Introduction to Photosynthesis

Chapter 35&10

Developed by Adam F. Sprague

Chapter 10

1.ALL LIFE REQUIRES ENERGY

2.Animals, fungi, and most protists obtain their energy by consuming, directly or indirectly, organic food stuffs from their environment (heterotrophs)

3.Some organisms (autotrophs) have the ability to capture the energy of the sun to synthesize their own organic food (green plants, algae)

4.THE ULTIMATE SOURCE OF ALL ENERGY ON EARTH IS THE SUN

5.PHOTOSYNTHESIS is the link between life on earth and the sun

6.It is a set of reactions which convert light energy from the sun into chemical bond energy of glucose and ATP

Photosynthesis can be summarized with this chemical equation:

6CO2 + 12H2O + LIGHT ENERGY --> C6H12O6 + 6O2 + 6H2O

6CO2 + 12H2O + LIGHT ENERGY --> C6H12O6 + 6O2 + 6H2O

The chemical change is the reverse of cellular respiration

The low energy inorganic compounds (CO2 and water) are converted into the high potential organic molecule (glucose)

The Chloroplasts: Sites of Photosynthesis

The primary function of this specialized organelle is to convert light energy into ATP and NADPH (nicotinamide adenine dinucleotide phosphate)

Chloroplasts are found mainly in the cells of the mesophyll (about 50/cell), the green tissue on the interior of the leaf

Leaf

Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata

The double membrane of the chloroplast regulates transport of materials in and out

Chloroplasts are filled with an aqueous solution called the stoma which contains all the necessary enzymes for photosynthesis

Chloroplast

The conversion from light energy to ATP and NADPH occurs in the thylakoid membranes within the stroma

The thylakoid membranes contain all of the pigments involved in the process including chlorophyll (green pigment) and other carotenoids

The thylakoids are organized into closely packed stacks called grana

Choloroplast

Within these thylakoids and grana, light energy is converted into ATP and NADPH – these are said to be LIGHT-DEPENDENT REACTIONS

The reactions that actually convert CO2 to carbohydrate are LIGHT-INDEPENDENT REACTIONS or DARK REACTIONS

The Light Reactions

Must take place in the presence of light Steps that convert solar energy to

chemical energy Light absorbed by chlorophyll drives a

transfer of electrons from water to an acceptor named NADP+ which temporarily stores the energized electrons

Light Reactions

Water is split in the process and thus it is the light reactions of photosynthesis that give off O2 as a by-product

The light reactions also generate ATP by powering the addition of a phosphate group to ADP, a process called photophosphorylation

THE LIGHT REACTIONS PRODUCE NO SUGAR

 

The Dark Reactions Light is not required directly for these reactions to occur These reactions incorporate CO2 from the air into organic

material through a process known as carbon fixation The fixed carbon is then reduced to carbohydrate by the

addition of electrons The reducing power is provided by NADPH and ATP

provided by the light reactions Dark reactions in most plants occur during daylight so

that the light reactions can regenerate NADPH and ATP These reactions occur in the stroma

Light and Pigments

The Nature of Sunlight

The Nature of Sunlight

light is a form of energy known as electromagnetic radiation

light travels in rhythmic waves which are disturbances of electrical and magnetic fields

The Nature of Sunlight

the distance between crests of electromagnetic waves is called the wavelength

the entire range of radiation is known as the electromagnetic spectrum

Light Energy the narrow range from about 380 to 750nm in wavelength

is detectable by the human eye and is called visible light the model of light as waves explains many of its

properties, but in certain respects it behaves as though it consists of discrete particles

these particles called photons 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 light (shorter wavelengths have more energy)

Photosynthetic Pigments

as light meets matter, it may be reflected, transmitted or absorbed

substances that absorb light are called pigments

if a pigment is illuminated in white light, the color we see is the color most reflected or transmitted by the pigment

Light perception

the major pigment in leaves, chlorophyll, appears green because it absorbs red and blue light while transmitted and reflecting green

chlorophyll is actually a family of pigments with similar chemical structures

Photoexcitation of Chlorophyll

when energy is absorbed by a molecule of pigment, one of the molecules electrons is elevated to from its ground state to a higher orbital around the nucleus (excited state)

Photoexcitation of Chlorophyll the only photons absorbed are those whose

energy is exactly equal to the energy difference between the ground state and an excited state

the energy of the photon is converted to the potential energy of an electron, making the electron less stable

generally, when pigments absorb light, their excited electrons drop back down to the ground state very quickly releasing their energy as heat and/or light (fluorescence)

Light Dependent Reactions

Photosynthetic Unit

Photosynthetic Unit in its native environment of the thylakoid membrane, chlorophyll

is organized along with proteins, pigments,  and other kinds of smaller organic molecules into photosystems

the proteins of these chloro-protein complexes affect the absorption properties of the photosystem

a photosystem has a light gathering "antenna complex" consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules

the number and variety of pigment molecules allows for the absorption of light over a larger surface area and larger portion of the spectrum

all of the antenna molecules absorb photons of light and the energy is transmitted from pigment molecule to pigment molecule until it reaches the reaction center

Photosystems I and II Emerson found that when plants were exposed to long (>680 nm) and

short (<680 nm) wavelengths of light, the rate of photosynthesis was much greater than the sum of the rates of production for each individual range (to explain this "Emerson Enhancement Effect", it must be assumed that there are two photosystems in the thylakoid membranes, photosystem I and photosystem II

the reaction center of photosystem I is known as P700 because its pigment is best at absorbing light with an average wavelength of 700 nm (far-red)

photosystem II has pigment in it reaction center, P680, which best absorbs light with an average wavelength of 680 nm (red)

the chlorophyll a in both photosystems is identical, it is their association with different proteins that affects their light absorbing properties

 

ATP Synthesis in Chloroplasts

ATP Synthesis in Chloroplasts

chloroplasts and mitochondria generate ATP by the same basic mechanism of chemiosmosis

an electron transport chain embedded in the thylakoid membrane pumps protons across the membrane as electrons are passed through a series of carriers producing a proton-motive force (potential energy stored in the proton gradient)

ATP Synthesis in Chloroplasts ATP synthase in the membrane couples the diffusion of

hydrogen ions down their gradient to the phosphorylation of ADP

in contrast to oxidative phosphorylation in mitochondria, chloroplasts use light energy (not chemical energy in food) to drive electrons to the top of the transport chain

the proton pump of the thylakoid membrane moves hydrogen ions from the stroma to the thylakoid space which functions as the H+ reservoir

the membrane makes ATP in the stroma as hydrogen ions diffuse back down their gradient through ATP synthase

Dark Reactions

Dark Reactions

The "Dark Reactions" include the biochemical, enzyme-catalyzed reactions involved in the synthesis of carbohydrate from carbon dioxide; these are collectively know as the Calvin-Benson cycle

 

The Reactions

The Reactions THE FIRST step (carbon fixation) of the reaction pathway is

when a molecule of CO2 is added to a compound named ribulose bisphosphate (RuBP), a five-carbon sugar with a phosphate group at each end

This reaction is catalyzed by the enzyme RuBP carboxylase-oxygenase, ("RUBISCO" for short) the most abundant protein in chloroplasts (and on earth!)

The product of the reaction is a six-carbon intermediate that is so unstable that it immediately splits in half to form two molecules of 3-phosphogrlyceric acid/phosphoglycerate

For every three CO2 that enter the Calvin-Benson cycle via rubisco, a total of six molecules of 3-phosphoglyerate are made

Dark reactions IN THE SECOND step (reduction) of the cycle, each molecule of 3-

phosophglyceric acid receives and additional phosphate group An enzyme transfers the phosphate group from ATP forming 1,3-

diphophoglyceric acid (glycolysis?) For every three (3) molecules of CO2 incorporated into the cycle, six molecules

of ATP must be used to produce six (6) molecules of 1,3-diphosphoglycerate IN THE NEXT step, the NADPH (from the light reactions) reduces the

diphosphoglycerate to phosphoglyceraldehyde (PGAL) (6 for every 3 CO2) Some of these molecules (1 PAL/3 CO2) are converted into glucose but most

are used to regenerate RuBP The stromal reactions to convert the 3-carbon PGAL to the 5-carbon RuBP are

dependant on the presence of 3 more molecules of ATP/3 CO2 in the cycle The five (5) remaining PGAL (3-C) are re-arranged into three (3) RuBP (5-C)

molecules

The Calvin-Benson cycle….

produces three-carbon intermediates used to synthesize glucose

produces three-carbon intermediates used to regenerate the initial carbon dioxide-acceptor molecule

The Calvin-Benson cycle….

Without the presence of ATP and NADPH from the light-dependent photo-chemical reactions, the conversion of carbon dioxide to glucose can not occur

 

The Metabolic Fates of Glucose: About 50% of the glucose formed is used immediately to meet the plants

energy needs Excess glucose can be converted to starch within the stroma of the chloroplast

or in specialized storage cells of roots, tubers, seeds, and fruits REMEMBER, plants actively metabolize glucose (cellular respiration) and grow

in the dark and in the light The glucose may be converted to sucrose (glucose + fructose) for transport

(via the phloem cells) to the non-photosynthetic leaves, roots, and stems The formation of sucrose takes place in the cytoplasm, NOT in the chloroplast the sucrose provides raw material for cellular respiration and many other

anabolic pathways that synthesize proteins, lipids, and other products The glucose may be converted to CELLULOSE, to build cell walls, especially in

plant cells that are still growig and maturing This conversion also takes place within the cytoplasm

Photosynthetic Induction In the dark, carbon fixation will stop in a plant when the

chloroplast has consumed all the ribulose bisphosphate and PGAL

When the plant is exposed to light, maximum rates of carbon dioxide fixation can not take place until all the intermediates of the Calvin cycle have been replenished to an optimal level

This lag time between exposure to light and maximum photosynthetic rates is called photosynthetic induction

The enzymes which catalyze the steps of the Calvin-Benson cycle also rely on products of the light-dependent reactions to maintain their "active" form

Photorespiration Plants that produce three-carbon phosphoglycerate as

the first product of the light-independent reactions are referred to as C3 plants

The active site of Rubisco can utilize O2 or CO2 with a preference for CO2

If the air spaces in a leaf have a much higher concentration of O2 than CO2, the active site of rubisco will accept O2

When this occurs, a two-carbon molecule of phosphoglycerate is produced, leaves the chloroplasts and is metabolized in the peroxisomes and mitochondria resulting in the release of carbon dioxide

Photorespiration Photorespiration consumes oxygen, released carbon

dioxide and generally occurs only in the light The environmental conditions that foster photorespiration

in C3 plants are hot, dry, bright days On such days, plants close their stomata to reduce water

loss and the plant soon depletes its CO2 and increases O2 within the leaf

Photorespiration generates no ATP, decreases photosynthetic output by siphoning organic material from the Calvin cycle, produces no food, and seemingly has no known benefit to plants

Alternate Photosynthetic Pathways

Photorespiration

Photorespiration Plants are constantly evolving to ensure they

are optimally adapted to their environments plants have adapted anatomically and

metabolically to thrive in their terrestrial domain of major concern to plants is dehydration via

transpiration through the stomata of the leaf surface

on hot, dry days, plants close their stomata to reduce water loss but at the same time, their limiting the intake of carbon dioxide which will reduce photosynthetic yield

Photorespiration with the stomata closed, carbon dioxide concentrations will quickly

decrease and oxygen concentrations will rise plants that produce three-carbon 3-phosphoglycerate as the first stable

product of the Calvin cycle are called C3 plants (ie. rice, wheat) these plants produce less food when their stomata close on hot, dry

days the active site of Rubisco can bind oxygen or carbon dioxide with a

preference for CO2 • Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known

by the shorter name RuBisCO, is an enzyme (EC 4.1.1.39) that is used in the Calvin cycle to catalyze the first major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the form of energy-rich molecules such as sucrose. RuBisCO catalyzes either the carboxylation or oxygenation of ribulose-1,5-bisphosphate (also known as RuBP) with carbon dioxide or oxygen.

if the air spaces in a leaf have a much higher concentration of oxygen than carbon dioxide, the active site of Rubisco will accept oxygen

C4 and CAM Plants

C4 and CAM Plants

certain plants have evolved alternate mode of carbon fixation forming a four-carbon compound as its first product

a unique leaf anatomy is correlated with the mechanism of C4 photosynthesis including two distinct types of photosynthetic cells; bundle sheath cells and mesophyll cells

C4 and CAM Plants bundle sheath cells are arranged into tightly packed sheaths

around the veins of the leaf between the bundle sheath and the leaf surface are the more

loosely arranged mesophyll cells the Calvin cycle is confined to the chloroplasts of the bundle

sheath the cycle is preceded by incorporation of carbon dioxide into

three-carbon phosphoenolpyruvate (PEP) to form four-carbon oxaloacetate

the enzyme involved, PEP carboxylase has a much higher affinity for carbon dioxide than does Rubisco

after the CO2 is "fixed", the mesophyll cells export oxaloacetate to the bundle sheath cells where the CO2 is released and is introduced into the Calvin cycle

C4 and CAM Plants CAM plants have adapted to dry conditions by opening

their stomata during the night and closing them during the day, opposite to how other plants behave

when the stomata are open CO2 is incorporated into a variety of organic acids in a method of carbon fixation call crassulacean acid metabolism (CAM)

the mesophyll cells of CAM plants store the organic acids they make during the night in their vacuoles until morning when the stomata close

CO2 is released from the acids during the day for incorporation into the Calvin cycle