context.

As for the fates of photosynthetic products, enzymes in the chloroplast and cytosol convert the G3P made in the Calvin cycle to many other organic compounds. In fact, the sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons for the synthesis of all the major organic molecules of plant cells. About 50% of the organic material made by photosynthesis is consumed as fuel for cellular respiration in plant cell mitochondria.

Technically, green cells are the only autotrophic parts of the plant. The rest of the plant depends on organic molecules exported from leaves via veins (see Figure 10.22, top). In most plants, carbohydrate is transported out of the leaves to the rest of the plant in the form of sucrose, a disaccharide. After arriving at nonphotosynthetic cells, the sucrose provides raw material for cellular respiration and a multitude of anabolic pathways that synthesize proteins, lipids, and other products. A consider-able amount of sugar in the form of glucose is linked together to make the polysaccharide cellulose (see Figure 5.6c), especially in plant cells that are still growing and maturing. Cellulose, the main ingredient of cell walls, is the most abundant organic mol-ecule in the plant—and probably on the surface of the planet.

Most plants and other photosynthesizers make more organic material each day than they need to use as respiratory

entering the leaves. During the night, when their stomata are open, these plants take up CO2 and incorporate it into a variety of organic acids. This mode of carbon fixation is called crassulacean acid metabolism, or CAM, after the plant family Crassulaceae, the succulents in which the process was first discovered. The mesophyll cells of CAM plants store the organic acids they make during the night in their vacu-oles until morning, when the stomata close. During the day, when the light reactions can supply ATP and NADPH for the Calvin cycle, CO2 is released from the organic acids made the night before to become incorporated into sugar in the chloroplasts.

Notice in Figure 10.21 that the CAM pathway is similar to the C4 pathway in that CO2 is first incorporated into organic intermediates before it enters the Calvin cycle. The difference is that in C4 plants, the initial steps of carbon fixation are separated structurally from the Calvin cycle, whereas in CAM plants, the two steps occur within the same cell but at separate times. (Keep in mind that CAM, C4, and C3 plants all eventu-ally use the Calvin cycle to make sugar from carbon dioxide.)

Sugar

CO2

CO2

CalvinCycle

CalvinCycle

Sugar

CO2

Organic acid

CO2

Bundle-sheathcell

Mesophyllcell

C4 CAM

Day

NightOrganic acid

Sugarcane Pineapple

(a) Spatial separation of steps.In C4 plants, carbon fixation and the Calvin cycle occur indifferent types of cells.

(b) Temporal separation of steps.In CAM plants, carbon fixationand the Calvin cycle occur in the same cell at different times.

11

22

Figure 10.21 C4 and CAM photosynthesis compared. The C4 and CAM pathways are two evolutionary solutions to the problem of maintaining photosynthesis with stomata partially or completely closed on hot, dry days. Both adaptations are characterized by 1 preliminary incorporation of CO2 into organic acids, followed by 2 transfer of CO2 to the Calvin cycle.

Animation: Photosynthesis in Dry Climates


would reach 17 times the distance from Earth to the sun! No chemical process is more important than photosynthesis to the welfare of life on Earth.

In Chapters 5 through 10, you have learned about many activities of cells. Figure 10.23 integrates these cellular processes into the context of a working plant cell. As you study the figure, reflect on how each process fits into the big picture: As the most basic unit of living organisms, a cell

performs all functions characteristic of life.

ConCept CHeCK 10.51. MAKe ConneCtIons can plants use the sugar they pro-

duce during photosynthesis to directly power the work of the cell? explain. (See Figures 8.10, 8.11, and 9.6.).


fuel and precursors for biosynthesis. They stockpile the extra sugar by synthesizing starch, storing some in the chloroplasts themselves and some in storage cells of roots, tubers, seeds, and fruits. In accounting for the consumption of the food molecules produced by photosynthesis, let’s not forget that most plants lose leaves, roots, stems, fruits, and sometimes their entire bodies to heterotrophs, including humans.

On a global scale, photosynthesis is the process responsible for the presence of oxygen in our atmosphere. Furthermore, although each chloroplast is minuscule, their collective productivity in terms of food production is prodigious: Photosynthesis makes an estimated 150 billion metric tons of carbohydrate per year (a metric ton is 1,000 kg, about 1.1 tons). That’s organic matter equivalent in mass to a stack of about 60 trillion biology textbooks—and the stack

• Take place in the stroma

• Use ATP and NADPH to convert CO2 to the sugar G3P

• Return ADP, inorganic phosphate, and NADP+ to the light reactions

Starch(storage)

Mesophyll cell

Chloroplast

O2

CALVINCYCLE

H2O

Sucrose (export)

Light

LIGHTREACTIONS:Photosystem II

Electron transport chainPhotosystem I

Electron transport chain

RuBP

3-Phosphoglycerate

G3P

NADPH

NADP+

ADP+P i

ATP

CO2

• Are carried out by molecules in the thylakoid membranes

• Convert light energy to the chemical energy of ATP and NADPH

• Split H2O and release O2 to the atmosphere

CO2

Sucrose(export)

H2O

O2

LIGHT REACTIONS CALVIN CYCLE REACTIONS

H2O

Figure 10.22 A review of photosynthesis. This diagram shows the main reactants and products of photosynthesis as they move through the tissues of a tree (left) and a chloroplast (right).

BioFlix® Animation: Photosynthesis MP3 Tutor: Photosynthesis

1

2

3

4

5

6

Golgiapparatus

Vesicleforming

Protein

Plasmamembrane

Cell wall

Proteinin vesicle

Rough endoplasmicreticulum (ER)

DNA

mRNA

Nuclearpore

Nucleus

Ribosome mRNA

Protein

1

2

3

4

5

6

Flow of Genetic Information in the Cell:DNA RNA Protein (Chapters 5–7)

This figure illustrates how a generalized plant cell functions, integrating the cellular activities you learned about in Chapters 5–10.

In the nucleus, DNA serves as a template for the synthesis of mRNA, which moves to the cytoplasm. (See Figures 5.22 and 6.9.)

mRNA attaches to a ribosome, which remains free in the cytosol or binds to the rough ER. Proteins are synthesized. (See Figures 5.22 and 6.10.)

Proteins and membrane produced by the rough ER flow in vesicles to the Golgi apparatus, where they are processed. (See Figures 6.15 and 7.9.)

Transport vesicles carrying proteins pinch off from the Golgi apparatus. (See Figure 6.15.)

Some vesicles merge with the plasma membrane, releasing proteins by exocytosis. (See Figure 7.9.)

Proteins synthesized on free ribosomes stay in the cell and perform specific functions; examples include the enzymes that catalyze the reactions of cellular respiration and photosynthesis. (See Figures 9.7, 9.9, and 10.19.)


Figure 10.23 MAKe ConneCtIons

The Working Cell

10

9

11

Vacuole

Photosynthesisin chloroplast

Organicmolecules

CO2

CO2

H2O

O2

O2

H2O

TransportpumpCellular respiration

in mitochondrion

7

8

9

11

ATP

ATP

ATP

ATP

10

Energy Transformations in the Cell:Photosynthesis and Cellular Respiration(Chapters 8–10)

Movement Across Cell Membranes(Chapter 7)

7

8

Water diffuses into and out of the cell directly through the plasma membrane and by facilitated diffusion through aquaporins. (See Figure 7.1.)

By passive transport, the CO2 used in photosynthesis diffuses into the cell and the O2 formed as a by-product of photosynthesis diffuses out of the cell. Both solutes move down their concentration gradients. (See Figures 7.10 and 10.22.)

Exocytosis (shown in step 5) and endocytosis move larger materials out of and into the cell. (See Figures 7.9 and 7.19.)

In active transport, energy (usually supplied by ATP) is used to transport a solute against its concentration gradient. (See Figure 7.16.)

In chloroplasts, the process of photosynthesis uses the energy of light to convert CO2 and H2O to organic molecules, with O2 as a by-product. (See Figure 10.22.)

In mitochondria, organic molecules are broken down by cellular respiration, capturing energy in molecules of ATP, which are used to power the work of the cell, such as protein synthesis and active transport. CO2 and H2O are by-products. (See Figures 8.9–8.11, 9.2, and 9.16.)


MAKe ConneCtIons The first enzyme that functions in glycolysis is hexokinase. In this plant cell, describe the entire process by which this enzyme is produced and where it functions, specifying the locations for each step. (See Figures 5.18, 5.22, and 9.9.)

BioFlix® Animation: Tour of an Animal CellBioFlix® Animation: Tour of a Plant Cell


Chapter Review10 Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, practice tests, and more in the Study Area.

VoCAB seLF-QUIZgoo.gl/6u55ks

sUMMARy oF Key ConCepts

ConCept 10.1photosynthesis converts light energy to the chemical energy of food (pp. 189–192)

In eukaryotes that are autotrophs, photosynthe-sis occurs in chloroplasts, organelles containing thylakoids. Stacks of thylakoids form grana. Photosynthesis is summarized as

6 CO2 + 12 H2O + Light energy S C6H12O6 + 6 O2 + 6 H2O.

Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules. Photosynthesis is a redox process: H2O is oxidized, and CO2 is reduced. The light reactions in the thylakoid membranes split water, releasing O2, producing ATP, and forming NADPH. The Calvin cycle in the stroma forms sugar from CO2, using ATP for energy and NADPH for reducing power.

? Compare the roles of CO2 and H2O in cellular respiration and photosynthesis.

ConCept 10.2the light reactions convert solar energy to the chemical energy of Atp and nADpH (pp. 192–201)

Light is a form of electromagnetic energy. The colors we see as visible light include those wavelengths that drive photosynthe-sis. A pigment absorbs light of specific wavelengths; chlorophyll a is the main photosynthetic pigment in plants. Other accessory pig-ments absorb different wavelengths of light and pass the energy on to chlorophyll a.

A pigment goes from a ground state to an excited state when a photon of light boosts one of the pigment’s electrons to a higher-energy orbital. This excited state is unstable. Electrons from isolated pigments tend to fall back to the ground state, giving off heat and/or light.

A photosystem is composed of a reaction-center complex surrounded by light-harvesting complexes that funnel the energy of photons to the reaction-center complex. When a special pair of reaction-center chlorophyll a molecules absorbs energy, one of its electrons is boosted to a higher energy level and transferred to the primary electron acceptor. Photosystem II contains P680 chlorophyll a molecules in the reaction-center complex; photosystem I contains P700 molecules.

Linear electron flow during the light reactions uses both photosystems and produces NADPH, ATP, and oxygen:

Photosystem II

Photosystem IATP

Pq

FdPrimaryelectronacceptor

Pc

Cytochromecomplex

NADP+

reductase

NADP+

+ H+

NADPH

Primaryelectronacceptor

O2

H2O

Electron transport

chain

Electron transportchain

Cyclic electron flow employs only one photosystem, produc-ing ATP but no NADPH or O2.

During chemiosmosis in both mitochondria and chloroplasts, electron transport chains generate an H+ gradient across a membrane. ATP synthase uses this proton-motive force to make ATP.

? The absorption spectrum of chlorophyll a differs from the action spectrum of photosynthesis. Explain this observation.

ConCept 10.3the Calvin cycle uses the chemical energy of Atp and nADpH to reduce Co2 to sugar (pp. 201–202)

The Calvin cycle occurs in the stroma, using electrons from NADPH and energy from ATP. One molecule of G3P exits the cycle per three CO2 molecules fixed and is converted to glucose and other organic molecules.

CalvinCycle

Carbon fixation

Regeneration ofCO2 acceptor

Reduction

3 x 5C 6 x 3C

5 x 3C

1 G3P (3C)

3 CO2

DRAW It On the diagram above, draw where ATP and NADPH are used and where rubisco functions. Describe these steps.

ConCept 10.4Alternative mechanisms of carbon fixation have evolved in hot, arid climates (pp. 203–206)

On dry, hot days, C3 plants close their stomata, conserving water but keeping CO2 out and O2 in. Under these conditions, photorespiration can occur: Rubisco binds O2 instead of CO2, consuming ATP and releasing CO2 without producing ATP or car-bohydrate. Photorespiration may be an evolutionary relic, and it may play a photoprotective role.

C4 plants minimize the cost of photorespiration by incorporat-ing CO2 into four-carbon compounds in mesophyll cells. These compounds are exported to bundle-sheath cells, where they release carbon dioxide for use in the Calvin cycle.

CAM plants open their stomata at night, incorporating CO2 into organic acids, which are stored in mesophyll cells. During the day, the stomata close, and the CO2 is released from the organic acids for use in the Calvin cycle.

Organic compounds produced by photosynthesis provide the energy and building material for Earth’s ecosystems.

? Why are C4 and CAM photosynthesis more energetically expensive than C3 photosynthesis? What climate conditions would favor C4 and CAM plants?


test yoUR UnDeRstAnDIng

Level 1: Knowledge/Comprehension

1. The light reactions of photosynthesis supply the Calvin cycle with(A) light energy.(B) CO2 and ATP.(C) H2O and NADPH.(D) ATP and NADPH.

2. Which of the following sequences correctly represents the flow of electrons during photosynthesis?(A) NADPH S O2 S CO2

(B) H2O S NADPH S Calvin cycle(C) H2O S photosystem I S photosystem II(D) NADPH S electron transport chain S O2

3. How is photosynthesis similar in C4 plants and CAM plants?(A) In both cases, only photosystem I is used.(B) Both types of plants make sugar without the Calvin cycle.(C) In both cases, rubisco is not used to fix carbon initially.(D) Both types of plants make most of their sugar in the dark.

4. Which of the following statements is a correct distinction between autotrophs and heterotrophs?(A) Autotrophs, but not heterotrophs, can nourish themselves

beginning with CO2 and other nutrients that are inorganic.(B) Only heterotrophs require chemical compounds from the

environment.(C) Cellular respiration is unique to heterotrophs.(D) Only heterotrophs have mitochondria.

5. Which of the following does not occur during the Calvin cycle?(A) carbon fixation(B) oxidation of NADPH(C) release of oxygen(D) regeneration of the CO2 acceptor

Level 2: Application/Analysis

6. In mechanism, photophosphorylation is most similar to(A) substrate-level phosphorylation in glycolysis.(B) oxidative phosphorylation in cellular respiration.(C) carbon fixation.(D) reduction of NADP+.

7. Which process is most directly driven by light energy?(A) creation of a pH gradient by pumping protons across the

thylakoid membrane(B) reduction of NADP+ molecules(C) transfer of energy from pigment molecule to pigment

molecule(D) ATP synthesis

Level 3: synthesis/evaluation

8. sCIenCe, teCHnoLogy, AnD soCIety Scientific evidence indicates that the CO2 added to the air by the burning of wood and fossil fuels is contributing to global warming, a rise in global temperature. Tropical rain forests are estimated to be responsible for approximately 20% of global photosynthesis, yet the consumption of large amounts of CO2 by living trees

pRACtICe test

goo.gl/cUYGKD

For additional practice questions, check out the Dynamic study Modules in MasteringBiology. You can use them to study on your smartphone, tablet, or computer anytime, anywhere!

ConCept 10.5

Life depends on photosynthesis (pp. 206–209) Organic compounds produced by photosynthesis provide the

energy and building material for Earth’s ecosystems.

? Explain how all life depends on photosynthesis.

is thought to make little or no net contribution to reduction of global warming. Explain why this might be the case. (Hint: What processes in both living and dead trees produce CO2?)

9. eVoLUtIon ConneCtIon Photorespiration can decrease soybeans’ photosynthetic output by about 50%. Would you expect this figure to be higher or lower in wild relatives of soybeans? Why?

10. sCIentIFIC InQUIRy • MAKe ConneCtIons The following diagram represents an experiment with isolated thylakoids. The thylakoids were first made acidic by soaking them in a solution at pH 4. After the thylakoid space reached pH 4, the thylakoids were transferred to a basic solution at pH 8. The thylakoids then made ATP in the dark. (See Concept 3.3 to review pH.)

pH 7

pH 4

pH 4

pH 8

ATP

Draw an enlargement of part of the thylakoid membrane in the beaker with the solution at pH 8. Draw ATP synthase. Label the areas of high H+ concentration and low H+ concentration. Show the direction protons flow through the enzyme, and show the reaction where ATP is synthesized. Would ATP end up in the thylakoid or outside of it? Explain why the thylakoids in the experiment were able to make ATP in the dark.

11. WRIte ABoUt A tHeMe: eneRgy AnD MAtteR Life is solar powered. Almost all the producers of the biosphere depend on energy from the sun to produce the organic molecules that supply the energy and carbon skeletons needed for life. In a short essay (100–150 words), describe how the process of photosynthesis in the chloroplasts of plants transforms the energy of sunlight into the chemical energy of sugar molecules.

12. syntHesIZe yoUR KnoWLeDge

“Watermelon snow” in Antarctica is caused by a species of photo-synthetic green algae that thrives in sub-zero temperatures (Chlamydomonas nivalis). These algae are also found in high-altitude year-round snowfields. In both loca-tions, UV light levels tend to be high. Based on what you learned in this chapter, propose an explanation for why this photosynthetic alga appears reddish-pink.

For selected answers, see Appendix A.

212

When you see this blue icon, log in to MasteringBiology and go to the Study Area for digital resources.

Key ConCepts11.1 External signals are converted

to responses within the cell

11.2 Reception: A signaling molecule binds to a receptor protein, causing it to change shape

11.3 Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell

11.4 Response: Cell signaling leads to regulation of transcription or cytoplasmic activities

11.5 Apoptosis integrates multiple cell-signaling pathways

Superset

Figure 11.1 How does cell signaling trigger the desperate flight of this impala?

11

Cellular MessagingThe impala in Figure 11.1 flees for its life, racing to escape the predatory cheetah nipping at its heels. The impala is breathing rapidly, its heart pounding and its legs pumping furiously. These physiological functions are all part of the impala’s “fight-or-flight” response, driven by hormones released from its adrenal glands at times of stress—in this case, upon sensing the cheetah. What systems allow the trillions of cells in the impala to “talk” to each other, coordinating their activities?

Cells can signal to each other and interpret the signals they receive from other cells and the environment. The signals may include light and touch, but are most often chemicals. The flight response shown here is triggered by a signaling molecule called epinephrine (also called adrenaline; see the space-filling model included here). In studying cell communication, biologists have discovered ample evidence for the evolutionary relatedness of all life. The same set of cell-signaling mechanisms shows up again and again in diverse species, in processes ranging from bacterial sig-naling to embryonic development to cancer. In this chapter, we focus on the main mechanisms by which cells receive, process, and respond to chemical signals sent from other cells. We will also consider apoptosis, a mechanism of programmed cell death that integrates input from multiple signaling pathways.

Cell Communication

Get Ready for This Chapter epinephrine (adrenaline)

chApter 11 Cell Communication 213

that the altered proteins would bind to each other but not to the original proteins of the parent cells. The genetically engineered cells were thus able to mate with one another but not with cells of the parent population. This evidence supports a model in which changes in the genes encoding receptor and mating factor proteins can lead to the establishment of new species.

How does the binding of a mating factor by the yeast cell surface receptor initiate a signal that brings about the cellular response of mating? This occurs in a series of steps called a signal transduction pathway. Many such pathways exist in both yeast and animal cells. In fact, the molecular details of signal transduction in yeasts and mammals are strikingly similar, even though it’s been over a billion years since they shared a common ancestor. This suggests that early versions of cell-signaling mechanisms evolved hundreds of millions of years before the first multicellular creatures appeared on Earth.

Scientists think that signaling mechanisms first evolved in ancient prokaryotes and single-celled eukaryotes like yeasts and then were adopted for new uses by their multicellular descen-dants. Cell signaling is also critical among prokaryotes. For example, bacterial cells secrete molecules that can be detected by other bacterial cells (Figure 11.3). Sensing the concentration of such signaling molecules allows bacteria to monitor their own local cell density, a phenomenon called quorum sensing.

Quorum sensing allows bacterial populations to coordinate their behaviors in activities that require a given number of cells

ConCept 11.1External signals are converted to responses within the cellWhat does a cell that is “talking” say to a “listening” cell, and how does the latter cell respond to the message—that is, how do cells communicate? Let’s approach these questions by first looking at communication among microorganisms.

Evolution of Cell Signaling evolution One topic cells communicate about is sex. Cells of the unicellular yeast Saccharomyces cerevisiae—which are used to make bread, wine, and beer—identify their sexual mates by chemical signaling. There are two sexes, or mating types, called a and � (Figure 11.2). Each type secretes a specific factor that binds only to receptors on the other type of cell. When exposed to each other’s mating factors, a pair of cells of opposite type change shape, grow toward each other, and fuse (mate). The new a/� cell contains all the genes of both original cells, a com-bination of genetic resources that provides advantages to the cell’s descendants, which arise by subsequent cell divisions.

The unique match between mating factor and receptor is key to ensuring mating only among cells of the same species of yeast. Recently, researchers were able to genetically engineer yeast cells with both receptors and mating factors altered so

a

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Exchange of mating factors. Each mating cell type secretes a mating factor that binds to receptors on the other mating type.

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Mating. Binding of the factors to receptors induces changes in the cells that lead to their fusion.

New a/α cell. The nucleus of the fused cell includes all the genes from the a and α cells.

a

Figure 11.2 Communication between mating yeast cells. Saccharomyces cerevisiae cells use chemical signaling to identify cells of opposite mating type and initiate the mating process. The two mating types and their corresponding chemical signaling molecules, or mating factors, are called a and �.

Individual rod-shaped cells

1

Fruiting bodies

Spore-forming structure(fruiting body)

3

Aggregation in progress2

0.5 mm

2.5 mm

Figure 11.3 Communication among bacteria. Soil-dwelling bacteria called myxobacteria (“slime bacteria”) use chemical signals to share information about nutrient availability. When food is scarce, starving cells secrete a signaling molecule that stimulates neighboring cells to aggregate. The cells form a structure called a fruiting body

that produces spores, thick-walled cells capable of surviving until the environment improves. The myxo-bacteria shown here are the species Myxococcus xanthus (steps 1–3, SEMs; lower photo, LM).


PRoblEm-SolvinG ExERCiSE

Can a skin wound turn deadly?“that scrape i got at the game last week looks infected. i wonder if i should go to the doctor?” contact sports can be hard on your body even if you are in top physi-cal condition. “contact” in many cases leads to skin wounds that can become infected—and even deadly, if infected with antibiotic-resistant bacteria.

Watch the video in the Mastering-Biology Study Area to see what happened when a strain of antibiotic-resistant bacteria called MrSA infected at least one high school student. MrSA stands for methicillin-resistant Staphylococcus aureus, a strain of bacteria that is resis-tant to several types of antibiotics, not just methicillin. Most “staph” infections are not antibiotic-resistant and can be treated with antibiotics.

in this exercise, you will investigate the mechanism by which cells sense their own population density (so-called quorum sensing) to analyze whether blocking it can stop S. aureus from producing toxin.

your Approach the facts you have in hand for your investigation are that quorum sensing in S. aureus involves two separate signal transduction pathways that can lead to toxin production. two candidate synthetic peptides (short proteins), called peptides 1 and 2, have been proposed to interfere with the S. aureus quorum-sensing pathways. Your job is to test these two potential inhibitors of quorum sensing to see if they block either or both of the pathways that lead to toxin production.

For your experiment, you grow four cultures of S. aureus to a standardized high density and measure the concentration of toxin in the culture. the control culture contains no peptide. the other cultures have one or both candidate inhibitory peptides mixed into the growth medium before starting the cultures.

your Data

AbC news video: mRSA outbreak

instructors: A version of this problem-Solving exercise can be assigned in MasteringBiology. or a more extensive investigation called “Solve it: is it possible to treat Bacterial infections Without traditional Antibiotics?” can be assigned.

Data from N. Balaban et al., Treatment of Staphylococcus aureus biofilm infection by the quorum-sensing inhibitor RIP, Antimicrobial Agents and Chemotherapy 51(6):2226–2229 (2007).

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your Analysis

Staphylococcus aureus (S. aureus) is a common bacterial species found on the surface of healthy skin that can turn into a serious pathogen if introduced into tissue through a cut or abrasion. once inside the body, a population of S. aureus that reaches a certain density will start to secrete a toxin, killing body cells and contributing significantly to inflamma-tion and damage. Because about 1 in 100 people carry a strain of S. aureus that is resistant to common antibiotics, a minor infection can turn permanently harmful or even deadly.

bbC video: brushing Your Teeth Can Save Your lifeinterview with bonnie bassler: Exploring how bacteria communicate with each otherHHmi video: interview with bonnie bassler

1. rank the cultures according to toxin production, from most to least.2. Which, if any, of the cultures with peptide(s) resulted in a toxin

concentration similar to the control culture? What is your evidence for this?

3. Was there an additive effect on toxin production when peptides 1 and 2 were both present in the growth medium? What is your evidence for this?

4. Based on these data, would you hypothesize that peptides 1 and 2 act on the same quorum-sensing pathway leading to toxin production or on two different pathways? What is your reasoning?

5. Do these data suggest a possible treatment for antibiotic-resistant S. aureus infections? What else would you want to know to investigate this further?


acting synchronously. One example is formation of a biofilm, an aggregation of bacterial cells adhered to a surface. The cells in the biofilm often derive nutrition from the surface they are on. You have probably encountered biofilms many times, per-haps without realizing it. The slimy coating on a fallen log or on leaves lying on a forest path, and even the film on your teeth each morning, are examples of bacterial biofilms. In fact, tooth-brushing and flossing disrupt biofilms that would otherwise cause cavities and gum disease.

Another example of bacterial behavior coordinated by quo-rum sensing is one that has serious medical implications: the secretion of toxins by infectious bacteria. Sometimes treatment by antibiotics doesn’t work with such infections due to antibi-otic resistance that has evolved in a particular strain of bacteria. Interfering with the signaling pathways used in quorum sensing represents a promising approach as an alternative treatment. In the problem-solving exercise, you can participate in the pro-cess of scientific thinking involved in this novel approach.

local and long-Distance SignalingLike bacteria or yeast cells, cells in a multicellular organism usually communicate via signaling molecules targeted for cells that may or may not be immediately adjacent. As we saw in Concepts 6.7 and 7.1, eukaryotic cells may commu-nicate by direct contact, which is one type of local signaling (Figure 11.4). Both animals and plants have cell junctions that, where present, directly connect the cytoplasms of adja-cent cells (Figure 11.4a). In these cases, signaling substances dissolved in the cytosol can pass freely between neighboring cells. Moreover, animal cells may communicate via direct contact between membrane-bound cell-surface molecules,

in a process called cell-cell recognition (Figure 11.4b). This sort of local signaling is especially important in embryonic development and the immune response.

In many other cases of local signaling, signaling molecules are secreted by the signaling cell. Some molecules travel only short distances; such local regulators influence cells in the vicinity. This type of local signaling in animals is called para-crine signaling (Figure 11.5a). One class of local regulators in

Plasma membranes Cell wall

Gap junctionsbetween animal cells

Plasmodesmatabetween plant cells

(a)

(b) Cell-cell recognition. Two cells in an animal may communicateby interaction between molecules protruding from their surfaces.

Cell junctions. Both animals and plants have cell junctions thatallow molecules, including signaling molecules, to pass readily between adjacent cells without crossing plasma membranes.

Figure 11.4 Communication by direct contact between cells.

Hormonetravels inbloodstream.

Bloodvessel

Endocrine cell Target cellspecificallybindshormone.

Local signaling Long-distance signaling

(a) (b)Paracrine signaling. A signaling cell acts on nearby target cells by secretingmolecules of a local regulator (a growth factor, for example).

Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stim-ulating the target cell, such as a muscle or another nerve cell.

(c) Endocrine (hormonal) signaling. Specialized endocrine cells secrete hormones into body fluids, often blood. Hormones reach most body cells, but are bound by and affect only some cells.

Electrical signal triggers release of neurotransmitter.

Target cell

Neurotransmitterdiffuses acrosssynapse.

Target cells

Signalingcell

Secretoryvesicles

Local regulator

Figure 11.5 local and long-distance cell signaling by secreted molecules in animals. In both local and long-distance signaling, only specific target cells that can recognize a given signaling molecule will respond to it.


in 1971. Sutherland and his colleagues at Vanderbilt University were investigating how the animal hormone epinephrine (also called adrenaline) triggers the “fight-or-flight” response in animals by stimulating the breakdown of the storage polysac-charide glycogen within liver cells and skeletal muscle cells. Glycogen breakdown releases the sugar glucose 1-phosphate, which the cell converts to glucose 6-phosphate. The liver or muscle cell can then use this compound, an early interme-diate in glycolysis, for energy production (see Figure 9.9). Alternatively, the compound can be stripped of phosphate and released from the cell into the blood as glucose, which can fuel cells throughout the body. Thus, one effect of epinephrine is the mobilization of fuel reserves, which can be used by the ani-mal to either defend itself (fight) or escape whatever elicited a scare (flight), as the impala in Figure 11.1 is clearly doing.

Sutherland’s research team discovered that epinephrine stimulates glycogen breakdown by somehow activating a cytosolic enzyme, glycogen phosphorylase. However, when epinephrine was added to a cell-free mixture containing the enzyme and its substrate, glycogen, no breakdown occurred. Glycogen phosphorylase could be activated by epinephrine only when the hormone was added to intact cells. This result told Sutherland two things. First, epinephrine does not interact directly with the enzyme responsible for glycogen breakdown; an intermediate step or series of steps must be occurring in the cell. Second, an intact, membrane-bound cell must be present for transmission of the signal to take place.

Sutherland’s work suggested that the process going on at the receiving end of a cellular communication can be dissected into three stages: reception, transduction, and response (Figure 11.6):

1 Reception. Reception is the target cell’s detection of a signaling molecule coming from outside the cell. A chem-ical signal is “detected” when the signaling molecule binds to a receptor protein located at the cell’s surface (or inside the cell, to be discussed later).

2 Transduction. The binding of the signaling molecule changes the receptor protein in some way, initiating the

animals, growth factors, are compounds that stimulate nearby target cells to grow and divide. Numerous cells can simultane-ously receive and respond to the growth factors produced by a single cell in their vicinity.

A more specialized type of local signaling called synaptic signaling occurs in the animal nervous system (Figure 11.5b). An electrical signal along a nerve cell triggers the secretion of neurotransmitter molecules. These molecules act as chemical signals, diffusing across the synapse—the narrow space between the nerve cell and its target cell—triggering a response in the target cell.

Both animals and plants use molecules called hormones for long-distance signaling. In hormonal signaling in animals, also known as endocrine signaling, specialized cells release hor-mones, which travel via the circulatory system to other parts of the body, where they reach target cells that can recognize and respond to them (Figure 11.5c). Plant hormones (often called plant growth regulators) sometimes travel in plant vessels (tubes) but more often reach their targets by moving through cells or by diffusing through the air as a gas (see Concept 39.2). Like local regulators, hormones vary widely in size and type. For instance, the plant hormone ethylene, a gas that promotes fruit ripening and helps regulate growth, is a hydrocarbon of only six atoms (C2H4), small enough to pass through cell walls. In contrast, the mammalian hormone insulin, which regulates sugar levels in the blood, is a protein with thousands of atoms.

What happens when a potential target cell is exposed to a secreted signaling molecule? The ability of a cell to respond is determined by whether it has a specific receptor molecule that can bind to the signaling molecule. The information conveyed by this binding, the signal, must then be changed into another form—transduced—inside the cell before the cell can respond. The remainder of the chapter discusses this process, primarily as it occurs in animal cells.

The Three Stages of Cell Signaling: A PreviewOur current understanding of how signaling molecules act via signal transduction pathways had its origins in the pioneering work of Earl W. Sutherland, whose research led to a Nobel Prize

Three relay molecules in a signal transduction pathway

Activationof cellularresponse, such as activation of glycogen phosphorylase

Transduction ResponseReception

Signaling molecule

Plasma membraneCYTOPLASMEXTRACELLULAR

FLUID

Receptor

1 2 3

1 2 3

Figure 11.6 overview of cell signaling. From the perspective of the cell receiving the message, cell signaling can be divided into three stages: signal reception, signal transduction, and cellular response. When reception occurs at the plasma membrane, as shown here, the transduction stage is usually a pathway of several steps (three are shown as an example), with each specific relay molecule in the pathway bringing about a change in the next molecule. The final molecule in the pathway triggers the cell’s response.

Animation: overview of Cell Signaling

visuAl sKills Where would the epinephrine in Sutherland’s experiment fit into this diagram of cell signaling?


acts as a ligand, the term for a molecule that specifically binds to another (often larger) molecule. Ligand binding generally causes a receptor protein to undergo a change in shape. For many receptors, this shape change directly acti-vates the receptor, enabling it to interact with other cellular molecules. For other kinds of receptors, the immediate effect of ligand binding is to cause the aggregation of two or more receptor proteins, which leads to further molecular events inside the cell. Most signal receptors are plasma membrane proteins, but others are located inside the cell. We discuss both of these types next.

Receptors in the Plasma membraneCell-surface transmembrane receptors play crucial roles in the biological systems of animals. The largest family of human cell-surface receptors is the G protein-coupled receptors (GPCRs). There are more than 800 GPCRs; an example is shown in Figure 11.7. Another example is the co-receptor hijacked by HIV to enter immune cells (see Figure 7.8); this GPCR is the target of the drug maraviroc, which has shown some success at treating AIDS.

Most water-soluble signaling molecules bind to specific sites on transmembrane receptor proteins that transmit infor-mation from the extracellular environment to the inside of the cell. We can see how cell-surface transmembrane recep-tors work by looking at three major types: G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and ion chan-nel receptors. These receptors are discussed and illustrated in Figure 11.8; study this figure before going on.

Animation: Reception

process of transduction. The transduction stage converts the signal to a form that can bring about a specific cellular re-sponse. In Sutherland’s system, the binding of epinephrine to a receptor protein in a liver cell’s plasma membrane leads to activation of glycogen phosphorylase in the cytosol. Transduction sometimes occurs in a single step but more often requires a sequence of changes in a series of different molecules—a signal transduction pathway. The mol-ecules in the pathway are often called relay molecules; three are shown as an example.

3 Response. In the third stage of cell signaling, the trans-duced signal finally triggers a specific cellular response. The response may be almost any imaginable cellular activity— such as catalysis by an enzyme (for example, glycogen phos-phorylase), rearrangement of the cytoskeleton, or activation of specific genes in the nucleus. The cell-signaling process helps ensure that crucial activities like these occur in the right cells, at the right time, and in proper coordination with the activities of other cells of the organism. We’ll now explore the mechanisms of cell signaling in more detail, including a discussion of regulation and termination of the process.

ConCept CHeCK 11.11. explain how signaling is involved in ensuring that yeast

cells fuse only with cells of the opposite mating type.

2. in liver cells, glycogen phosphorylase acts in which of the three stages of the signaling pathway associated with an epinephrine-initiated signal?

3. WHAt iF? if epinephrine were mixed with glycogen phosphorylase and glycogen in a cell-free mixture in a test tube, would glucose 1-phosphate be generated? Why or why not?


β2-adrenergicreceptors

Moleculethat mimics ligand

CholesterolPlasma membrane

Figure 11.7 the structure of a G protein-coupled receptor (GpCR). Shown here is a model of the human β2-adrenergic receptor, which binds adrenaline (epinephrine) and was able to be crystallized in the presence of both a molecule that mimics adrenaline (green in the model) and cholesterol in the membrane (orange). Two receptor molecules (blue) are shown as ribbon models in a side view. Caffeine can also bind to this receptor; see question 10 at the end of the chapter.

ConCept 11.2Reception: A signaling molecule binds to a receptor protein, causing it to change shapeA wireless router may broadcast its network signal indis-criminately, but often it can be joined only by computers with the correct password: Reception of the signal depends on the receiver. Similarly, the signals emitted by an a mating type yeast cell are “heard” only by its prospective mates, � cells. In the case of the epinephrine circulating throughout the bloodstream of the impala in Figure 11.1, the hormone encounters many types of cells, but only certain target cells detect and react to the epinephrine molecule. A receptor protein on or in the target cell allows the cell to “hear” the signal and respond to it. The signaling molecule is comple-mentary in shape to a specific site on the receptor and attaches there, like a hand in a glove. The signaling molecule


Figure 11.8 Exploring Cell-Surface Transmembrane Receptors

Signaling moleculeActivatedreceptor

Inactiveenzyme

GTPGDP

GTP

G Protein-Coupled Receptors

A G protein-coupled receptor (GPCR) is a cell-surface transmembrane receptor that works with the help of a G protein, a pro-tein that binds the energy-rich molecule GTP. Many different signaling molecules—including yeast mating factors, neurotransmitters, and epinephrine (adrenaline) and many other hormones—use GPCRs. G protein-coupled receptors vary in the binding sites for their ligands and also for different types of G proteins inside the cell. Nevertheless, GPCR proteins are all remarkably similar in structure. In fact, they make up a large family of eukaryotic receptor proteins with a secondary structure in which the single polypeptide, represented here in a ribbon model, has seven transmembrane α helices (outlined with cylinders and depicted in a row for clarity). Specific loops between the helices (here, the loops on the right) form binding

sites for signaling molecules (outside the cell) and G proteins (on the cytoplasmic side). GPCR-based signaling systems are extremely widespread and diverse in their functions, including roles in embryonic development and sensory reception. In humans, for example, vision, smell, and taste depend on GPCRs (see Concept 50.4). Similarities in structure in G proteins and GPCRs in diverse organisms suggest that G proteins and their associated receptors evolved very early among eukaryotes. Malfunctions of the associated G proteins themselves are involved in many human diseases,including bacterial infections. The bacteria that cause cholera, pertussis (whooping cough), and botulism, among others, make their victims ill by producing toxins that interfere with G protein function. Up to 60% of all medicines used today exert their effects by influencing G protein pathways.

GDP

P i

P i

Activatedenzyme

Cellular response

GTP

2 When the appropriate signaling molecule binds to the extracellular side of the receptor, the receptor is activated and changes shape. Its cytoplasmic side then binds an inactive G protein, causing a GTP to displace the GDP. This activates the G protein.

3 The activated G protein dissociates from the receptor, diffuses along the membrane, and then binds to an enzyme, altering the enzyme’s shape and activity. Once activated, the enzyme can trigger the next step leading to a cellular response. Binding of signaling molecules is reversible: Like other ligands, they bind and dissociate many times. The ligand concentration outside the cell determines how often a ligand is bound and initiates signaling.

4 The changes in the enzyme and G protein are only temporary because the G protein also functions as a GTPase enzyme—in other words, it then hydrolyzes its bound GTP to GDP and . Now inactive again, the G protein leaves the enzyme, which returns to its original state. The G protein is now available for reuse. The GTPase function of the G protein allows the pathway to shut down rapidly when the signaling molecule is no longer present.

Plasma membraneG protein-coupledreceptor

G protein(inactive)CYTOPLASM

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EXTRACELLULAR

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Enzyme

1 Attached but able to move along the cytoplasmic side of the membrane, a G protein functions as a molecular switch that is either on or off, depending on whether GDP or GTP is attached —hence the term G protein. (GTP, or guanosine triphosphate, is similar to ATP.) When GDP is bound to the G protein, as shown above, the G protein is inactive. The receptor and G protein work together with another protein, usually an enzyme.

GDP

Segment thatinteracts with G proteins inside the cell

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Figure 11.8 Exploring Cell-Surface Transmembrane Receptors

Receptor Tyrosine Kinases

Receptor tyrosine kinases (RTKs) belong to a major class of plasma membrane receptors characterized by having enzymatic activity. An RTK is a protein kinase—an enzyme that catalyzes the transfer of phosphate groups from ATP to another protein. The part of the receptor protein extending into the cytoplasm functions more specifically as a tyrosine kinase, an enzyme that catalyzes the transfer of a phosphate group from ATP to the amino acid tyrosine of a substrate protein. Thus, RTKs are membrane receptors that attach phosphates to tyrosines.

Upon binding a ligand such as a growth factor, one RTK may activate ten or more different transduction pathways and cell-ular responses. Often, more than one signal transduction path-way can be triggered at once, helping the cell regulate and coordinate many aspects of cell growth and cell reproduction. The ability of a single ligand-binding event to trigger so many pathways is a key difference between RTKs and GPCRs, which generally activate a single transduction pathway. Abnormal RTKs that function even in the absence of signaling molecules are associated with many kinds of cancer.

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Many receptor tyrosine kinases have the structure depicted schematically here. Before the signaling molecule binds, thereceptors exist as individual units referred to as monomers. Notice that each monomer has an extracellular ligand-binding site, an α helix spanning the membrane, and an intracellular tail containing multiple tyrosines.

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21 The binding of a signaling molecule (such as a growth factor) causes two receptor monomers to associate closely with each other, forming a complex known as a dimer, in a process called dimerization. (In some cases, larger clusters form. The details of monomer association are a focus of current research.)

3 Dimerization activates the tyrosine kinase region of each monomer; each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine that is part of the tail of the other monomer.

4 Now that the receptor is fully activated, it is recognized by specific relay proteins inside the cell. Each such protein binds to a specific phosphorylated tyrosine, undergoing a resulting structural change that activates the bound relay protein. Each activated protein triggers a transduction pathway, leading to a cellular response.

Activated relayproteins

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Continued on next page


Given the many important functions of cell-surface recep-tors, it is not surprising that their malfunctions are associated with many human diseases, including cancer, heart disease, and asthma. To better understand and treat these conditions, a major focus of both university research teams and the phar-maceutical industry has been to analyze the structure of these receptors.

Although cell-surface receptors represent 30% of all human proteins, determining their structures has proved challenging: They make up only 1% of the proteins whose structures have been determined by X-ray crystallography (see Figure 5.21). For one thing, cell-surface receptors tend to be flexible and inherently unstable, thus difficult to crystallize. It took years of persistent efforts for researchers to determine the first few of these structures, such as the GPCR shown in Figure 11.7. In that case, the β-adrenergic receptor was stable enough to be crystallized only while it was among membrane molecules and in the presence of a molecule mimicking its ligand.

Abnormal functioning of receptor tyrosine kinases (RTKs) is associated with many types of cancers. For example, breast cancer patients have a poor prognosis if their tumor cells harbor excessive levels of a receptor tyrosine kinase called HER2 (see the end of Concept 12.3 and Figure 18.27). Using molecular biological techniques, researchers have developed a protein called Herceptin that binds to HER2 on cells and inhibits cell division, thus thwarting further tumor develop-ment. In some clinical studies, treatment with Herceptin improved patient survival rates by more than one-third. One goal of ongoing research into these cell-surface recep-tors and other cell-signaling proteins is development of additional successful treatments.

intracellular ReceptorsIntracellular receptor proteins are found in either the cyto-plasm or nucleus of target cells. To reach such a receptor, a signaling molecule passes through the target cell’s plasma membrane. A number of important signaling molecules can do this because they are either hydrophobic enough or small enough to cross the hydrophobic interior of the membrane (see Concept 7.1). The hydrophobic signaling molecules include both steroid hormones and thyroid hormones of ani-mals. Another chemical signaling molecule that possesses an intracellular receptor is nitric oxide (NO), a gas; this very small molecule readily passes between the membrane phospholipids. Once a hormone has entered a cell, its binding to an intracel-lular receptor changes the receptor into a hormone-receptor complex that is able to cause a response—in many cases, the turning on or off of particular genes.

The behavior of aldosterone is a representative example of how steroid hormones work. This hormone is secreted by cells of the adrenal gland, a gland that lies above the kidney. Aldosterone then travels through the blood and enters cells all over the body. However, a response occurs only in kidney cells, which contain receptor molecules

Ion Channel Receptors

A ligand-gated ion channel is a type of membrane channel receptor containing a region that can act as a “gate,” opening or closing the channel when the receptor changes shape. When a signaling molecule binds as a ligand to the channel receptor, the channel opens or closes, allowing or blocking the flow of specific ions, such as Na+ or Ca2+. Like the other receptors we have discussed, these proteins bind the ligand at a specific site on their extracellular sides.

Ligand-gated ion channels are very important in the nervoussystem. For example, the neurotransmitter molecules releasedat a synapse between two nerve cells (see Figure 11.5b) bind asligands to ion channels on the receiving cell, causing the chan-nels to open. Ions flow in (or, in some cases, out), triggering anelectrical signal that propagates down the length of the receiv-ing cell. Some gated ion channels are controlled by electricalsignals instead of ligands; these voltage-gated ion channels arealso crucial to the functioning of the nervous system, as wewill discuss in Chapter 48. Some ion channels are present onmembranes of organelles, such as the ER.

2

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When the ligand binds to the receptor and the channel opens, specific ions can flow through the channel and rapidly change the concentration of that particular ion inside the cell. This change may directly affect the activity of the cell in some way.

When the ligand dissociates from this receptor, the channel closes and ions no longer enter the cell.

Here we show a ligand-gated ion channel receptor in which the channel remains closed until a ligand binds to the receptor.

Figure 11.8 (continued)

Animation: Acetylcholine Receptor

MAKe ConneCtions Is the flow of ions through a ligand-gated channel an example of active or passive transport? (Review Concepts 7.3 and 7.4.)


for aldosterone. In these cells, the hormone binds to and activates the receptor protein. With aldosterone attached, the active form of the receptor protein then enters the nucleus and turns on specific genes that control water and sodium flow in kidney cells, ultimately affecting blood volume (Figure 11.9).

How does the activated hormone-receptor complex turn on genes? Recall that the genes in a cell’s DNA function by being transcribed and processed into messenger RNA (mRNA), which leaves the nucleus and is translated into a specific pro-tein by ribosomes in the cytoplasm (see Figure 5.22). Special proteins called transcription factors control which genes are turned on—that is, which genes are transcribed into mRNA—in a particular cell at a particular time. When the aldosterone receptor is activated, it acts as a transcription factor that turns on specific genes. (You’ll learn more about transcription fac-tors in Chapters 17 and 18.)

By acting as a transcription factor, the aldosterone recep-tor itself carries out the transduction part of the signaling pathway. Most other intracellular receptors function in the same way, although many of them, such as the thyroid hormone receptor, are already in the nucleus before the

Animation: Signal Transduction Pathways

Hormone(aldosterone)

EXTRACELLULARFLUID

Plasmamembrane

Receptorprotein

Hormone-receptorcomplex

mRNA

DNA

NUCLEUS

CYTOPLASM

New protein

The steroidhormone aldosteronepasses through theplasma membrane.

1

Aldosterone bindsto a receptor proteinin the cytoplasm,activating it.

2

The hormone-receptor complexenters the nucleusand binds to specific genes.

3

The bound proteinacts as a transcriptionfactor, stimulating thetranscription ofthe gene into mRNA.

4

The mRNA istranslated into aspecific protein.

5

Figure 11.9 steroid hormone interacting with an intracellular receptor.

MAKe ConneCtions Why is a cell-surface receptor protein not required for this steroid hormone to enter the cell? (See Concept 7.2.)

signaling molecule reaches them. Interestingly, many of these intracellular receptor proteins are structurally similar, suggesting an evolutionary kinship.

ConCept CHeCK 11.21. nerve growth factor (nGF) is a water-soluble signaling

molecule. Would you expect the receptor for nGF to be intracellular or in the plasma membrane? Why?

2. WHAt iF? What would the effect be if a cell made defective receptor tyrosine kinase proteins that were unable to dimerize?

3. MAKe ConneCtions how is ligand binding similar to the process of allosteric regulation of enzymes? (See Figure 8.20.)


ConCept 11.3Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cellWhen receptors for signaling molecules are plasma mem-brane proteins, like most of those we have discussed, the transduction stage of cell signaling is usually a multistep pathway involving many molecules. Steps often include activation of proteins by addition or removal of phosphate groups or release of other small molecules or ions that act as signaling molecules. One benefit of multiple steps is the possibility of greatly amplifying a signal. If each molecule transmits the signal to numerous molecules at the next step in the series, the result is a geometric increase in the num-ber of activated molecules by the end (see Figure 11.16). Moreover, multistep pathways provide more opportunities for coordination and control than do simpler systems. This allows regulation of the response, as we’ll discuss later in the chapter.

Signal Transduction PathwaysThe binding of a specific signaling molecule to a receptor in the plasma membrane triggers the first step in the signal transduction pathway—the chain of molecular interac-tions that leads to a particular response within the cell. Like falling dominoes, the signal-activated receptor activates another molecule, which activates yet another molecule, and so on, until the protein that produces the final cellular response is activated. The molecules that relay a signal from receptor to response, which we call relay molecules in this book, are often proteins. Protein-protein interactions are a major theme of cell signaling—indeed, a unifying theme of all cellular activities.

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