183 10 Photosynthesis Concept Outline 10.1 What is photosynthesis? The Chloroplast as a Photosynthetic Machine. The highly organized system of membranes in chloroplasts is essential to the functioning of photosynthesis. 10.2 Learning about photosynthesis: An experimental journey. The Role of Soil and Water. The added mass of a growing plant comes mostly from photosynthesis. In plants, water supplies the electrons used to reduce carbon dioxide. Discovery of the Light-Independent Reactions. Photosynthesis is a two-stage process. Only the first stage directly requires light. The Role of Light. The oxygen released during green plant photosynthesis comes from water, and carbon atoms from carbon dioxide are incorporated into organic molecules. The Role of Reducing Power. Electrons released from the splitting of water reduce NADP + ; ATP and NADPH are then used to reduce CO 2 and form simple sugars. 10.3 Pigments capture energy from sunlight. The Biophysics of Light. The energy in sunlight occurs in “packets” called photons, which are absorbed by pigments. Chlorophylls and Carotenoids. Photosynthetic pigments absorb light and harvest its energy. Organizing Pigments into Photosystems. A photosystem uses light energy to eject an energized electron. How Photosystems Convert Light to Chemical Energy. Some bacteria rely on a single photosystem to produce ATP. Plants use two photosystems in series to generate enough energy to reduce NADP + and generate ATP. How the Two Photosystems of Plants Work Together. Photosystems II and I drive the synthesis of the ATP and NADPH needed to form organic molecules. 10.4 Cells use the energy and reducing power captured by the light reactions to make organic molecules. The Calvin Cycle. ATP and NADPH are used to build organic molecules, a process reversed in mitochondria. Reactions of the Calvin Cycle. Ribulose bisphosphate binds CO 2 in the process of carbon fixation. Photorespiration. The enzyme that catalyzes carbon fixation also affects CO 2 release. L ife on earth would be impossible without photosyn- thesis. Every oxygen atom in the air we breathe was once part of a water molecule, liberated by photosynthesis. The energy released by the burning of coal, firewood, gasoline, and natural gas, and by our bodies’ burning of all the food we eat—all, directly or indirectly, has been cap- tured from sunlight by photosynthesis. It is vitally impor- tant that we understand photosynthesis. Research may en- able us to improve crop yields and land use, important goals in an increasingly crowded world. In the previous chapter we described how cells extract chemical energy from food molecules and use that energy to power their activities. In this chapter, we will examine photosynthesis, the process by which organisms capture energy from sun- light and use it to build food molecules rich in chemical energy (figure 10.1). FIGURE 10.1 Capturing energy. These sunflower plants, growing vigorously in the August sun, are capturing light energy for conversion into chemical energy through photosynthesis.
Transcript
183
10Photosynthesis
Concept Outline
10.1 What is photosynthesis?
The Chloroplast as a Photosynthetic Machine. Thehighly organized system of membranes in chloroplasts isessential to the functioning of photosynthesis.
10.2 Learning about photosynthesis: An experimentaljourney.
The Role of Soil and Water. The added mass of agrowing plant comes mostly from photosynthesis. In plants,water supplies the electrons used to reduce carbon dioxide.Discovery of the Light-Independent Reactions.Photosynthesis is a two-stage process. Only the first stagedirectly requires light.The Role of Light. The oxygen released during greenplant photosynthesis comes from water, and carbon atomsfrom carbon dioxide are incorporated into organic molecules.The Role of Reducing Power. Electrons released fromthe splitting of water reduce NADP+; ATP and NADPHare then used to reduce CO2 and form simple sugars.
10.3 Pigments capture energy from sunlight.
The Biophysics of Light. The energy in sunlight occursin “packets” called photons, which are absorbed by pigments.Chlorophylls and Carotenoids. Photosyntheticpigments absorb light and harvest its energy.Organizing Pigments into Photosystems. Aphotosystem uses light energy to eject an energized electron.How Photosystems Convert Light to Chemical Energy.Some bacteria rely on a single photosystem to produceATP. Plants use two photosystems in series to generateenough energy to reduce NADP+ and generate ATP.How the Two Photosystems of Plants Work Together.Photosystems II and I drive the synthesis of the ATP andNADPH needed to form organic molecules.
10.4 Cells use the energy and reducing power capturedby the light reactions to make organic molecules.
The Calvin Cycle. ATP and NADPH are used to buildorganic molecules, a process reversed in mitochondria.Reactions of the Calvin Cycle. Ribulose bisphosphatebinds CO2 in the process of carbon fixation.Photorespiration. The enzyme that catalyzes carbonfixation also affects CO2 release.
Life on earth would be impossible without photosyn-thesis. Every oxygen atom in the air we breathe was
once part of a water molecule, liberated by photosynthesis.The energy released by the burning of coal, firewood,gasoline, and natural gas, and by our bodies’ burning of allthe food we eat—all, directly or indirectly, has been cap-tured from sunlight by photosynthesis. It is vitally impor-tant that we understand photosynthesis. Research may en-able us to improve crop yields and land use, importantgoals in an increasingly crowded world. In the previouschapter we described how cells extract chemical energyfrom food molecules and use that energy to power theiractivities. In this chapter, we will examine photosynthesis,the process by which organisms capture energy from sun-light and use it to build food molecules rich in chemicalenergy (figure 10.1).
FIGURE 10.1Capturing energy. These sunflower plants, growing vigorouslyin the August sun, are capturing light energy for conversion intochemical energy through photosynthesis.
184 Part III Energetics
10.1 What is photosynthesis?
Cuticle
Epidermis
Mesophyll
Vascularbundle
Stoma
Bundlesheath
Chloroplasts
Vacuole
Nucleus
Cell wall
Outermembrane
Innermembrane
Stroma
Granum
Thylakoid
The Chloroplast as a Photosynthetic MachineLife is powered by sunshine. The energy used by most liv-ing cells comes ultimately from the sun, captured by plants,algae, and bacteria through the process of photosynthesis.The diversity of life is only possible because our planet isawash in energy streaming earthward from the sun. Eachday, the radiant energy that reaches the earth equals about1 million Hiroshima-sized atomic bombs. Photosynthesiscaptures about 1% of this huge supply of energy, using it toprovide the energy that drives all life.
The Photosynthetic Process: A Summary
Photosynthesis occurs in many kinds of bacteria and algae,and in the leaves and sometimes the stems of green plants.Figure 10.2 describes the levels of organization in a plantleaf. Recall from chapter 5 that the cells of plant leavescontain organelles called chloroplasts that actually carryout the photosynthetic process. No other structure in aplant cell is able to carry out photosynthesis. Photosynthe-
FIGURE 10.2Journey into a leaf. A plant leaf possesses a thick layer of cells (the mesophyll) rich in chloroplasts. The flattened thylakoids in thechloroplast are stacked into columns called grana (singular, granum). The light reactions take place on the thylakoid
sis takes place in three stages: (1) capturing energy fromsunlight; (2) using the energy to make ATP and reducingpower in the form of a compound called NADPH; and(3) using the ATP and NADPH to power the synthesis oforganic molecules from CO2 in the air (carbon fixation).
The first two stages take place in the presence of lightand are commonly called the light reactions. The thirdstage, the formation of organic molecules from atmos-pheric CO2, is called the Calvin cycle. As long as ATP andNADPH are available, the Calvin cycle may occur in theabsence of light.
The following simple equation summarizes the overallprocess of photosynthesis:
6 CO2 + 12 H2O + light —→ C6H12O6 + 6 H2O + 6 O2
carbon water glucose water oxygendioxide
Inside the Chloroplast
The internal membranes of chloroplasts are organized intosacs called thylakoids, and often numerous thylakoids arestacked on one another in columns called grana. The thy-lakoid membranes house the photosynthetic pigments forcapturing light energy and the machinery to make ATP.Surrounding the thylakoid membrane system is a semiliq-uid substance called stroma. The stroma houses the en-zymes needed to assemble carbon molecules. In the mem-
branes of thylakoids, photosynthetic pigments are clusteredtogether to form a photosystem.
Each pigment molecule within the photosystem is capa-ble of capturing photons, which are packets of energy. A lat-tice of proteins holds the pigments in close contact withone another. When light of a proper wavelength strikes apigment molecule in the photosystem, the resulting excita-tion passes from one chlorophyll molecule to another. Theexcited electron is not transferred physically—it is the en-ergy that passes from one molecule to another. A crudeanalogy to this form of energy transfer is the initial “break”in a game of pool. If the cue ball squarely hits the point ofthe triangular array of 15 pool balls, the two balls at the farcorners of the triangle fly off, but none of the central ballsmove. The energy passes through the central balls to themost distant ones.
Eventually the energy arrives at a key chlorophyll mole-cule that is touching a membrane-bound protein. The en-ergy is transferred as an excited electron to that protein,which passes it on to a series of other membrane proteinsthat put the energy to work making ATP and NADPH andbuilding organic molecules. The photosystem thus acts as alarge antenna, gathering the light harvested by many indi-vidual pigment molecules.
The reactions of photosynthesis take place withinthylakoid membranes within chloroplasts in leaf cells.
Chapter 10 Photosynthesis 185
Sunlight
Light reactions
Organicmolecules
CO2
H2O
O2
Photosystem
ADP NADPH NADP+
Stroma
Thylakoid
Thylakoid
Stroma
Granum
ATP
Calvincycle
FIGURE 10.2 (continued)membrane and generate the ATP and NADPH that fuel the Calvin cycle. The fluid interior matrix of a chloroplast, the stroma, contains theenzymes that carry out the Calvin cycle.
The Role of Soil and WaterThe story of how we learned about photosynthesis is one ofthe most interesting in science and serves as a good intro-duction to this complex process. The story starts over 300years ago, with a simple but carefully designed experimentby a Belgian doctor, Jan Baptista van Helmont(1577–1644). From the time of the Greeks, plants werethought to obtain their food from the soil, literally suckingit up with their roots; van Helmont thought of a simpleway to test the idea. He planted a small willow tree in a potof soil after weighing the tree and the soil. The tree grew inthe pot for several years, during which time van Helmontadded only water. At the end of five years, the tree wasmuch larger: its weight had increased by 74.4 kilograms.However, all of this added mass could not have come from thesoil, because the soil in the pot weighed only 57 grams lessthan it had five years earlier! With this experiment, vanHelmont demonstrated that the substance of the plant wasnot produced only from the soil. He incorrectly concludedthat mainly the water he had been adding accounted for theplant’s increased mass.
A hundred years passed before the story became clearer.The key clue was provided by the English scientist JosephPriestly, in his pioneering studies of the properties of air.On the 17th of August, 1771, Priestly “accidentally hitupon a method of restoring air that had been injured by theburning of candles.” He “put a [living] sprig of mint intoair in which a wax candle had burnt out and found that, onthe 27th of the same month, another candle could beburned in this same air.” Somehow, the vegetation seemedto have restored the air! Priestly found that while a mousecould not breathe candle-exhausted air, air “restored” byvegetation was not “at all inconvenient to a mouse.” Thekey clue was that living vegetation adds something to the air.
How does vegetation “restore” air? Twenty-five yearslater, Dutch physician Jan Ingenhousz solved the puzzle.Working over several years, Ingenhousz reproduced andsignificantly extended Priestly’s results, demonstrating thatair was restored only in the presence of sunlight, and onlyby a plant’s green leaves, not by its roots. He proposed thatthe green parts of the plant carry out a process (which wenow call photosynthesis) that uses sunlight to split carbondioxide (CO2) into carbon and oxygen. He suggested thatthe oxygen was released as O2 gas into the air, while thecarbon atom combined with water to form carbohydrates.His proposal was a good guess, even though the later stepwas subsequently modified. Chemists later found that theproportions of carbon, oxygen, and hydrogen atoms in car-bohydrates are indeed about one atom of carbon per mole-cule of water (as the term carbohydrate indicates). A Swissbotanist found in 1804 that water was a necessary reactant.By the end of that century the overall reaction for photo-synthesis could be written as:
CO2 + H2O + light energy —→ (CH2O) + O2
It turns out, however, that there’s more to it than that.When researchers began to examine the process in moredetail in the last century, the role of light proved to be un-expectedly complex.
Van Helmont showed that soil did not add mass to agrowing plant. Priestly and Ingenhousz and others thenworked out the basic chemical reaction.
Discovery of the Light-IndependentReactionsIngenhousz’s early equation for photosynthesis includesone factor we have not discussed: light energy. What roledoes light play in photosynthesis? At the beginning of theprevious century, the English plant physiologist F. F.Blackman began to address the question of the role of lightin photosynthesis. In 1905, he came to the startling conclu-sion that photosynthesis is in fact a two-stage process, onlyone of which uses light directly.
Blackman measured the effects of different light inten-sities, CO2 concentrations, and temperatures on photo-synthesis. As long as light intensity was relatively low, hefound photosynthesis could be accelerated by increasingthe amount of light, but not by increasing the tempera-ture or CO2 concentration (figure 10.3). At high light in-tensities, however, an increase in temperature or CO2concentration greatly accelerated photosynthesis. Black-man concluded that photosynthesis consists of an initialset of what he called “light” reactions, that are largely in-dependent of temperature, and a second set of “dark” re-actions, that seemed to be independent of light but lim-ited by CO2. Do not be confused by Blackman’slabels—the so-called “dark” reactions occur in the light(in fact, they require the products of the light reactions);their name simply indicates that light is not directly in-volved in those reactions.
Blackman found that increased temperature increasesthe rate of the dark carbon-reducing reactions, but only upto about 35°C. Higher temperatures caused the rate to falloff rapidly. Because 35°C is the temperature at which manyplant enzymes begin to be denatured (the hydrogen bondsthat hold an enzyme in its particular catalytic shape beginto be disrupted), Blackman concluded that enzymes mustcarry out the dark reactions.
Blackman showed that capturing photosynthetic energyrequires sunlight, while building organic moleculesdoes not.
186 Part III Energetics
10.2 Learning about photosynthesis: An experimental journey.
The Role of LightThe role of light in the so-called light anddark reactions was worked out in the 1930sby C. B. van Niel, then a graduate student atStanford University studying photosynthesisin bacteria. One of the types of bacteria hewas studying, the purple sulfur bacteria, doesnot release oxygen during photosynthesis;instead, they convert hydrogen sulfide (H2S)into globules of pure elemental sulfur thataccumulate inside themselves. The processthat van Niel observed was
CO2 + 2 H2S + light energy → (CH2O) + H2O + 2 S
The striking parallel between this equationand Ingenhousz’s equation led van Niel topropose that the generalized process ofphotosynthesis is in fact
CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A
In this equation, the substance H2A servesas an electron donor. In photosynthesisperformed by green plants, H2A is water,while among purple sulfur bacteria, H2A ishydrogen sulfide. The product, A, comesfrom the splitting of H2A. Therefore, theO2 produced during green plant photosyn-thesis results from splitting water, not car-bon dioxide.
When isotopes came into common use in biology in theearly 1950s, it became possible to test van Niel’s revolu-tionary proposal. Investigators examined photosynthesis ingreen plants supplied with 18O water; they found that the18O label ended up in oxygen gas rather than in carbohy-drate, just as van Niel had predicted:
CO2 + 2 H218O + light energy —→ (CH2O) + H2O + 18O2
In algae and green plants, the carbohydrate typically pro-duced by photosynthesis is the sugar glucose, which has sixcarbons. The complete balanced equation for photosynthe-sis in these organisms thus becomes
6 CO2 + 12 H2O + light energy —→ C6H12O6 + 6 O2 + 6 H2O.
We now know that the first stage of photosynthesis, thelight reactions, uses the energy of light to reduce NADP(an electron carrier molecule) to NADPH and to manufac-ture ATP. The NADPH and ATP from the first stage ofphotosynthesis are then used in the second stage, theCalvin cycle, to reduce the carbon in carbon dioxide andform a simple sugar whose carbon skeleton can be used tosynthesize other organic molecules.
Van Niel discovered that photosynthesis splits watermolecules, incorporating the carbon atoms of carbondioxide gas and the hydrogen atoms of water intoorganic molecules and leaving oxygen gas.
The Role of Reducing PowerIn his pioneering work on the light reactions, van Niel hadfurther proposed that the reducing power (H+) generated bythe splitting of water was used to convert CO2 into organicmatter in a process he called carbon fixation. Was he right?
In the 1950s Robin Hill demonstrated that van Niel wasindeed right, and that light energy could be used to generatereducing power. Chloroplasts isolated from leaf cells wereable to reduce a dye and release oxygen in response to light.Later experiments showed that the electrons released fromwater were transferred to NADP+. Arnon and coworkersshowed that illuminated chloroplasts deprived of CO2 accu-mulate ATP. If CO2 is then introduced, neither ATP norNADPH accumulate, and the CO2 is assimilated into organicmolecules. These experiments are important for three rea-sons. First, they firmly demonstrate that photosynthesis oc-curs only within chloroplasts. Second, they show that thelight-dependent reactions use light energy to reduce NADP+
and to manufacture ATP. Thirdly, they confirm that theATP and NADPH from this early stage of photosynthesis arethen used in the later light-independent reactions to reducecarbon dioxide, forming simple sugars.
Hill showed that plants can use light energy to generatereducing power. The incorporation of carbon dioxideinto organic molecules in the light-independentreactions is called carbon fixation.
Chapter 10 Photosynthesis 187
Maximum rate
Excess CO2; 35°C
Temperature limited
Excess CO2; 20°C
CO2 limited
Light intensity (foot-candles)
Rat
e of
pho
tosy
nthe
sis
Ligh
t lim
ited
500
(b)
1000 1500 2000 2500
Insufficient CO2 (0.01%); 20°C
FIGURE 10.3Discovery of the dark reactions. (a) Blackman measured photosynthesis rates underdiffering light intensities, CO2 concentrations, and temperatures. (b) As this graphshows, light is the limiting factor at low light intensities, while temperature and CO2concentration are the limiting factors at higher light intensities.
(a)
The Biophysics of LightWhere is the energy in light? What isthere in sunlight that a plant can use toreduce carbon dioxide? This is themystery of photosynthesis, the one fac-tor fundamentally different fromprocesses such as respiration. To an-swer these questions, we will need toconsider the physical nature of light it-self. James Clerk Maxwell had theo-rized that light was an electromagneticwave—that is, that light movedthrough the air as oscillating electricand magnetic fields. Proof of this camein a curious experiment carried out in alaboratory in Germany in 1887. Ayoung physicist, Heinrich Hertz, wasattempting to verify a highly mathe-matical theory that predicted the exis-tence of electromagnetic waves. To seewhether such waves existed, Hertz de-signed a clever experiment. On oneside of a room he constructed a powerful spark generatorthat consisted of two large, shiny metal spheres standingnear each other on tall, slender rods. When a very high sta-tic electrical charge was built up on one sphere, sparkswould jump across to the other sphere.
After constructing this device, Hertz set out to investigatewhether the sparking would create invisible electromagneticwaves, so-called radio waves, as predicted by the mathemati-cal theory. On the other side of the room, he placed theworld’s first radio receiver, a thin metal hoop on an insulat-ing stand. There was a small gap at the bottom of the hoop,so that the hoop did not quite form a complete circle. WhenHertz turned on the spark generator across the room, he sawtiny sparks passing across the gap in the hoop! This was thefirst demonstration of radio waves. But Hertz noted anothercurious phenomenon. When UV light was shining acrossthe gap on the hoop, the sparks were produced more readily.This unexpected facilitation, called the photoelectric effect,puzzled investigators for many years.
The photoelectric effect was finally explained using aconcept proposed by Max Planck in 1901. Planck devel-oped an equation that predicted the blackbody radiationcurve based upon the assumption that light and other formsof radiation behaved as units of energy called photons. In1905 Albert Einstein explained the photoelectric effect uti-lizing the photon concept. Ultraviolet light has photons ofsufficient energy that when they fell on the loop, electronswere ejected from the metal surface. The photons hadtransferred their energy to the electrons, literally blastingthem from the ends of the hoop and thus facilitating the
passage of the electric spark induced by the radio waves.Visible wavelengths of light were unable to remove theelectrons because their photons did not have enough en-ergy to free the electrons from the metal surface at the endsof the hoop.
The Energy in Photons
Photons do not all possess the same amount of energy (fig-ure 10.4). Instead, the energy content of a photon is in-versely proportional to the wavelength of the light: short-wavelength light contains photons of higher energy thanlong-wavelength light. X rays, which contain a great deal ofenergy, have very short wavelengths—much shorter than visi-ble light, making them ideal for high-resolution microscopes.
Hertz had noted that the strength of the photoelectriceffect depends on the wavelength of light; short wave-lengths are much more effective than long ones in produc-ing the photoelectric effect. Einstein’s theory of the photo-electric effect provides an explanation: sunlight containsphotons of many different energy levels, only some ofwhich our eyes perceive as visible light. The highest energyphotons, at the short-wavelength end of the electromag-netic spectrum (see figure 10.4), are gamma rays, withwavelengths of less than 1 nanometer; the lowest energyphotons, with wavelengths of up to thousands of meters,are radio waves. Within the visible portion of the spectrum,violet light has the shortest wavelength and the most ener-getic photons, and red light has the longest wavelength andthe least energetic photons.
188 Part III Energetics
10.3 Pigments capture energy from sunlight.
1 nm
400 nm
0.001 nm 10 nm 1000 nmIncreasing wavelength
Visible light
Increasing energy
0.01 cm 1 cm 1 m
Radio wavesInfraredX raysGamma raysUVlight
100 m
430 nm 500 nm 560 nm 600 nm 650 nm 740 nm
FIGURE 10.4The electromagnetic spectrum. Light is a form of electromagnetic energy convenientlythought of as a wave. The shorter the wavelength of light, the greater its energy. Visiblelight represents only a small part of the electromagnetic spectrum between 400 and 740nanometers.
Ultraviolet Light
The sunlight that reaches the earth’s surfacecontains a significant amount of ultraviolet(UV) light, which, because of its shorterwavelength, possesses considerably more en-ergy than visible light. UV light is thought tohave been an important source of energy onthe primitive earth when life originated. To-day’s atmosphere contains ozone (derivedfrom oxygen gas), which absorbs most of theUV photons in sunlight, but a considerableamount of UV light still manages to pene-trate the atmosphere. This UV light is a po-tent force in disrupting the bonds of DNA,causing mutations that can lead to skin can-cer. As we will describe in a later chapter,loss of atmospheric ozone due to human ac-tivities threatens to cause an enormous jumpin the incidence of human skin cancersthroughout the world.
Absorption Spectra and Pigments
How does a molecule “capture” the energyof light? A photon can be envisioned as avery fast-moving packet of energy. When itstrikes a molecule, its energy is either lost asheat or absorbed by the electrons of the mol-ecule, boosting those electrons into higherenergy levels. Whether or not the photon’senergy is absorbed depends on how muchenergy it carries (defined by its wavelength)and on the chemical nature of the molecule ithits. As we saw in chapter 2, electrons occupydiscrete energy levels in their orbits aroundatomic nuclei. To boost an electron into a different energylevel requires just the right amount of energy, just as reach-ing the next rung on a ladder requires you to raise yourfoot just the right distance. A specific atom can, therefore,absorb only certain photons of light—namely, those thatcorrespond to the atom’s available electron energy levels.As a result, each molecule has a characteristic absorptionspectrum, the range and efficiency of photons it is capableof absorbing.
Molecules that are good absorbers of light in the visiblerange are called pigments. Organisms have evolved a vari-ety of different pigments, but there are only two generaltypes used in green plant photosynthesis: carotenoids andchlorophylls. Chlorophylls absorb photons within narrowenergy ranges. Two kinds of chlorophyll in plants, chloro-phylls a and b, preferentially absorb violet-blue and redlight (figure 10.5). Neither of these pigments absorbs pho-tons with wavelengths between about 500 and 600nanometers, and light of these wavelengths is, therefore,reflected by plants. When these photons are subsequently
absorbed by the pigment in our eyes, we perceive them asgreen.
Chlorophyll a is the main photosynthetic pigment and isthe only pigment that can act directly to convert light en-ergy to chemical energy. However, chlorophyll b, acting asan accessory or secondary light-absorbing pigment, com-plements and adds to the light absorption of chlorophyll a.Chlorophyll b has an absorption spectrum shifted towardthe green wavelengths. Therefore, chlorophyll b can absorbphotons chlorophyll a cannot. Chlorophyll b thereforegreatly increases the proportion of the photons in sunlightthat plants can harvest. An important group of accessorypigments, the carotenoids, assist in photosynthesis by cap-turing energy from light of wavelengths that are not effi-ciently absorbed by either chlorophyll.
In photosynthesis, photons of light are absorbed bypigments; the wavelength of light absorbed dependsupon the specific pigment.
Chapter 10 Photosynthesis 189
Chlorophyll b Chlorophyll a
Rel
ativ
e lig
ht a
bsor
ptio
n
Wavelength (nm)
400 450 500 550 600 650 700
Carotenoids
FIGURE 10.5The absorption spectrum of chlorophyll. The peaks represent wavelengths ofsunlight that the two common forms of photosynthetic pigment, chlorophyll a (solidline) and chlorophyll b (dashed line), strongly absorb. These pigments absorbpredominately violet-blue and red light in two narrow bands of the spectrum andreflect the green light in the middle of the spectrum. Carotenoids (not shown here)absorb mostly blue and green light and reflect orange and yellow light.
Chlorophylls and CarotenoidsChlorophylls absorb photons by means of an excitationprocess analogous to the photoelectric effect. These pigmentscontain a complex ring structure, called a porphyrin ring,with alternating single and double bonds. At the center of thering is a magnesium atom. Photons absorbed by the pigmentmolecule excite electrons in the ring, which are then chan-neled away through the alternating carbon-bond system. Sev-eral small side groups attached to the outside of the ring alterthe absorption properties of the molecule in different kinds ofchlorophyll (figure 10.6). The precise absorption spectrum isalso influenced by the local microenvironment created by theassociation of chlorophyll with specific proteins.
Once Ingenhousz demonstrated that only the green partsof plants can “restore” air, researchers suspected chlorophyllwas the primary pigment that plants employ to absorb lightin photosynthesis. Experiments conducted in the 1800sclearly verified this suspicion. One such experiment, per-formed by T. W. Englemann in 1882 (figure 10.7), serves asa particularly elegant example, simple in design and clear inoutcome. Englemann set out to characterize the actionspectrum of photosynthesis, that is, the relative effective-ness of different wavelengths of light in promoting photo-synthesis. He carried out the entire experiment utilizing asingle slide mounted on a microscope. To obtain different
wavelengths of light, he placed a prism under his micro-scope, splitting the light that illuminated the slide into aspectrum of colors. He then arranged a filament of greenalgal cells across the spectrum, so that different parts of thefilament were illuminated with different wavelengths, andallowed the algae to carry out photosynthesis. To assesshow fast photosynthesis was proceeding, Englemann choseto monitor the rate of oxygen production. Lacking a massspectrometer and other modern instruments, he addedaerotactic (oxygen-seeking) bacteria to the slide; he knewthey would gather along the filament at locations whereoxygen was being produced. He found that the bacteria ac-cumulated in areas illuminated by red and violet light, thetwo colors most strongly absorbed by chlorophyll.
All plants, algae, and cyanobacteria use chlorophyll a astheir primary pigments. It is reasonable to ask why thesephotosynthetic organisms do not use a pigment like retinal(the pigment in our eyes), which has a broad absorptionspectrum that covers the range of 500 to 600 nanometers.The most likely hypothesis involves photoefficiency. Al-though retinal absorbs a broad range of wavelengths, itdoes so with relatively low efficiency. Chlorophyll, in con-trast, absorbs in only two narrow bands, but does so withhigh efficiency. Therefore, plants and most other photo-synthetic organisms achieve far higher overall photon cap-ture rates with chlorophyll than with other pigments.
Chlorophyll moleculesembedded in a protein complex in the thylakoidmembrane
FIGURE 10.6Chlorophyll.Chlorophyllmolecules consistof a porphyrinhead and ahydrocarbon tailthat anchors thepigment moleculeto hydrophobicregions of proteinsembedded withinthe membranes ofthylakoids. Theonly differencebetween the twochlorophyllmolecules is thesubstitution of a —CHO(aldehyde) groupin chlorophyll bfor a —CH3(methyl) group inchlorophyll a.
Carotenoids consist of carbon rings linked to chainswith alternating single and double bonds. They can absorbphotons with a wide range of energies, although they arenot always highly efficient in transferring this energy.Carotenoids assist in photosynthesis by capturing energyfrom light of wavelengths that are not efficiently absorbedby chlorophylls (figure 10.8; see figure 10.5).
A typical carotenoid is β-carotene, whose two carbonrings are connected by a chain of 18 carbon atoms with al-ternating single and double bonds. Splitting a molecule of
β-carotene into equal halves produces two molecules of vit-amin A. Oxidation of vitamin A produces retinal, the pig-ment used in vertebrate vision. This explains why carrots,which are rich in β-carotene, enhance vision.
A pigment is a molecule that absorbs light. Thewavelengths absorbed by a particular pigment dependon the available energy levels to which light-excitedelectrons can be boosted in the pigment.
Chapter 10 Photosynthesis 191
Abs
orba
nce
Filament ofgreen alga
Oxygen-seeking bacteria
T.W. Englemann revealed the action spectrum of photosynthesis in the filamentous alga Spirogyra in 1882. Englemann used the rate ofoxygen production to measure the rate of photosynthesis. As his oxygen indicator, he chose bacteria that are attracted by oxygen. In placeof the mirror and diaphragm usually used to illuminate objects under view in his microscope, he substituted a "microspectral apparatus,"which, as its name implies, produced a tiny spectrum of colors that it projected upon the slide under the microscope. Then he arranged afilament of algal cells parallel to the spread of the spectrum. The oxygen-seeking bacteria congregated mostly in the areas where the violetand red wavelengths fell upon the algal filament.
FIGURE 10.7Constructing an action spectrum for photosynthesis. As you can see, the action spectrum for photosynthesis that Englemann revealedin his experiment parallels the absorption spectrum of chlorophyll (see figure 10.5).
Oak leaf in summer
Oak leaf in autumn
FIGURE 10.8Fall colors are produced by carotenoids and other accessory pigments. During the spring and summer, chlorophyll in leaves masks thepresence of carotenoids and other accessory pigments. When cool fall temperatures cause leaves to cease manufacturing chlorophyll, thechlorophyll is no longer present to reflect green light, and the leaves reflect the orange and yellow light that carotenoids and otherpigments do not absorb.
Organizing Pigments intoPhotosystemsThe light reactions of photosynthesis occur in membranes.In bacteria like those studied by van Niel, the plasma mem-brane itself is the photosynthetic membrane. In plants andalgae, by contrast, photosynthesis is carried out by or-ganelles that are the evolutionary descendants of photosyn-thetic bacteria, chloroplasts—the photosynthetic mem-branes exist within the chloroplasts. The light reactionstake place in four stages:
1. Primary photoevent. A photon of light is capturedby a pigment. The result of this primary photoeventis the excitation of an electron within the pigment.
2. Charge separation. This excitation energy is trans-ferred to a specialized chlorophyll pigment termed areaction center, which reacts by transferring an ener-getic electron to an acceptor molecule, thus initiatingelectron transport.
3. Electron transport. The excited electron is shut-tled along a series of electron-carrier molecules em-bedded within the photosynthetic membrane. Severalof them react by transporting protons across themembrane, generating a gradient of proton concen-tration. Its arrival at the pump induces the transportof a proton across the membrane. The electron isthen passed to an acceptor.
4. Chemiosmosis. The protons that accumulate onone side of the membrane now flow back across themembrane through specific protein complexes wherechemiosmotic synthesis of ATP takes place, just as itdoes in aerobic respiration.
Discovery of Photosystems
One way to study how pigments absorb light is to measurethe dependence of the output of photosynthesis on the in-tensity of illumination—that is, how much photosynthesisis produced by how much light. When experiments of thissort are done on plants, they show that the output of pho-tosynthesis increases linearly at low intensities but lessensat higher intensities, finally saturating at high-intensitylight (figure 10.9). Saturation occurs because all of thelight-absorbing capacity of the plant is in use; additionallight doesn’t increase the output because there is nothingto absorb the added photons.
It is tempting to think that at saturation, all of a plant’spigment molecules are in use. In 1932 plant physiologistsRobert Emerson and William Arnold set out to test thishypothesis in an organism where they could measure boththe number of chlorophyll molecules and the output ofphotosynthesis. In their experiment, they measured theoxygen yield of photosynthesis when Chlorella (unicellulargreen algae) were exposed to very brief light flashes lasting
only a few microseconds. Assuming the hypothesis of pig-ment saturation to be correct, they expected to find that asthey increased the intensity of the flashes, the yield perflash would increase, until each chlorophyll molecule ab-sorbed a photon, which would then be used in the light re-actions, producing a molecule of O2.
Unexpectedly, this is not what happened. Instead, satu-ration was achieved much earlier, with only one moleculeof O2 per 2500 chlorophyll molecules! This led Emersonand Arnold to conclude that light is absorbed not by inde-pendent pigment molecules, but rather by clusters ofchlorophyll and accessory pigment molecules which havecome to be called photosystems. Light is absorbed by any oneof the hundreds of pigment molecules in a photosystem,which transfer their excitation energy to one with a lowerenergy level than the others. This reaction center of thephotosystem acts as an energy sink, trapping the excitationenergy. It was the saturation of these reaction centers, notindividual molecules, that was observed by Emerson andArnold.
Architecture of a Photosystem
In chloroplasts and all but the most primitive bacteria, lightis captured by such photosystems. Each photosystem is anetwork of chlorophyll a molecules, accessory pigments,and associated proteins held within a protein matrix on thesurface of the photosynthetic membrane. Like a magnify-ing glass focusing light on a precise point, a photosystemchannels the excitation energy gathered by any one of itspigment molecules to a specific molecule, the reaction cen-ter chlorophyll. This molecule then passes the energy out
192 Part III Energetics
Expected
Observed
Saturation when all photosystems are in use
Intensity of light flashes
Out
put (
O2
yiel
d pe
r fla
sh)
Saturation when all chlorophyll molecules are in use
FIGURE 10.9Emerson and Arnold’s experiment. When photosyntheticsaturation is achieved, further increases in intensity cause noincrease in output.
of the photosystem so it can be put to work driving the syn-thesis of ATP and organic molecules.
A photosystem thus consists of two closely linkedcomponents: (1) an antenna complex of hundreds of pig-ment molecules that gather photons and feed the cap-tured light energy to the reaction center; and (2) a reac-tion center, consisting of one or more chlorophyll amolecules in a matrix of protein, that passes the energyout of the photosystem.
The Antenna Complex. The antenna complex capturesphotons from sunlight (figure 10.10). In chloroplasts, theantenna complex is a web of chlorophyll molecules linkedtogether and held tightly on the thylakoid membrane by amatrix of proteins. Varying amounts of carotenoid acces-sory pigments may also be present. The protein matrixserves as a sort of scaffold, holding individual pigment mol-ecules in orientations that are optimal for energy transfer.The excitation energy resulting from the absorption of aphoton passes from one pigment molecule to an adjacentmolecule on its way to the reaction center. After the trans-fer, the excited electron in each molecule returns to thelow-energy level it had before the photon was absorbed.Consequently, it is energy, not the excited electrons them-selves, that passes from one pigment molecule to the next.The antenna complex funnels the energy from many elec-trons to the reaction center.
The Reaction Center. The reaction center is a trans-membrane protein-pigment complex. In the reaction cen-ter of purple photosynthetic bacteria, which is simpler thanin chloroplasts but better understood, a pair of chlorophylla molecules acts as a trap for photon energy, passing an ex-cited electron to an acceptor precisely positioned as itsneighbor. Note that here the excited electron itself is trans-ferred, not just the energy as we saw in pigment-pigmenttransfers. This allows the photon excitation to move awayfrom the chlorophylls and is the key conversion of light tochemical energy.
Figure 10.11 shows the transfer of energy from the reac-tion center to the primary electron acceptor. By energizingan electron of the reaction center chlorophyll, light createsa strong electron donor where none existed before. Thechlorophyll transfers the energized electron to the primaryacceptor, a molecule of quinone, reducing the quinone andconverting it to a strong electron donor. A weak electrondonor then donates a low-energy electron to the chloro-phyll, restoring it to its original condition. In plant chloro-plasts, water serves as the electron donor.
Photosystems contain pigments that capture photonenergy from light. The pigments transfer the energy toreaction centers. There, the energy excites electrons,which are channeled away to do chemical work.
Chapter 10 Photosynthesis 193
Electrondonor
Electronacceptor
PhotonReactioncenterchlorophyll
Chlorophyllmolecules
Photosystem
FIGURE 10.10How the antenna complex works. When light of the properwavelength strikes any pigment molecule within a photosystem,the light is absorbed by that pigment molecule. The excitationenergy is then transferred from one molecule to another withinthe cluster of pigment molecules until it encounters the reactioncenter chlorophyll a. When excitation energy reaches the reactioncenter chlorophyll, electron transfer is initiated.
Electron donor
Electronacceptor
Acceptorreduced
Chlorophylloxidized
Acceptorreduced
Donoroxidized
Excited chlorophyllmolecule
+ – + –
FIGURE 10.11Converting light to chemical energy. The reaction centerchlorophyll donates a light-energized electron to the primaryelectron acceptor, reducing it. The oxidized chlorophyll then fillsits electron “hole” by oxidizing a donor molecule.
How Photosystems Convert Lightto Chemical EnergyBacteria Use a Single Photosystem
Photosynthetic pigment arrays are thought to have evolvedmore than 3 billion years ago in bacteria similar to the sul-fur bacteria studied by van Niel.
1. Electron is joined with a proton to make hydrogen.In these bacteria, the absorption of a photon of light at apeak absorption of 870 nanometers (near infrared, not visi-ble to the human eye) by the photosystem results in thetransmission of an energetic electron along an electrontransport chain, eventually combining with a proton toform a hydrogen atom. In the sulfur bacteria, the proton isextracted from hydrogen sulfide, leaving elemental sulfur asa by-product. In bacteria that evolved later, as well as inplants and algae, the proton comes from water, producingoxygen as a by-product.
2. Electron is recycled to chlorophyll. The ejection ofan electron from the bacterial reaction center leaves itshort one electron. Before the photosystem of the sulfurbacteria can function again, an electron must be re-turned. These bacteria channel the electron back to thepigment through an electron transport system similar tothe one described in chapter 9; the electron’s passage drivesa proton pump that promotes the chemiosmotic synthesisof ATP. One molecule of ATP is produced for everythree electrons that follow this path. Viewed overall (fig-ure 10.12), the path of the electron is thus a circle.Chemists therefore call the electron transfer processleading to ATP formation cyclic photophosphorylation.Note, however, that the electron that left the P870 reac-tion center was a high-energy electron, boosted by theabsorption of a photon of light, while the electron thatreturns has only as much energy as it had before the pho-ton was absorbed. The difference in the energy of thatelectron is the photosynthetic payoff, the energy that drivesthe proton pump.
For more than a billion years, cyclic photophosphory-lation was the only form of photosynthetic light reactionthat organisms used. However, its major limitation isthat it is geared only toward energy production, not to-ward biosynthesis. Most photosynthetic organisms incor-porate atmospheric carbon dioxide into carbohydrates.Because the carbohydrate molecules are more reduced(have more hydrogen atoms) than carbon dioxide, asource of reducing power (that is, hydrogens) must beprovided. Cyclic photophosphorylation does not do this.The hydrogen atoms extracted from H2S are used as asource of protons, and are not available to join to carbon.Thus bacteria that are restricted to this process mustscavenge hydrogens from other sources, an inefficientundertaking.
Why Plants Use Two Photosystems
After the sulfur bacteria appeared, other kinds of bacteriaevolved an improved version of the photosystem that over-came the limitation of cyclic photophosphorylation in aneat and simple way: a second, more powerful photosystemusing another arrangement of chlorophyll a was combinedwith the original.
In this second photosystem, called photosystem II,molecules of chlorophyll a are arranged with a differentgeometry, so that more shorter wavelength, higher energyphotons are absorbed than in the ancestral photosystem,which is called photosystem I. As in the ancestral photo-system, energy is transmitted from one pigment moleculeto another within the antenna complex of these photosys-tems until it reaches the reaction center, a particular pig-ment molecule positioned near a strong membrane-boundelectron acceptor. In photosystem II, the absorption peak(that is, the wavelength of light most strongly absorbed) ofthe pigments is approximately 680 nanometers; therefore,the reaction center pigment is called P680. The absorptionpeak of photosystem I pigments in plants is 700 nanome-ters, so its reaction center pigment is called P700. Workingtogether, the two photosystems carry out a noncyclic elec-tron transfer.
When the rate of photosynthesis is measured using twolight beams of different wavelengths (one red and the
194 Part III Energetics
Electronacceptor
PhotonEne
rgy
of e
lect
rons
Plastocyanin
Photosystem
ATP
ADP
pC
Ferredoxin
b6-fcomplex
b6-f complex
Fd
e–
e–
e–
Excitedreactioncenter
Reactioncenter
P870
FIGURE 10.12The path of an electron in purple sulfur bacteria. When alight-energized electron is ejected from the photosystem reactioncenter (P870), it passes in a circle, eventually returning to thephotosystem from which it was ejected.
other far-red), the rate was greater than the sum of therates using individual beams of red and far-red light (fig-ure 10.13). This surprising result, called the enhancementeffect, can be explained by a mechanism involving twophotosystems acting in series (that is, one after the other),one of which absorbs preferentially in the red, the otherin the far-red.
The use of two photosystems solves the problem of ob-taining reducing power in a simple and direct way, by har-nessing the energy of two photosystems. The schemeshown in figure 10.14, called a Z diagram, illustrates thetwo electron-energizing steps, one catalyzed by each pho-tosystem. The electrons originate from water, which holdsonto its electrons very tightly (redox potential = +820 mV),and end up in NADPH, which holds its electrons muchmore loosely (redox potential = –320 mV).
In sulfur bacteria, excited electrons ejected from thereaction center travel a circular path, driving a protonpump and then returning to their original photosystem.Plants employ two photosystems in series, whichgenerates power to reduce NADP+ to NADPH withenough left over to make ATP.
Chapter 10 Photosynthesis 195
Far-redlight on
Bothlights on
Red lighton
Off Off
Time
Rel
ativ
e ra
te o
fph
otos
ynth
esis
Off
FIGURE 10.13The “enhancement effect.” The rate of photosynthesis whenred and far-red light are provided together is greater than the sumof the rates when each wavelength is provided individually. Thisresult baffled researchers in the 1950s. Today it provides the keyevidence that photosynthesis is carried out by two photochemicalsystems with slightly different wavelength optima.
Proton gradientformed for ATPsynthesis
Water-splittingenzyme
Photon
Photon
Ene
rgy
of e
lect
rons
Plastocyanin
Ferredoxin
Plastoquinone
Photosystem II
NADP+ + H+
b6-fcomplex
NADPreductase
b6-f complex Photosystem I NADP reductase
e–
e–
H2O
2H+ + �O2
e–
H+
e–
e–
pC
Q
P700
P680
NADPH
Fd
12
Reactioncenter
Excitedreactioncenter
Reactioncenter
Excitedreactioncenter
FIGURE 10.14A Z diagram of photosystems I and II. Two photosystems work sequentially. First, a photon of light ejects a high-energy electron fromphotosystem II; that electron is used to pump a proton across the membrane, contributing chemiosmotically to the production of amolecule of ATP. The ejected electron then passes along a chain of cytochromes to photosystem I. When photosystem I absorbs a photonof light, it ejects a high-energy electron used to drive the formation of NADPH.
How the TwoPhotosystems of PlantsWork TogetherPlants use the two photosystems dis-cussed earlier in series, first one andthen the other, to produce both ATPand NADPH. This two-stage processis called noncyclic photophosphory-lation, because the path of the elec-trons is not a circle—the electronsejected from the photosystems do notreturn to it, but rather end up inNADPH. The photosystems are re-plenished instead with electrons ob-tained by splitting water. PhotosystemII acts first. High-energy electronsgenerated by photosystem II are usedto synthesize ATP and then passed tophotosystem I to drive the productionof NADPH. For every pair of elec-trons obtained from water, one mole-cule of NADPH and slightly morethan one molecule of ATP are pro-duced.
Photosystem II
The reaction center of photosystem II,called P680, closely resembles the reac-tion center of purple bacteria. It con-sists of more than 10 transmembraneprotein subunits. The light-harvestingantenna complex consists of some 250molecules of chlorophyll a and acces-sory pigments bound to several proteinchains. In photosystem II, the oxygenatoms of two water molecules bind to acluster of manganese atoms which areembedded within an enzyme andbound to the reaction center. In a way that is poorly under-stood, this enzyme splits water, removing electrons one at atime to fill the holes left in the reaction center by departureof light-energized electrons. As soon as four electrons havebeen removed from the two water molecules, O2 is released.
The Path to Photosystem I
The primary electron acceptor for the light-energized elec-trons leaving photosystem II is a quinone molecule, as itwas in the bacterial photosystem described earlier. The re-duced quinone which results (plastoquinone, symbolized Q)is a strong electron donor; it passes the excited electron to aproton pump called the b6-f complex embedded within thethylakoid membrane (figure 10.15). This complex closelyresembles the bc1 complex in the respiratory electron trans-
port chain of mitochondria discussed in chapter 9. Arrivalof the energetic electron causes the b6-f complex to pump aproton into the thylakoid space. A small copper-containingprotein called plastocyanin (symbolized pC) then carries theelectron to photosystem I.
Making ATP: Chemiosmosis
Each thylakoid is a closed compartment into which pro-tons are pumped from the stroma by the b6-f complex.The splitting of water also produces added protons thatcontribute to the gradient. The thylakoid membrane isimpermeable to protons, so protons cross back out almostexclusively via the channels provided by ATP synthases.These channels protrude like knobs on the external sur-face of the thylakoid membrane. As protons pass out of
196 Part III Energetics
NADPH
Photosystem II Photosystem Ib6-f complex
Photon
Stroma
Thylakoidspace
H2O
2H+H+
H+ + NADP+
NADPreductase
Thylakoidmembrane
Antennacomplex
PlastoquinoneWater-splittingenzyme
Photon
Protongradient
Plastocyanin Ferredoxin
Fd
pC
Q
�O212
FIGURE 10.15The photosynthetic electron transport system. When a photon of light strikes a pigmentmolecule in photosystem II, it excites an electron. This electron is coupled to a protonstripped from water by an enzyme and is passed along a chain of membrane-boundcytochrome electron carriers (red arrow). When water is split, oxygen is released from thecell, and the hydrogen ions remain in the thylakoid space. At the proton pump (b6-f complex), the energy supplied by the photon is used to transport a proton across themembrane into the thylakoid. The concentration of hydrogen ions within the thylakoidthus increases further. When photosystem I absorbs another photon of light, its pigmentpasses a second high-energy electron to a reduction complex, which generates NADPH.
the thylakoid through the ATP syn-thase channel, ADP is phosphorylatedto ATP and released into the stroma,the fluid matrix inside the chloroplast(figure 10.16). The stroma containsthe enzymes that catalyze the reac-tions of carbon fixation.
Photosystem I
The reaction center of photosystem I,called P700, is a transmembrane complexconsisting of at least 13 protein sub-units. Energy is fed to it by an antennacomplex consisting of 130 chlorophyll aand accessory pigment molecules. Pho-tosystem I accepts an electron fromplastocyanin into the hole created bythe exit of a light-energized electron.This arriving electron has by no meanslost all of its light-excited energy; al-most half remains. Thus, the absorp-tion of a photon of light energy byphotosystem I boosts the electron leav-ing the reaction center to a very highenergy level. Unlike photosystem IIand the bacterial photosystem, photo-system I does not rely on quinones aselectron acceptors. Instead, it passeselectrons to an iron-sulfur proteincalled ferredoxin (Fd).
Making NADPH
Photosystem I passes electrons toferredoxin on the stromal side of themembrane (outside the thylakoid). Thereduced ferredoxin carries a very-high-potential electron. Two of them, fromtwo molecules of reduced ferredoxin, are then donated to amolecule of NADP+ to form NADPH. The reaction is cat-alyzed by the membrane-bound enzyme NADP reductase.Because the reaction occurs on the stromal side of themembrane and involves the uptake of a proton in formingNADPH, it contributes further to the proton gradient es-tablished during photosynthetic electron transport.
Making More ATP
The passage of an electron from water to NADPH in thenoncyclic photophosphorylation described previously gen-erates one molecule of NADPH and slightly more than onemolecule of ATP. However, as you will learn later in thischapter, building organic molecules takes more energy thanthat—it takes one-and-a-half ATP molecules per NADPHmolecule to fix carbon. To produce the extra ATP, many
plant species are capable of short-circuiting photosystem I,switching photosynthesis into a cyclic photophosphorylationmode, so that the light-excited electron leaving photosystemI is used to make ATP instead of NADPH. The energeticelectron is simply passed back to the b6-f complex ratherthan passing on to NADP+. The b6-f complex pumps out aproton, adding to the proton gradient driving the chemios-motic synthesis of ATP. The relative proportions of cyclicand noncyclic photophosphorylation in these plants deter-mines the relative amounts of ATP and NADPH availablefor building organic molecules.
The electrons that photosynthesis strips from watermolecules provide the energy to form ATP andNADPH. The residual oxygen atoms of the watermolecules combine to form oxygen gas.
Chapter 10 Photosynthesis 197
Photosystem II b6-f complex ATP synthase
H+
H+H+
H+
H+
H+
H+
H+
Photon
Stroma
Thylakoidspace
ATPADP
Q
Chloroplast
Plant cell
�O212
H2O
2
FIGURE 10.16Chemiosmosis in a chloroplast. The b6-f complex embedded in the thylakoid membranepumps protons into the interior of the thylakoid. ATP is produced on the outside surface ofthe membrane (stroma side), as protons diffuse back out of the thylakoid through ATPsynthase channels.
The Calvin CyclePhotosynthesis is a way of making organic molecules fromcarbon dioxide (CO2). These organic molecules containmany C—H bonds and are highly reduced compared withCO2. To build organic molecules, cells use raw materialsprovided by the light reactions:
1. Energy. ATP (provided by cyclic and noncyclic pho-tophosphorylation) drives the endergonic reactions.
2. Reducing power. NADPH (provided by photosys-tem I) provides a source of hydrogens and the energeticelectrons needed to bind them to carbon atoms. Muchof the light energy captured in photosynthesis ends upinvested in the energy-rich C—H bonds of sugars.
Carbon Fixation
The key step in the Calvin cycle—the event that makes thereduction of CO2 possible—is the attachment of CO2 to avery special organic molecule. Photosynthetic cells producethis molecule by reassembling the bonds of two intermedi-ates in glycolysis, fructose 6-phosphate and glyceraldehyde3-phosphate, to form the energy-rich five-carbon sugar,ribulose1,5-bisphosphate (RuBP), and a four-carbon sugar.
CO2 binds to RuBP in the key process called carbonfixation, forming two three-carbon molecules of phospho-glycerate (PGA) (figure 10.17). The enzyme that carriesout this reaction, ribulose bisphosphate carboxylase/oxygenase(usually abbreviated rubisco) is a very large four-subunitenzyme present in the chloroplast stroma. This enzymeworks very sluggishly, processing only about three mole-cules of RuBP per second (a typical enzyme processesabout 1000 substrate molecules per second). Because itworks so slowly, many molecules of rubisco are needed.In a typical leaf, over 50% of all the protein is rubisco. Itis thought to be the most abundant protein on earth.
Discovering the Calvin Cycle
Nearly 100 years ago, Blackman concluded that, because ofits temperature dependence, photosynthesis might involveenzyme-catalyzed reactions. These reactions form a cycleof enzyme-catalyzed steps similar to the Krebs cycle. Thiscycle of reactions is called the Calvin cycle, after its dis-coverer, Melvin Calvin of the University of California,Berkeley. Because the cycle begins when CO2 binds RuBPto form PGA, and PGA contains three carbon atoms, thisprocess is also called C3 photosynthesis.
198 Part III Energetics
10.4 Cells use the energy and reducing power captured by the lightreactions to make organic molecules.
H2C
Rubisco
2 molecules ofphosphoglycerate(PGA)
C O
O
HC OH
HC OH
P
H2C O
Ribulose1,5-bisphosphate
(RuBP)
P H2C
C
O–
O
O
HC OH
P
C O2 + H2O
H2C
C
O–
O
O
HC OH
P
FIGURE 10.17The key step in the Calvin cycle.Melvin Calvin and his coworkers at theUniversity of California worked out thefirst step of what later became known asthe Calvin cycle. They exposedphotosynthesizing algae to radioactivecarbon dioxide (14CO2). By following thefate of a radioactive carbon atom, theyfound that it first binds to a molecule ofribulose 1,5-bisphosphate (RuBP), thenimmediately splits, forming twomolecules of phosphoglycerate (PGA).One of these PGAs contains theradioactive carbon atom. In 1948,workers isolated the enzyme responsiblefor this remarkable carbon-fixingreaction: rubisco.
The Energy Cycle
The energy-capturing metabolisms of the chloroplastsstudied in this chapter and the mitochondria studied in theprevious chapter are intimately related. Photosynthesis usesthe products of respiration as starting substrates, and respi-ration uses the products of photosynthesis as its startingsubstrates (figure 10.18). The Calvin cycle even uses part ofthe ancient glycolytic pathway, run in reverse, to produceglucose. And, the principal proteins involved in electrontransport in plants are related to those in mitochondria,and in many cases are actually the same.
Photosynthesis is but one aspect of plant biology, al-though it is an important one. In chapters 37 through 43,
we will examine plants in more detail. We have treatedphotosynthesis here, in a section devoted to cell biology,because photosynthesis arose long before plants did, and allorganisms depend directly or indirectly on photosynthesisfor the energy that powers their lives.
Chloroplasts put ATP and NADPH to work buildingcarbon-based molecules, a process that essentiallyreverses the breakdown of such molecules that occursin mitochondria. Taken together, chloroplasts andmitochondria carry out a cycle in which energy entersfrom the sun and leaves as heat and work.
Chapter 10 Photosynthesis 199
ATPPhoto-system
I
NADP+
ADP ATP
Calvincycle
ATP
ATP
NADPH
Sunlight
Glucose
Chloroplast Mitochondrion
Heat
Pyruvate
Krebscycle
Electrontransportsystem
NAD+ NADH
Photo-system
II
CO2
H2O
O2
FIGURE 10.18Chloroplasts and mitochondria: Completing an energy cycle. Water and oxygen gas cycle between chloroplasts and mitochondriawithin a plant cell, as do glucose and CO2. Cells with chloroplasts require an outside source of CO2 and water and generate glucose andoxygen. Cells without chloroplasts, such as animal cells, require an outside source of glucose and oxygen and generate CO2 and water.
Reactions of the Calvin CycleIn a series of reactions (figure 10.19), three molecules ofCO2 are fixed by rubisco to produce six molecules of PGA(containing 6 × 3 = 18 carbon atoms in all, three from CO2and 15 from RuBP). The 18 carbon atoms then undergo acycle of reactions that regenerates the three molecules ofRuBP used in the initial step (containing 3 × 5 = 15 carbonatoms). This leaves one molecule of glyceraldehyde 3-phosphate (three carbon atoms) as the net gain.
The net equation of the Calvin cycle is:3 CO2 + 9 ATP + 6 NADPH + water —→
With three full turns of the cycle, three molecules ofcarbon dioxide enter, a molecule of glyceraldehyde 3-phosphate (G3P) is produced, and three molecules ofRuBP are regenerated (figure 10.20).
We now know that light is required indirectly for differ-ent segments of the CO2 reduction reactions. Five of theCalvin cycle enzymes—including rubisco—are light acti-vated; that is, they become functional or operate more effi-ciently in the presence of light. Light also promotes trans-port of three-carbon intermediates across chloroplastmembranes that are required for Calvin cycle reactions. Andfinally, light promotes the influx of Mg++ into the chloro-plast stroma, which further activates the enzyme rubisco.
200 Part III Energetics
THE CALVIN CYCLE1 2 3
3
3CO2
63-phosphoglycerate
P
The Calvin cycle begins when a carbonatom from a CO2 molecule is added to a five-carbon molecule (the starting material). The resulting six-carbonmolecule is unstable and immediately splits into three-carbon molecules.
Then, through a series of reactions,energy from ATP and hydrogens fromNADPH (the products of the lightreactions) are added to the three-carbon molecules. The now-reducedthree-carbon molecules either combineto make glucose or are used to makeother molecules.
Most of the reduced three-carbonmolecules are used to regenerate thefive-carbon starting material, thuscompleting the cycle.
5Glyceraldehyde
3-phosphate
3
RuBP3
P
6
6
6
Glyceraldehyde3-phosphate
Glucose
NADPH
3-phosphoglycerate
ATP
ATP
P
6Glyceraldehyde3-phosphate
P
1
P
(Starting material)RuBP
(Starting material)
FIGURE 10.19How the Calvin cycle works.
Output of the Calvin Cycle
The glyceraldehyde 3-phosphate that is the product of theCalvin cycle is a three-carbon sugar that is a key intermedi-ate in glycolysis. Much of it is exported from the chloro-plast to the cytoplasm of the cell, where the reversal of sev-eral reactions in glycolysis allows it to be converted tofructose 6-phosphate and glucose 1-phosphate, and fromthat to sucrose, a major transport sugar in plants (sucrose,common table sugar, is a disaccharide made of fructose andglucose).
In times of intensive photosynthesis, glyceraldehyde 3-phosphate levels in the stroma of the chloroplast rise. As a
consequence, some glyceraldehyde 3-phosphate in thechloroplast is converted to glucose 1-phosphate, in an anal-ogous set of reactions to those done in the cytoplasm, byreversing several reactions similar to those of glycolysis.The glucose 1-phosphate is then combined into an insolu-ble polymer, forming long chains of starch stored as bulkystarch grains in chloroplasts.
Plants incorporate carbon dioxide into sugars by means ofa cycle of reactions called the Calvin cycle, which is drivenby the ATP and NADPH produced in the light reactionswhich are consumed as CO2 is reduced to G3P.
FIGURE 10.20The Calvin cycle. For every three molecules of CO2 that enter the cycle, one molecule of the three-carbon compound, glyceraldehyde 3-phosphate (G3P), is produced. Notice that the process requires energy stored in ATP and NADPH, which are generated by the lightreactions. This process occurs in the stroma of the chloroplast.
PhotorespirationEvolution does not necessarily result in optimum solutions.Rather, it favors workable solutions that can be derivedfrom others that already exist. Photosynthesis is no excep-tion. Rubisco, the enzyme that catalyzes the key carbon-fixing reaction of photosynthesis, provides a decidedly sub-optimal solution. This enzyme has a second enzyme activitythat interferes with the Calvin cycle, oxidizing ribulose 1,5-bisphosphate. In this process, called photorespiration, O2is incorporated into ribulose 1,5-bisphosphate, which un-dergoes additional reactions that actually release CO2.Hence, photorespiration releases CO2—essentially undoingthe Calvin cycle which reduces CO2 to carbohydrate.
The carboxylation and oxidation of ribulose 1,5-bispho-sphate are catalyzed at the same active site on rubisco, andcompete with each other. Under normal conditions at25°C, the rate of the carboxylation reaction is four timesthat of the oxidation reaction, meaning that 20% of photo-synthetically fixed carbon is lost to photorespiration. Thisloss rises substantially as temperature increases, because therate of the oxidation reaction increases with temperaturefar faster than the carboxylation reaction rate.
Plants that fix carbon using only C3 photosynthesis (theCalvin cycle) are called C3 plants. In C3 photosynthesis,ribulose 1,5-bisphosphate is carboxylated to form a three-carbon compound via the activity of rubisco. Other plantsuse C4 photosynthesis, in which phosphoenolpyruvate, orPEP, is carboxylated to form a four-carbon compoundusing the enzyme PEP carboxylase. This enzyme has nooxidation activity, and thus no photorespiration. Further-more, PEP carboxylase has a much greater affinity for CO2than does rubisco. In the C4 pathway, the four-carboncompound undergoes further modification, only to be de-carboxylated. The CO2 which is released is then capturedby rubisco and drawn into the Calvin cycle. Because an or-ganic compound is donating the CO2, the effective concen-tration of CO2 relative to O2 is increased, and photorespi-ration is minimized.
The loss of fixed carbon as a result of photorespiration isnot trivial. C3 plants lose between 25 and 50% of theirphotosynthetically fixed carbon in this way. The rate de-pends largely upon the temperature. In tropical climates,especially those in which the temperature is often above28°C, the problem is severe, and it has a major impact ontropical agriculture.
The C4 Pathway
Plants that adapted to these warmer environments haveevolved two principal ways that use the C4 pathway todeal with this problem. In one approach, plants conductC4 photosynthesis in the mesophyll cells and the Calvincycle in the bundle sheath cells. This creates high locallevels of CO2 to favor the carboxylation reaction of ru-bisco. These plants are called C4 plants and include corn,
sugarcane, sorghum, and a number of other grasses. In theC4 pathway, the three-carbon metabolite phospho-enolpyruvate is carboxylated to form the four-carbonmolecule oxaloacetate, which is the first product of CO2fixation (figure 10.21). In C4 plants, oxaloacetate is in turnconverted into the intermediate malate, which is trans-ported to an adjacent bundle-sheath cell. Inside the bundle-sheath cell, malate is decarboxylated to produce pyruvate,releasing CO2. Because bundle-sheath cells are imperme-able to CO2, the CO2 is retained within them in high con-centrations. Pyruvate returns to the mesophyll cell, wheretwo of the high-energy bonds in an ATP molecule aresplit to convert the pyruvate back into phosphoenolpyru-vate, thus completing the cycle.
The enzymes that carry out the Calvin cycle in a C4plant are located within the bundle-sheath cells, where theincreased CO2 concentration decreases photorespiration.Because each CO2 molecule is transported into the bundle-sheath cells at a cost of two high-energy ATP bonds, andbecause six carbons must be fixed to form a molecule ofglucose, 12 additional molecules of ATP are required toform a molecule of glucose. In C4 photosynthesis, the ener-getic cost of forming glucose is almost twice that of C3photosynthesis: 30 molecules of ATP versus 18. Neverthe-less, C4 photosynthesis is advantageous in a hot climate:photorespiration would otherwise remove more than halfof the carbon fixed.
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CO2
Phosphoenol-pyruvate (PEP) Oxaloacetate
Pyruvate Malate
Bundle-sheathcell
Mesophyllcell
PPi + AMP
ATPPi +
Calvincycle
Glucose
CO2
MalatePyruvate
FIGURE 10.21Carbon fixation in C4 plants. This process is called the C4pathway because the starting material, oxaloacetate, is a moleculecontaining four carbons.
The Crassulacean Acid Pathway
A second strategy to decrease photorespiration in hot re-gions has been adopted by many succulent (water-storing)plants such as cacti, pineapples, and some members ofabout two dozen other plant groups. This mode of initialcarbon fixation is called crassulacean acid metabolism(CAM), after the plant family Crassulaceae (thestonecrops or hens-and-chicks), in which it was first dis-covered. In these plants, the stomata (singular, stoma),specialized openings in the leaves of all plants throughwhich CO2 enters and water vapor is lost, open during thenight and close during the day. This pattern of stomatalopening and closing is the reverse of that in most plants.CAM plants open stomata at night and initially fix CO2into organic compounds using the C4 pathway. These or-ganic compounds accumulate throughout the night and
are decarboxylated during the day to yield high levels ofCO2. In the day, these high levels of CO2 drive the Calvincycle and minimize photorespiration. Like C4 plants,CAM plants use both C4 and C3 pathways. They differfrom C4 plants in that they use the C4 pathway at nightand the C3 pathway during the day within the same cells. InC4 plants, the two pathways take place in different cells(figure 10.22).
Photorespiration results in decreased yields ofphotosynthesis. C4 and CAM plants circumvent thisproblem through modifications of leaf architecture andphotosynthetic chemistry that locally increase CO2concentrations. C4 plants isolate CO2 productionspatially, CAM plants temporally.
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C4 plants
Mesophyllcell
Bundle-sheath
cell
CO2
Calvincycle
Glucose
C4pathway
CO2
CAM plants
Mesophyllcell
Night
Day
CO2
Calvincycle
C4pathway
Glucose
CO2
FIGURE 10.22A comparison of C4 and CAM plants. Both C4 and CAM plants utilize the C4 and the C3 pathways. In C4 plants, the pathways areseparated spatially: the C4 pathway takes place in the mesophyll cells and the C3 pathway in the bundle-sheath cells. In CAM plants, thetwo pathways are separated temporally: the C4 pathway is utilized at night and the C3 pathway during the day.
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Chapter 10Summary Questions Media Resources
10.1 What is photosynthesis?
• Light is used by plants, algae, and some bacteria, in aprocess called photosynthesis, to convert atmosphericcarbon (CO2) into carbohydrate.
1. Where do the oxygen atomsin the O2 produced duringphotosynthesis come from?
• A series of simple experiments demonstrated thatplants capture energy from light and use it to convertthe carbon atoms of CO2 and the hydrogen atoms ofwater into organic molecules.
2. How did van Helmontdetermine that plants do notobtain their food from the soil?
10.2 Learning about photosynthesis: An experimental journey.
• Light consists of energy packets called photons; theshorter the wavelength of light, the more its energy.When photons are absorbed by a pigment, electronsin the pigment are boosted to a higher energy level.
• Photosynthesis channels photon excitation energyinto a single pigment molecule. In bacteria, thatmolecule then donates an electron to an electrontransport chain, which drives a proton pump andultimately returns the electron to the pigment.
• Plants employ two photosystems. Light is firstabsorbed by photosystem II and passed tophotosystem I, driving a proton pump and bringingabout the chemiosmotic synthesis of ATP.
• When the electron arrives at photosystem I, anotherphoton of light is absorbed, and energized electronsare channeled to a primary electron acceptor, whichreduces NADP+ to NADPH. Use of NADPH ratherthan NADH allows plants and algae to keep theprocesses of photosynthesis and oxidative respirationseparate from each other.
3. How is the energy of lightcaptured by a pigment molecule?Why does light reflected by thepigment chlorophyll appeargreen?4. What is the function of thereaction center chlorophyll?What is the function of theprimary electron acceptor?5. Explain how photosynthesis inthe sulfur bacteria is a cyclicprocess. What is its energy yieldin terms of ATP moleculessynthesized per electron?6. How do the two photosystemsin plants and algae work? Whichstage generates ATP and whichgenerates NADPH?
10.3 Pigments capture energy from sunlight.
• The ATP and reducing power produced by the lightreactions are used to fix carbon in a series of reactionscalled the Calvin cycle.
• RuBP carboxylase, the enzyme that fixes carbon inthe Calvin cycle, also carries out an oxidative reactionthat uses the products of photosynthesis, a processcalled photorespiration.
• Many tropical plants inhibit photorespiration byexpending ATP to increase the intracellularconcentration of CO2. This process, called the C4pathway, nearly doubles the energetic cost ofsynthesizing glucose.
7. In a C3 plant, where do thelight reactions occur? Wheredoes the Calvin cycle occur?8. What is photorespiration?What advantage do C4 plantshave over C3 plants with respectto photorespiration? Whatdisadvantage do C4 plants havethat limits their distributionprimarily to warm regions of theearth?
10.4 Cells use the energy and reducing power captured by the light reactions to make organic molecules.
