PHOTOSYNTHESIS

After reading this chapter and attending lecture, the student should be able to:

• Photosynthesis transforms solar light energy trapped by chloroplasts into chemical bond energy stored in sugar and other organic molecules. This process:
• Synthesizes energy-rich organic molecules from the energy-poor molecules, CO₂ and H₂O.
• Uses CO₂ as a carbon source and light energy as the energy source.
• Directly or indirectly supplies energy to most living organisms.

• Organisms acquire organic molecules used for energy and carbon skeletons by one of two nutritional modes: 1) Autotrophic Nutrition or 2) Heterotrophic Nutrition.
• Autotrophic nutrition = (Auto = self; trophos = feed) Nutritional mode of synthesizing organic molecules from inorganic raw materials.
• Examples of autotrophic organisms are plants, which require only CO₂, H₂O and minerals as nutrients.
• Because autotrophic organisms produce organic molecules that enter an ecosystem’s food store, autotrophs are also known as producers.
• Autotrophic organisms require an energy source to synthesize organic molecules. That energy source may be from light (photoautotrophic) or from the oxidation of inorganic substances (chemoautotrophic).
• Photoautotrophs = Autotrophic organisms that use light as an energy source to synthesize organic molecules. Examples are photosynthetic organisms such as plants, algae and some prokaryotes.
• Chemoautotrophs = Autotrophic organisms that use the oxidation of inorganic substances, such as sulfur or ammonia, as an energy source to synthesize organic molecules. Unique to some bacteria, this is a rarer form of autotrophic nutrition.
• Heterotrophic nutrition = (Heteros = other; trophos = feed) Nutritional mode of acquiring organic molecules from compounds produced by other organisms; heterotrophs are unable to synthesize organic molecules from inorganic raw materials.
• Heterotrophs are also known as consumers.
• Examples are animals that eat plants or other animals.
• Examples also include decomposers, heterotrophs that decompose and feed on organic litter. Most fungi and many bacteria are decomposers.
• Most heterotrophs depend on photoautotrophs for food and oxygen (a by-product of photosynthesis).

• Although all green plant parts have chloroplasts, leaves are the major organs of photosynthesis in most plants.
• Chlorophyll is the green pigment in chloroplasts that gives a leaf its color and that absorbs the light energy used to drive photosynthesis.

LEAF CROSS SECTION

• Chloroplasts are primarily in cells of mesophyll, green tissue in the leaf's interior.
• CO₂ enters and O₂ exits the leaf through microscopic pores called stomata.
• Water absorbed by the roots is transported to leaves through veins or vascular bundles which also export sugar from leaves to nonphotosynthetic parts of the plant.
• Chloroplasts are lens-shaped organelles measuring about 2 – 4 µm by 4 – 7 µm. These organelles are divided into three functional compartments by a system of membranes:
• Intermembrane Space . The chloroplast is bound by a double membrane which partitions its contents from the cytosol. A narrow intermembrane space separates the two membranes.
• Thylakoids. Thylakoids form another membranous system within the chloroplast. The thylakoid membrane segregates the interior of the chloroplast into two compartments: thylakoid space and stroma.
• Thylakoids = Flattened membranous sacs inside the chloroplast.
• Chlorophyll is found in the thylakoid membranes.
• Thylakoids function in the steps of photosynthesis that initially convert light energy to chemical energy.
• Thylakoid space = Space inside the thylakoid.
• Grana = (Singular, granum) Stacks of thylakoids in a chloroplast.
• Stroma. Reactions that use chemical energy to convert carbon dioxide to sugar occur in the stroma, viscous fluid outside the thylakoids.
• Photosynthetic prokaryotes lack chloroplasts, but have chlorophyll built into the plasma membrane or into membranes of numerous vesicles within the cell.
• These membranes function in a manner similar to the thylakoid membranes of chloroplasts.
• Photosynthetic membranes of cyanobacteria are usually arranged in parallel stacks of flattened sacs similar to the thylakoids of chloroplasts.

• Some steps in photosynthesis are not yet understood, but the following summary equation has been known since the early 1800's:
• 6 CO₂ + 12 H₂O + light energy ® C₆H₁₂O₆ + 6 O₂ + 6 H₂O
• Glucose (C₆H₁₂O₆) is shown in the summary equation, though the main products of photosynthesis are other carbohydrates.
• Water is on both sides of the equation, because photosynthesis consumes 12 molecules and forms 6.
• Indicating the net consumption of water simplifies the equation:
• 6 CO₂ + 6 H₂O + light energy ® C₆H₁₂O₆ + 6 O₂
• In this form, the summary equation for photosynthesis is the reverse of that for cellular respiration.
• Photosynthesis and cellular respiration both occur in plant cells, but plants do not simply reverse the steps of respiration to make food.
• The simplest form of the equation is: CO₂ + H₂O ® CH₂O + O₂
• CH₂O symbolizes the general formula for a carbohydrate.
• In this form, the summary equation emphasizes the production of a sugar molecule, one carbon at a time. Six repetitions produces a glucose molecule.

• The discovery that O₂ released by plants is derived from H₂O and not from CO₂, was one of the earliest clues to the mechanism of photosynthesis.
• In the 1930's, C.B. van Niel from Stanford University challenged an early model, which predicted that:
• O₂ released during photosynthesis comes from CO₂.
• CO₂ is split and water is added to the carbon.
• Van Niel studied bacteria that use hydrogen sulfide (H₂S) rather than H₂O for photosynthesis and that produce yellow sulfur globules as a by-product.
• Van Niel deduced that these bacteria split H₂S and use H to make sugar. He generalized that all photosynthetic organisms require hydrogen, but that the source varies.
• Van Niel thus hypothesized that plants split water as a source of hydrogen and release oxygen as a by-product.
• Scientists later confirmed van Niel's hypothesis by using a heavy isotope of oxygen (¹⁸O) as a tracer to follow oxygen's fate during photosynthesis.
• If water was labeled with tracer, released oxygen was ¹⁸O.
• If the ¹⁸O was introduced to the plant as CO₂, the tracer did not appear in the released oxygen.
• An important result of photosynthesis is the extraction of hydrogen from water and its incorporation into sugar.
• Electrons associated with hydrogen have more potential energy in organic molecules than they do in water, where the electrons are closer to electronegative oxygen.
• Energy is stored in sugar and other food molecules in the form of these high-energy electrons.

• Respiration is an exergonic redox process; energy is released from the oxidation of sugar.
• Electrons associated with sugar's hydrogens lose potential energy as carriers transport them to oxygen, forming water.
• Electronegative oxygen pulls electrons down the electron transport chain, and the potential energy released is used by the mitochondrion to produce ATP.
• Photosynthesis is an endergonic redox process; energy is required to reduce carbon dioxide.
• Light is the energy source that boosts potential energy of electrons as they are moved from water to sugar.
• When water is split, electrons are transferred from the water to carbon dioxide, reducing it to sugar.

IV. The light dependent reactions and the Calvin cycle cooperate in transforming light to the chemical energy of food: an overview

• Photosynthesis occurs in two stages: the light dependent reactions and the light independent reactions (known as the Calvin cycle).
• Light dependent reactions = In photosynthesis, the reactions that convert light energy to chemical bond energy in ATP and NADPH. These reactions:
• Occur in the thylakoid membranes of chloroplasts.
• Reduce NADP⁺ to NADPH.
• Light absorbed by chlorophyll provides the energy to reduce NADP⁺ to NADPH, which temporarily stores the energized electrons transferred from water.
• NADP⁺ (nicotinamide adenine dinucleotide phosphate), a coenzyme similar to NAD⁺ in respiration, is reduced by adding a pair of electrons along with a hydrogen nucleus, or H⁺.
• Give off O₂ as a by-product from the splitting of water.
• Generate ATP. The light reactions power the addition of a phosphate group to ADP in a process called photophosphorylation.
• Calvin cycle = In photosynthesis, the carbon-fixation reactions that assimilate atmospheric CO₂ and then reduce it to a carbohydrate; named for Melvin Calvin. These reactions:
• Occur in the stroma of the chloroplast.
• First incorporate atmospheric CO₂ into existing organic molecules by a process called carbon fixation, and then reduce fixed carbon to carbohydrate.
• Carbon fixation = The process of incorporating CO₂ into organic molecules.
• The Calvin cycle reactions do not require light directly, but reduction of CO₂ to sugar requires the products of the light reactions:
• NADPH provides the reducing power.
• ATP provides the chemical energy.
• Chloroplasts thus use light energy to make sugar by coordinating the two stages of photosynthesis.
• Light reactions occur in the thylakoids of chloroplasts.
• Calvin cycle reactions occur in the stroma.
• As NADP⁺ and ADP contact thylakoid membranes, they pick up electrons and phosphate respectively, and then transfer their high-energy cargo to the Calvin cycle.

• To understand how the thylakoids of chloroplasts transform light energy into the chemical energy of ATP and NADPH, it is necessary to know some important properties of light.

• Sunlight is electromagnetic energy. The quantum mechanical model of electromagnetic radiation describes light as having a behavior that is both wavelike and particlelike.
• Wavelike properties of light.
• Electromagnetic energy is a form of energy that travels in rhythmic waves which are disturbances of electric and magnetic fields.
• A wavelength is the distance between the crests of electromagnetic waves.
• The electromagnetic spectrum ranges from wavelengths that are less than a nanometer (gamma rays) to those that are more than a kilometer (radio waves). (See Campbell, Figure 10.5)
• Visible light, which is detectable by the human eye, is only a small portion of the electromagnetic spectrum and ranges from about 380 to 750 nm. The wavelengths most important for photosynthesis are within this range of visible light.
• Particlelike properties of light.
• Light also behaves as if it consists of discrete particles or quanta called photons.
• Each photon has a fixed quantity of energy which is inversely proportional to the wavelength of light. For example, a photon of violet light has nearly twice as much energy as a photon of red light.
• The sun radiates the full spectrum of electromagnetic energy.
• The atmosphere acts as a selective window that allows visible light to pass through while screening out a substantial fraction of other radiation.
• The visible range of light is the radiation that drives photosynthesis.
• Blue and red, the two wavelengths most effectively absorbed by chlorophyll, are the colors most useful as energy for the light reactions.

• Light may be reflected, transmitted or absorbed when it contacts matter. (See Campbell, Figure 10.6)
• Pigments = Substances that absorb visible light.
• Different pigments absorb different wavelengths of light.
• Wavelengths that are absorbed disappear, so a pigment that absorbs all wavelengths appears black.
• When white light, which contains all the wavelengths of visible light, illuminates a pigment, the color you see is the color most reflected or transmitted by the pigment. For example, a leaf appears green because chlorophyll absorbs red and blue light but transmits and reflects green light.
• Each pigment has a characteristic absorption spectrum or pattern of wavelengths that it absorbs. It is expressed as a graph of absorption versus wavelength.
• The absorption spectrum for a pigment in solution can be determined by using a spectrophotometer, an instrument used to measure what proportion of a specific wavelength of light is absorbed or transmitted by the pigment. (See Campbell Methods Box)
• Since chlorophyll a is the light-absorbing pigment that participates directly in the light reactions, the absorption spectrum of chlorophyll a provides clues as to which wavelengths of visible light are most effective for photosynthesis.
• Chlorophyll a has absorption peaks at approximately 440 nm and 680 nm.
• A graph of wavelength versus rate of photosynthesis is called an action spectrum and profiles the relative effectiveness of different wavelengths of visible light for driving photosynthesis.
• The action spectrum of photosynthesis can be determined by illuminating chloroplasts with different wavelengths of light and measuring some indicator of photosynthetic rate, such as oxygen release or carbon dioxide consumption.
• It is apparent from the action spectrum of photosynthesis that blue and red light are the most effective wavelengths for photosynthesis and green light is the least effective.
• The action spectrum for photosynthesis does not exactly match the absorption spectrum for chlorophyll a.
• Since chlorophyll a is not the only pigment in chloroplasts that absorb light, the absorption spectrum for chlorophyll a underestimates the effectiveness of some wavelengths.
• Even though only special chlorophyll a molecules can participate directly in the light reactions, other pigments, called accessory pigments, can absorb light and transfer the energy to chlorophyll a.
• The accessory pigments expand the range of wavelengths available for photosynthesis. These pigments include:
• Chlorophyll b, a yellow-green pigment with a structure similar to chlorophyll a. This minor structural difference gives the pigments slightly different absorption spectra.
• Carotenoids, yellow and orange hydrocarbons that are built into the thylakoid membrane with the two types of chlorophyll.

• What happens when chlorophyll or accessory pigments absorb photons?
• Colors of absorbed wavelengths disappear from the spectrum of transmitted and reflected light.
• The absorbed photon boosts one of the pigment molecule's electrons in its lowest-energy state (ground state) to an orbital of higher potential energy (excited state).
• The only photons absorbed by a molecule are those with an energy state equal to the difference in energy between the ground state and excited state.
• This energy difference varies from one molecule to another. Pigments have unique absorption spectra because pigments only absorb photons corresponding to specific wavelengths.
• The photon energy absorbed is converted to potential energy of an electron elevated to the excited state.
• The excited state is unstable, so excited electrons quickly fall back to the ground state orbital, releasing excess energy in the process. This released energy may be:
• Dissipated as heat.
• Reradiated as light of a lower energy and longer wavelength than the original light that excited the pigment. This afterglow is called fluorescence.
• Pigment molecules do not fluoresce when in the thylakoid membranes, because nearby primary electron acceptor molecules trap excited state electrons that have absorbed photons.
• In this redox reaction, chlorophyll is photooxidized by the absorption of light energy and the electron acceptor is reduced.
• Because no primary electron acceptor is present, isolated chlorophyll fluoresces in the red part of the spectrum and dissipates heat.

• Chlorophyll a, chlorophyll b and the carotenoids are assembled into photosystems located within the thylakoid membrane. Each photosystem is composed of:
• antenna complex.
• Several hundred chlorophyll a, chlorophyll b and carotenoid molecules are light-gathering antennae that absorb photons and pass the energy from molecule to molecule.
• Different pigments within the antennal complex have slightly different absorption spectra, so collectively they can absorb photons from a wider range of the light spectrum than would be possible with only one type of pigment molecule.
• reaction-center chlorophyll.
• Only one of the many chlorophyll a molecules in each complex can actually transfer an excited electron to initiate the light reactions. This specialized chlorophyll a is located in the reaction center.
• primary electron acceptor.
• Located near the reaction center, a primary electron acceptor molecule traps excited state electrons released from the reaction center chlorophyll.
• The transfer of excited state electrons from chlorophyll to primary electron acceptor molecules is the first step of the light reactions. The energy stored in the trapped electrons powers the synthesis of ATP and NADPH in subsequent steps.
• Two types of photosystems are located in the thylakoid membranes, photosystem I and photosystem II.
• The reaction center of photosystem I has a specialized chlorophyll a molecule known as P700, which absorbs best at 700 nm (the far red portion of the spectrum).
• The reaction center of photosystem II has a specialized chlorophyll a molecule known as P680, which absorbs best at a wavelength of 680 nm.
• P700 and P680 are identical chlorophyll a molecules, but each is associated with a different protein. This affects their electron distribution and results in slightly different absorption spectra.

E. Noncyclic Electron Flow

• There are two possible routes for electron flow during the light reactions: noncyclic flow and cyclic flow.
• Both photosystem I and photosystem II function and cooperate in noncyclic electron flow which transforms light energy to chemical energy stored in the bonds of NADPH and ATP. This process:
• Occurs in the thylakoid membrane.
• Passes electrons continuously from water to NADP⁺.
• Produces ATP by noncyclic photophosphorylation.
• Produces NADPH.
• Produces O₂.

Electron Flow in Photosystem I

• Light excites electrons from P700, the reaction center chlorophyll in photosystem I. These excited state electrons do not return to the reaction center chlorophyll, but are ultimately stored in NADPH, which will later be the electron donor in the Calvin Cycle.
• Initially, the excited state electrons are transferred from P700 to the primary electron acceptor for photosystem I.
• The primary electron acceptor passes these excited state electrons to ferredoxin (Fd), an iron-containing protein.
• NADP⁺ reductase catalyzes the redox reaction that transfers these electrons from ferredoxin to NADP⁺, producing reduced coenzyme – NADPH.
• The oxidized P700 chlorophyll becomes an oxidizing agent as its electron "holes" must be filled; photosystem II supplies the electrons to fill these holes.

Electron Flow in Photosystem II

• When the antenna assembly of photosystem II absorbs light, the energy is transferred to the P680 reaction center.
• Electrons ejected from P680 are trapped by the photosystem II primary electron acceptor.
• The electrons are then transferred from this primary electron acceptor to an electron transport chain embedded in the thylakoid membrane. The first carrier in the chain, plastoquinone (Pq) receives the electrons from the primary electron acceptor. In a cascade of redox reactions, the electrons travel from Pq to a complex of two cytochromes to plastocyanin (Pc) to P700 of photosystem I.
• As these electrons pass down the electron transport chain, they lose potential energy until they reach the ground state of P700.
• These electrons then fill the electron vacancies left in photosystem I when NADP⁺ was reduced.
• Electrons from P680 flow to P700 during noncyclic electron flow, restoring the missing electrons in P700. This, however, leaves the P680 reaction center of photosystem II with missing electrons; the oxidized P680 chlorophyll thus becomes a strong oxidizing agent.
• A water-splitting enzyme extracts electrons from water and passes them to oxidized P680, which has a high affinity for electrons.
• As water is oxidized, the removal of electrons splits water into two hydrogen ions and an oxygen atom.
• The oxygen atom immediately combines with a second oxygen atom to form O₂. It is this water-splitting step of photosynthesis that releases O₂.

Photophosphorylation

• As excited electrons give up energy along the transport chain to P700, the thylakoid membrane couples the exergonic flow of electrons to the endergonic reactions that phosphorylate ADP to ATP.
• This coupling mechanism is chemiosmosis.
• Some electron carriers can only transport electrons in the company of protons.
• The protons are picked up on one side of the thylakoid membrane and deposited on the opposite side as the electrons move to the next member of the transport chain.
• The electron flow thus stores energy in the form of a proton gradient across the thylakoid membrane – a proton-motive force.
• An ATP synthase enzyme in the thylakoid membrane uses the proton-motive force to make ATP. This process is called photophosphorylation because the energy required is light.
• This form of ATP production is called noncyclic photophosphorylation.

F. Cyclic Electron Flow

• Cyclic electron flow is the simplest pathway, but involves only photosystem I and generates ATP without producing NADPH or evolving oxygen.
• It is cyclic because excited electrons that leave from chlorophyll a at the reaction center return to the reaction center.
• As photons are absorbed by Photosystem I, the P700 reaction center chlorophyll releases excited-state electrons to the primary electron acceptor; which, in turn, passes them to ferredoxin. From there the electrons take an alternate path that sends them tumbling down the electron transport chain to P700. This is the same electron transport chain used in noncyclic electron flow.
• With each redox reaction along the electron transport chain, electrons lose potential energy until they return to their ground-state orbital in the P700 reaction center.
• The exergonic flow of electrons is coupled to ATP production by the process of chemiosmosis. This process of ATP production is called cyclic photo-phosphorylation.
• Absorption of another two photons of light by the pigments send a second pair of electrons through the cyclic pathway.
• The function of the cyclic pathway is to produce additional ATP.
• It does so without the production of NADPH or O₂.
• Cyclic photophosphorylation supplements the ATP supply required for the Calvin cycle and other metabolic pathways. The noncyclic pathway produces approximately equal amounts of ATP and NADPH, which is not enought ATP to meet demand.
• NADPH concentration might influence whether electrons flow through cyclic or noncyclic pathways.

G. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• Chemiosmosis = The coupling of exergonic electron flow down an electron transport chain to endergonic ATP production by the creation of an electrochemical proton gradient across a membrane. The proton gradient drives ATP synthesis as protons diffuse back across the membrane.
• Chemiosmosis in chloroplasts and chemiosmosis in mitochondria are similar in several ways:
• An electron transport chain assembled in a membrane translocates protons across the membrane as electrons pass through a series of carriers that are progressively more electronegative.
• An ATP synthase complex built into the same membrane, couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP.
• The ATP synthase complexes and some electron carriers (including quinones and cytochromes) are very similar in both chloroplasts and mitochondria.
• Oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts differ in the following ways:

1. Electron Transport Chain

• Mitochondria transfer chemical energy from food molecules to ATP. The high-energy electrons that pass down the transport chain are extracted by the oxidation of food molecules.
• Chloroplasts transform light energy into chemical energy. Photosystems capture light energy and use it to drive electrons to the top of the transport chain.

2. Spatial Organization

• The inner mitochondrial membrane pumps protons from the matrix out to the intermembrane space, which is a reservoir of protons that power ATP synthase.
• The chloroplast's thylakoid membrane pumps protons from the stroma into the thylakoid compartment, which functions as a proton reservoir. ATP is produced as protons diffuse from the thylakoid compartment back to the stroma through ATP synthase complexes that have catalytic heads on the membrane's stroma side. Thus, ATP forms in the stroma where it drives sugar synthesis during the Calvin cycle.
• There is a large proton or pH gradient across the thylakoid membrane.
• When chloroplasts are illuminated, there is a thousand-fold difference in H⁺ concentration. The pH in the thylakoid compartment is reduced to about 5 while the pH in the stroma increases to about 8.
• When chloroplasts are in the dark, the pH gradient disappears, but can be reestablished if chloroplasts are illuminated.
• Andre Jagendorf (1960's) produced compelling evidence for chemiosmosis when he induced chloroplasts to produce ATP in the dark by using artificial means to create a pH gradient. His experiments demonstrated that during photophosphorylation, the function of the photosystems and the electron transport chain is to create a proton-motive force that drives ATP synthesis.
• For a tentative model of the organization of the thylakoid membrane see Campbell, Figure 10.15.
• Proton pumping by the thylakoid membrane depends on an asymmetric placement of electron carriers that accept and release protons (H⁺).

• During noncyclic electron flow, the photosystems of the thylakoid membrane transform light energy to the chemical energy stored in NADPH and ATP. This process:
• Pushes low energy-state electrons from water to NADPH, where they are stored at a higher state of potential energy. NADPH, in turn, is the electron donor used to reduce carbon dioxide to sugar (Calvin Cycle).
• Produces ATP from this light driven electron current.
• Produces oxygen as a by-product.
• During cyclic electron flow, electrons ejected from P700 reach ferredoxin and flow back to P700. This process:
• Produces ATP.
• Unlike noncyclic electron flow, does not produce NADPH or O₂.

VI. The Calvin cycle uses ATP and NADPH to convert CO₂ to sugar: a closer look

• ATP and NADPH produced by the light reactions are used in the Calvin cycle to reduce carbon dioxide to sugar.
• The Calvin cycle is similar to the Krebs cycle in that the starting material is regenerated by the end of the cycle.
• Carbon enters the Calvin cycle as CO₂ and leaves as sugar.
• ATP is the energy source, while NADPH is the reducing agent that adds high-energy electrons to form sugar.
• The Calvin cycle actually produces a three-carbon sugar glyceraldehyde 3-phosphate (G3P).
• For the Calvin cycle to synthesize one molecule of sugar (G3P), three molecules of CO₂ must enter the cycle. The cycle may be divided into three phases:
• Phase 1: Carbon Fixation.
• The Calvin cycle begins when each molecule of CO₂ is attached to a five-carbon sugar, ribulose biphosphate (RuBP).
• This reaction is catalyzed by the enzyme RuBP carboxylase (rubisco) – one of the most abundant proteins on Earth.
• The product of this reaction is an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate.
• For every three CO₂ molecules that enter the Calvin cycle via rubisco, three RuBP molecules are carboxylated forming six molecules of 3-phosphoglycerate.
• Phase 2: Reduction.
• This endergonic reduction phase is a two-step process that couples ATP hydrolysis with the reduction of 3-phosphoglycerate to glyceraldehyde phosphate.
• An enzyme phosphorylates 3-phosphoglycerate by transferring a phosphate group from ATP. This reaction:
• produces 1, 3-bisphosphoglycerate.
• uses six ATP molecules to produce six molecules of 1,3-bisphosphoglycerate.
• primes 1,3-bisphosphoglycerate for the addition of high-energy electrons from NADPH.
• Electrons from NADPH reduce the carboxyl group of 1,3-bisphosphoglycerate to the aldehyde group of glyceraldehyde 3-phosphate (G3P).
• The product, G3P, stores more potential energy than the initial reactant, 3-phosphoglycerate.
• G3P is the same three-carbon sugar produced when glycolysis splits glucose.
• For every three CO₂ molecules that enter the Calvin cycle, six G3P molecules are produced, only one of which can be counted as net gain.
• The cycle begins with three five-carbon RuBP molecules – a total of 15 carbons.
• The six G3P molecules produced contain 18 carbons, a net gain of three carbons from CO₂.
• One G3P molecule exits the cycle; the other five are recycled to regenerate three molecules of RuBP.

• A complex series of reactions rearranges the carbon skeletons of five G3P molecules into three RuBP molecules.
• These reactions require three ATP molecules.
• RuBP is thus regenerated to begin the cycle again.
• For the net synthesis of one G3P molecule, the Calvin cycle uses the products of the light reactions:
• 9 ATP molecules
• 6 NADPH molecules
• G3P produced by the Calvin cycle is the raw material used to synthesize glucose and other carbohydrates.
• The Calvin cycle uses 18 ATP and 12 NADPH molecules to produce one glucose molecule.

• When the O₂ concentration in the leaf's air spaces is higher than CO₂ concentration, rubisco accepts O₂ and transfers it to RuBP.
• This pathway produces no ATP molecules and reduces the number of organic molecules that can be reduced by the Calvin cycle.
• This metabolic pathway is called photorespiration and it reduces photosynthetic output.
• Photorespiration = In plants, a metabolic pathway that consumes oxygen, evolves carbon dioxide, produces no ATP and decreases photosynthetic output.
• Occurs because the active site of rubisco can accept O₂ as well as CO₂.
• The "respiration" in photorespiration refers to the fact that this process uses O₂ and releases CO₂.
• Some scientists believe that photorespiration is a metabolic relic from earlier times when the atmosphere contained less oxygen and more carbon dioxide than is present today.
• Under these conditions, when rubisco evolved, the inability of the enzyme's active site to distinguish carbon dioxide from oxygen would have made little difference.
• This affinity for oxygen has been retained by rubisco and some photorespiration is bound to occur.
• Whether photorespiration is beneficial to plants is not known.
• It is known that some crop plants (e.g. soybeans) lose as much as 50% of the carbon fixed by the Calvin cycle to photorespiration.
• If photorespiration could be reduced in some agricultural plants, crop yields and food supplies would increase.
• Photorespiration is fostered by hot, dry, bright days.
• Under these conditions, plants close their stomata to prevent dehydration by reducing water loss from the leaf.
• Photosynthesis then depletes available carbon dioxide and increases oxygen within the leaf air spaces. This condition favors photorespiration.

Certain species of plants, which live in hot arid climates, have evolved alternate modes of carbon fixation that minimize photorespiration. C₄ and CAM are the two most important of these photosynthetic adaptations.

• The Calvin cycle occurs in most plants and produces 3-phosphoglycerate, a three-carbon compound, as the first stable intermediate.
• These plants are called C₃ plants, because the first stable intermediate has three carbons.
• Agriculturally important C₃ plants include rice, wheat and soybeans.
• Many plant species precede the Calvin cycle with reactions that incorporate carbon dioxide into four-carbon compounds.
• These plants are called C₄ plants.
• The C₄ pathway is used by several thousand species in at least 19 families including corn and sugarcane, important agricultural grasses.
• This pathway is adaptive, because it enhances carbon fixation under conditions that favor photorespiration, such as hot, arid environments.
• Leaf anatomy of C4 plants spatially segregates the Calvin cycle from the initial incorporation of CO₂ into organic compounds.
• In C₄ plants there are two distinct types of photosynthetic cells:

1. Bundle-sheath cells.

• Are arranged into tightly packed sheaths around the veins of the leaf.
• Thylakoids in the chloroplasts of bundle-sheath cells are not stacked into grana.
• The Calvin cycle is confined to the chloroplasts of the bundle sheath.

2. Mesophyll cells.

• Are more loosely arranged in the area between the bundle sheath and the leaf surface.
• The Calvin cycle of C₄ plants is preceded by incorporation of CO₂ into organic compounds in the mesophyll. (See Campbell, Figure 10.18)

Step 1: CO₂ is added to phosphoenolpyruvate (PEP) to form oxaloacetate, a four-carbon product.

• PEP carboxylase is the enzyme that adds CO₂ to PEP. Compared to rubisco, it has a much greater affinity for CO₂ and has no affinity for O₂.
• Thus, PEP carboxylase can fix CO₂ efficiently when rubisco cannot ¾ under hot, dry conditions that cause stomata to close, CO₂ concentrations to drop and O₂ concentrations to rise.

Step 2: After CO₂ has been fixed by mesophyll cells, they convert oxaloacetate to another four-carbon compound (usually malate).

Step 3: Mesophyll cells then export the four-carbon products (i.e. malate) to the bundle-sheath cells.

• In the bundle-sheath cells, the four carbon compounds release CO₂, which is then fixed by rubisco in the Calvin cycle.
• Mesophyll cells thus pump CO₂ into bundle-sheath cells, minimizing photorespiration and enhancing sugar production by maintaining a CO₂ concentration sufficient for rubisco to accept CO₂ rather than oxygen.

• A second photosynthetic adaptation exists in succulent plants adapted to very arid conditions. These plants open their stomata primarily at night and close them during the day (opposite of most plants).
• This conserves water during the day, but prevents CO₂ from entering the leaves.
• When stomata are open at night, CO₂ is taken up and incorporated into a variety of organic acids. This mode of carbon fixation is called crassulacean acid metabolism (CAM).
• The organic acids made at night are stored in vacuoles of mesophyll cells until morning, when the stomata close.
• During daytime, light reactions supply ATP and NADPH for the Calvin cycle. At this time, CO₂ is released from the organic acids made the previous night and is incorporated into sugar in the chloroplasts.
• The CAM and C4 pathways:
• are similar in that CO₂ is first incorporated into organic intermediates before it enters the Calvin cycle.
• differ in that the initial steps of carbon fixation in C₄ plants are structurally separate from the Calvin cycle; in CAM plants, the two steps occur at separate times.
• Regardless of whether the plant uses a C₃, C₄ or CAM pathway, all plants use the Calvin cycle to produce sugar from CO₂.

VIII. Photosynthesis is the biosphere’s metabolic foundation: a review

• On a global scale, photosynthesis makes about 160 billion metric tons of carbohydrate per year. No other chemical process on Earth is more productive or is as important to life.
• Light reactions capture solar energy and use it to:
• produce ATP
• transfer electrons from water to NADP⁺ to form NADPH
• The Calvin cycle uses ATP and NADPH to fix CO₂ and produce sugar.
• Photosynthesis transforms light energy to chemical bond energy in sugar molecules.
• Sugars made in chloroplasts supply the entire plant with chemical energy and carbon skeletons to synthesize organic molecules.
• Nonphotosynthetic parts of a plant depend on organic molecules exported from leaves in veins.
• The disaccharide sucrose is the transport form of carbohydrate in most plants.
• Sucrose is the raw material for cellular respiration and many anabolic pathways in nonphotosynthetic cells.
• Much of the sugar is glucose – the monomer linked to form cellulose, the main constituent of plant cell walls.
• Most plants make more organic material than needed for respiratory fuel and for precursors of biosynthesis.
• Plants consume about 50% of the photosynthate as fuel for cellular respiration.
• Extra sugars are synthesized into starch and stored in storage cells of roots, tubers, seeds, and fruits.
• Heterotrophs also consume parts of plants as food.
• Photorespiration can reduce photosynthetic yield of C3 plants in hot dry climates. Alternate methods of carbon fixation minimize photorespiration.
• C4 plants spatially separate carbon fixation from the Calvin cycle.
• CAM plants temporally separate carbon fixation from the Calvin cycle.

Atkins, P.W. Atoms, Electrons, and Change. New York, Oxford: W.H. Freeman and Company, 1991. Chapter 9, "Light and Life" is a witty, imaginative description of photosynthesis. Though written for a lay audience, it is probably best appreciated by someone already familiar with photosynthesis.

Campbell, N. Biology. 4th ed. Menlo Park, California: Benjamin/Cummings, 1996.

Lehninger, A.L., D.L. Nelson and M.M. Cox. Principles of Biochemistry. 2nd ed. New York: Worth, 1993.

Matthews, C.K. and K.E. van Holde. Biochemistry. 2nd ed. Redwood City, California: Benjamin/Cummings, 1996.