The Chloroplast Is One Member of the Plastid Family of Organelles

Chloroplasts are the most prominent members of the plastid family of organelles. Plastids are present in all living plant cells, each cell type having its own characteristic complement. All plastids share certain features. Most notably, all plastids in a particular plant species contain multiple copies of the same relatively small genome. In addition, each is enclosed by an envelope composed of two concentric membranes.

As discussed in Chapter 12 (see Figure 12-3), all plastids develop from proplastids, small organelles in the immature cells of plant meristems (Figure 14-33A). Proplastids develop according to the requirements of each differentiated cell, and the type that is present is determined in large part by the nuclear genome. If a leaf is grown in darkness, its proplastids enlarge and develop into etioplasts, which have a semicrystalline array of internal membranes containing a yellow chlorophyll precursor instead of chlorophyll. When exposed to light, the etioplasts rapidly develop into chloroplasts by converting this precursor to chlorophyll and by synthesizing new membrane pigments, photosynthetic enzymes, and components of the electron-transport chain.

Figure 14-33

Plastid diversity. (A) A proplastid from a root tip cell of a bean plant. Note the double membrane; the inner membrane has also generated the relatively sparse internal membranes present. (B) Three amyloplasts (a form of leucoplast), or starch-storing (more...)

Leucoplasts are plastids present in many epidermal and internal tissues that do not become green and photosynthetic. They are little more than enlarged proplastids. A common form of leucoplast is the amyloplast (Figure 14-33B), which accumulates the polysaccharide starch in storage tissues—a source of sugar for future use. In some plants, such as potatoes, the amyloplasts can grow to be as large as an average animal cell.

It is important to realize that plastids are not just sites for photosynthesis and the deposition of storage materials. Plants have also used their plastids to compartmentalize their intermediary metabolism. Purine and pyrimidine synthesis, most amino acid synthesis, and all of the fatty acid synthesis of plants takes place in the plastids, whereas in animal cells these compounds are produced in the cytosol.

Chloroplasts Resemble Mitochondria But Have an Extra Compartment

Chloroplasts carry out their energy interconversions by chemiosmotic mechanisms in much the same way that mitochondria do. Although much larger (Figure 14-34A), they are organized on the same principles. They have a highly permeable outer membrane; a much less permeable inner membrane, in which membrane transport proteins are embedded; and a narrow intermembrane space in between. Together, these membranes form the chloroplast envelope (Figure 14-34B,C). The inner membrane surrounds a large space called the stroma, which is analogous to the mitochondrial matrix and contains many metabolic enzymes. Like the mitochondrion, the chloroplast has its own genome and genetic system. The stroma therefore also contains a special set of ribosomes, RNAs, and the chloroplast DNA.

Figure 14-34

Electron micrographs of chloroplasts. (A) In a wheat leaf cell, a thin rim of cytoplasm—containing chloroplasts, the nucleus, and mitochondria—surrounds a large vacuole. (B) A thin section of a single chloroplast, showing the chloroplast (more...)

There is, however, an important difference between the organization of mitochondria and that of chloroplasts. The inner membrane of the chloroplast is not folded into cristae and does not contain electron-transport chains. Instead, the electron-transport chains, photosynthetic light-capturing systems, and ATP synthase are all contained in the thylakoid membrane, a third distinct membrane that forms a set of flattened disclike sacs, the thylakoids (Figure 14-35). The lumen of each thylakoid is thought to be connected with the lumen of other thylakoids, thereby defining a third internal compartment called the thylakoid space, which is separated by the thylakoid membrane from the stroma that surrounds it.

Figure 14-35

The chloroplast. This photosynthetic organelle contains three distinct membranes (the outer membrane, the inner membrane, and the thylakoid membrane) that define three separate internal compartments (the intermembrane space, the stroma, and the thylakoid (more...)

The structural similarities and differences between mitochondria and chloroplasts are illustrated in Figure 14-36. The head of the chloroplast ATP synthase, where ATP is made, protrudes from the thylakoid membrane into the stroma, whereas it protrudes into the matrix from the inner mitochondrial membrane.

Figure 14-36

A mitochondrion and chloroplast compared. A chloroplast is generally much larger than a mitochondrion and contains, in addition to an outer and inner membrane, a thylakoid membrane enclosing a thylakoid space. Unlike the chloroplast inner membrane, the (more...)

Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon

The many reactions that occur during photosynthesis in plants can be grouped into two broad categories:

1.

In the photosynthetic electron-transfer reactions (also called the “light reactions”), energy derived from sunlight energizes an electron in the green organic pigment chlorophyll, enabling the electron to move along an electron-transport chain in the thylakoid membrane in much the same way that an electron moves along the respiratory chain in mitochondria. The chlorophyll obtains its electrons from water (H₂O), producing O₂ as a by-product. During the electron-transport process, H⁺ is pumped across the thylakoid membrane, and the resulting electrochemical proton gradient drives the synthesis of ATP in the stroma. As the final step in this series of reactions, high-energy electrons are loaded (together with H⁺) onto NADP⁺, converting it to NADPH. All of these reactions are confined to the chloroplast.

2.

In the carbon-fixation reactions (also called the “dark reactions”), the ATP and the NADPH produced by the photosynthetic electron-transfer reactions serve as the source of energy and reducing power, respectively, to drive the conversion of CO₂ to carbohydrate. The carbon-fixation reactions, which begin in the chloroplast stroma and continue in the cytosol, produce sucrose and many other organic molecules in the leaves of the plant. The sucrose is exported to other tissues as a source of both organic molecules and energy for growth.

Thus, the formation of ATP, NADPH, and O₂ (which requires light energy directly) and the conversion of CO₂ to carbohydrate (which requires light energy only indirectly) are separate processes (Figure 14-37), although elaborate feedback mechanisms interconnect the two. Several of the chloroplast enzymes required for carbon fixation, for example, are inactivated in the dark and reactivated by light-stimulated electron-transport processes.

Figure 14-37

The reactions of photosynthesis in a chloroplast. Water is oxidized and oxygen is released in the photosynthetic electron-transfer reactions, while carbon dioxide is assimilated (fixed) to produce sugars and a variety of other organic molecules in the (more...)

Carbon Fixation Is Catalyzed by Ribulose Bisphosphate Carboxylase

We have seen earlier in this chapter how cells produce ATP by using the large amount of free energy released when carbohydrates are oxidized to CO₂ and H₂O. Clearly, therefore, the reverse reaction, in which CO₂ and H₂O combine to make carbohydrate, must be a very unfavorable one that can only occur if it is coupled to other, very favorable reactions that drive it.

The central reaction of carbon fixation, in which an atom of inorganic carbon is converted to organic carbon, is illustrated in Figure 14-38: CO₂ from the atmosphere combines with the five-carbon compound ribulose 1,5-bisphosphate plus water to yield two molecules of the three-carbon compound 3-phosphoglycerate. This “carbon-fixing” reaction, which was discovered in 1948, is catalyzed in the chloroplast stroma by a large enzyme called ribulose bisphosphate carboxylase. Since each molecule of the complex works sluggishly (processing only about 3 molecules of substrate per second compared to 1000 molecules per second for a typical enzyme), many enzyme molecules are needed. Ribulose bisphosphate carboxylase often constitutes more than 50% of the total chloroplast protein, and it is thought to be the most abundant protein on Earth.

Figure 14-38

The initial reaction in carbon fixation. This reaction, in which carbon dioxide is converted into organic carbon, is catalyzed in the chloroplast stroma by the abundant enzyme ribulose bisphosphate carboxylase. The product is 3-phosphoglycerate, which (more...)

Three Molecules of ATP and Two Molecules of NADPH Are Consumed for Each CO₂ Molecule That Is Fixed

The actual reaction in which CO₂ is fixed is energetically favorable because of the reactivity of the energy-rich compound ribulose 1,5-bisphosphate, to which each molecule of CO₂ is added (see Figure 14-38). The elaborate metabolic pathway that produces ribulose 1,5-bisphosphate requires both NADPH and ATP; it was worked out in one of the first successful applications of radioisotopes as tracers in biochemistry. This carbon-fixation cycle (also called the Calvin cycle) is outlined in Figure 14-39. It starts when 3 molecules of CO₂ are fixed by ribulose bisphosphate carboxylase to produce 6 molecules of 3-phosphoglycerate (containing 6 × 3 = 18 carbon atoms in all: 3 from the CO₂ and 15 from ribulose 1,5-bisphosphate). The 18 carbon atoms then undergo a cycle of reactions that regenerates the 3 molecules of ribulose 1,5-bisphosphate used in the initial carbon-fixation step (containing 3 × 5 = 15 carbon atoms). This leaves 1 molecule of glyceraldehyde 3-phosphate (3 carbon atoms) as the net gain.

Figure 14-39

The carbon-fixation cycle, which forms organic molecules from CO₂ and H₂O. The number of carbon atoms in each type of molecule is indicated in the white box. There are many intermediates between glyceraldehyde 3-phosphate and ribulose 5-phosphate, but (more...)

A total of 3 molecules of ATP and 2 molecules of NADPH are consumed for each CO₂ molecule converted into carbohydrate. The net equation is:

Thus, both phosphate-bond energy (as ATP) and reducing power (as NADPH) are required for the formation of organic molecules from CO₂ and H₂O. We return to this important point later.

The glyceraldehyde 3-phosphate produced in chloroplasts by the carbon-fixation cycle is a three-carbon sugar that also serves as a central intermediate in glycolysis. Much of it is exported to the cytosol, where it can be converted into fructose 6-phosphate and glucose 1-phosphate by the reversal of several reactions in glycolysis (see Panel 2-8, pp. 124–125). The glucose 1-phosphate is then converted to the sugar nucleotide UDP-glucose, and this combines with the fructose 6-phosphate to form sucrose phosphate, the immediate precursor of the disaccharide sucrose. Sucrose is the major form in which sugar is transported between plant cells: just as glucose is transported in the blood of animals, sucrose is exported from the leaves via vascular bundles, providing the carbohydrate required by the rest of the plant.

Most of the glyceraldehyde 3-phosphate that remains in the chloroplast is converted to starch in the stroma. Like glycogen in animal cells, starch is a large polymer of glucose that serves as a carbohydrate reserve (see Figure 14-33B). The production of starch is regulated so that it is produced and stored as large grains in the chloroplast stroma during periods of excess photosynthetic capacity. This occurs through reactions in the stroma that are the reverse of those in glycolysis: they convert glyceraldehyde 3-phosphate to glucose 1-phosphate, which is then used to produce the sugar nucleotide ADP-glucose, the immediate precursor of starch. At night the starch is broken down to help support the metabolic needs of the plant. Starch provides an important part of the diet of all animals that eat plants.

Carbon Fixation in Some Plants Is Compartmentalized to Facilitate Growth at Low CO₂ Concentrations

Although ribulose bisphosphate carboxylase preferentially adds CO₂ to ribulose 1,5-bisphosphate, it can use O₂ as a substrate in place of CO₂, and if the concentration of CO₂ is low, it will add O₂ to ribulose 1,5-bisphosphate instead (see Figure 14-38). This is the first step in a pathway called photorespiration, whose ultimate effect is to use up O₂ and liberate CO₂ without the production of useful energy stores. In many plants, about one-third of the CO₂ fixed is lost again as CO₂ because of photorespiration.

Photorespiration can be a serious liability for plants in hot, dry conditions, which cause them to close their stomata (the gas exchange pores in their leaves) to avoid excessive water loss. This in turn causes the CO₂ levels in the leaf to fall precipitously, thereby favoring photorespiration. A special adaptation, however, occurs in the leaves of many plants, such as corn and sugar cane that live in hot, dry environments. In these plants, the carbon-fixation cycle occurs only in the chloroplasts of specialized bundle-sheath cells, which contain all of the plant's ribulose bisphosphate carboxylase. These cells are protected from the air and are surrounded by a specialized layer of mesophyll cells that use the energy harvested by their chloroplasts to “pump” CO₂ into the bundle-sheath cells. This supplies the ribulose bisphosphate carboxylase with a high concentration of CO₂, thereby greatly reducing photorespiration.

The CO₂ pump is produced by a reaction cycle that begins in the cytosol of the mesophyll cells. A CO₂-fixation step is catalyzed by an enzyme that binds carbon dioxide (as bicarbonate) and combines it with an activated three-carbon molecule to produce a four-carbon molecule. The four-carbon molecule diffuses into the bundle-sheath cells, where it is broken down to release the CO₂ and generate a molecule with three carbons. The pumping cycle is completed when this three-carbon molecule is returned to the mesophyll cells and converted back to its original activated form. Because the CO₂ is initially captured by converting it into a compound containing four carbons, the CO₂-pumping plants are called C₄ plants. All other plants are called C₃ plants because they capture CO₂ into the three-carbon compound 3-phosphoglycerate (Figure 14-40).

Figure 14-40

Comparative leaf anatomy in a C₃ plant and a C₄ plant. The cells with green cytosol in the leaf interior contain chloroplasts that perform the normal carbon-fixation cycle. In C₄ plants, the mesophyll cells are specialized for CO₂ pumping rather than (more...)

As for any vectorial transport process, pumping CO₂ into the bundle-sheath cells in C₄ plants costs energy. In hot, dry environments, however, this cost can be much less than the energy lost by photorespiration in C₃ plants, so C₄ plants have a potential advantage. Moreover, because C₄ plants can perform photosynthesis at a lower concentration of CO₂ inside the leaf, they need to open their stomata less often and therefore can fix about twice as much net carbon as C₃ plants per unit of water lost. Although the vast majority of plant species are C₃ plants, C₄ plants such as corn and sugar cane are much more effective at converting sunlight energy into biomass than C₃ plants such as cereal grains. They are therefore of special importance in world agriculture.

Photosynthesis Depends on the Photochemistry of Chlorophyll Molecules

Having discussed the carbon-fixation reactions, we now return to the question of how the photosynthetic electron-transfer reactions in the chloroplast generate the ATP and the NADPH needed to drive the production of carbohydrates from CO₂ and H₂O. The required energy is derived from sunlight absorbed by chlorophyll molecules (Figure 14-41). The process of energy conversion begins when a chlorophyll molecule is excited by a quantum of light (a photon) and an electron is moved from one molecular orbital to another of higher energy. As illustrated in Figure 14-42, such an excited molecule is unstable and tends to return to its original, unexcited state in one of three ways:

Figure 14-41

The structure of chlorophyll. A magnesium atom is held in a porphyrin ring, which is related to the porphyrin ring that binds iron in heme (see Figure 14-22). Electrons are delocalized over the bonds shown in blue.

Figure 14-42

Three ways for an excited chlorophyll molecule to return to its original, unexcited state. The light energy absorbed by an isolated chlorophyll molecule is completely released as light and heat by process 1. In photosynthesis, by contrast, chlorophylls (more...)

1.

By converting the extra energy into heat (molecular motions) or to some combination of heat and light of a longer wavelength (fluorescence), which is what happens when light energy is absorbed by an isolated chlorophyll molecule in solution.

2.

By transferring the energy—but not the electron—directly to a neighboring chlorophyll molecule by a process called resonance energy transfer.

3.

By transferring the high-energy electron to another nearby molecule, an electron acceptor, and then returning to its original state by taking up a low-energy electron from some other molecule, an electron donor.

The last two mechanisms are exploited in the process of photosynthesis.

A Photosystem Consists of a Reaction Center Plus an Antenna Complex

Multiprotein complexes called photosystems catalyze the conversion of the light energy captured in excited chlorophyll molecules to useful forms. A photosystem consists of two closely linked components: an antenna complex, consisting of a large set of pigment molecules that capture light energy and feed it to the reaction center; and a photochemical reaction center, consisting of a complex of proteins and chlorophyll molecules that enable light energy to be converted into chemical energy (Figure 14-43).

Figure 14-43

The antenna complex and photochemical reaction center in a photosystem. The antenna complex is a collector of light energy in the form of excited electrons. The energy of the excited electrons is funneled, through a series of resonance energy transfers, (more...)

The antenna complex is important for capturing light. In chloroplasts it consists of a number of distinct membrane protein complexes (known as light-harvesting complexes); together, these proteins bind several hundred chlorophyll molecules per reaction center, orienting them precisely in the thylakoid membrane. Depending on the plant, different amounts of accessory pigments called carotenoids, which protect the chlorophylls from oxidation and can help collect light of other wavelengths, are also located in each complex. When a chlorophyll molecule in the antenna complex is excited, the energy is rapidly transferred from one molecule to another by resonance energy transfer until it reaches a special pair of chlorophyll molecules in the photochemical reaction center. Each antenna complex thereby acts as a funnel, collecting light energy and directing it to a specific site where it can be used effectively (see Figure 14-43).

The photochemical reaction center is a transmembrane protein-pigment complex that lies at the heart of photosynthesis. It is thought to have evolved more than 3 billion years ago in primitive photosynthetic bacteria. The special pair of chlorophyll molecules in the reaction center acts as an irreversible trap for excitation quanta because its excited electron is immediately passed to a chain of electron acceptors that are precisely positioned as neighbors in the same protein complex (Figure 14-44). By moving the high-energy electron rapidly away from the chlorophylls, the photochemical reaction center transfers it to an environment where it is much more stable. The electron is thereby suitably positioned for subsequent reactions, which require more time to complete.

Figure 14-44

The arrangement of the electron carriers in a bacterial photochemical reaction center, as determined by x-ray crystallography. The pigment molecules shown are held in the interior of a transmembrane protein and are surrounded by the lipid bilayer of the (more...)

In a Reaction Center, Light Energy Captured by Chlorophyll Creates a Strong Electron Donor from a Weak One

The electron transfers involved in the photochemical reactions just outlined have been analyzed extensively by rapid spectroscopic methods. An enormous amount of detailed information is available for the photosystem of purple bacteria, which is somewhat simpler than the evolutionarily related photosystems in chloroplasts. The reaction center in this photosystem is a large protein-pigment complex that can be solubilized with detergent and purified in active form. In 1985, its complete three-dimensional structure was determined by x-ray crystallography (see Figure 10-38). This structure, combined with kinetic data, provides the best picture we have of the initial electron-transfer reactions that underlie photosynthesis.

The sequence of electron transfers that take place in the reaction center of purple bacteria is shown in Figure 14-45. As outlined previously for the general case (see Figure 14-43), light causes a net electron transfer from a weak electron donor (a molecule with a strong affinity for electrons) to a molecule that is a strong electron donor in its reduced form. The excitation energy in chlorophyll that would normally be released as fluorescence or heat is thereby used instead to create a strong electron donor (a molecule carrying a high-energy electron) where none had been before. In the purple bacterium, the weak electron donor used to fill the electron-deficient hole created by a light-induced charge separation is a cytochrome (see orange box in Figure 14-45); the strong electron donor produced is a quinone. In the chloroplasts of higher plants, a quinone is similarly produced. However, as we discuss next, water serves as the initial weak electron donor, which is why oxygen gas is released by photosynthesis in plants.

Figure 14-45

The electron transfers that occur in the photochemical reaction center of a purple bacterium. A similar set of reactions occurs in the evolutionarily related photosystem II in plants. At the top left is an orientating diagram showing the molecules that (more...)

Noncyclic Photophosphorylation Produces Both NADPH and ATP

Photosynthesis in plants and cyanobacteria produces both ATP and NADPH directly by a two-step process called noncyclic photophosphorylation. Because two photosystems—called photosystems I and II—are used in series to energize an electron, the electron can be transferred all the way from water to NADPH. As the high-energy electrons pass through the coupled photosystems to generate NADPH, some of their energy is siphoned off for ATP synthesis.

The first of the two photosystems—paradoxically called photosystem II for historical reasons—has the unique ability to withdraw electrons from water. The oxygens of two water molecules bind to a cluster of manganese atoms in a poorly understood water-splitting enzyme. This enzyme enables electrons to be removed one at a time from the water, as required to fill the electron-deficient holes created by light in chlorophyll molecules in the reaction center. As soon as four electrons have been removed from the two water molecules (requiring four quanta of light), O₂ is released. Photosystem II thus catalyzes the reaction 2H₂O + 4 photons → 4H⁺ + 4e ^- + O₂. As we discussed for the electron-transport chain in mitochondria, which uses O₂ and produces water, the mechanism ensures that no partly oxidized water molecules are released as dangerous, highly reactive oxygen radicals. Essentially all the oxygen in the Earth's atmosphere has been produced in this way.

The core of the reaction center in photosystem II is homologous to the bacterial reaction center just described, and it likewise produces strong electron donors in the form of reduced quinone molecules dissolved in the lipid bilayer of the membrane. The quinones pass their electrons to a H⁺ pump called the cytochrome b₆-f complex, which resembles the cytochrome b-c₁ complex in the respiratory chain of mitochondria. The cytochrome b₆-f complex pumps H⁺ into the thylakoid space across the thylakoid membrane (or out of the cytosol across the plasma membrane in cyanobacteria), and the resulting electrochemical gradient drives the synthesis of ATP by an ATP synthase (Figure 14-46).

Figure 14-46

Electron flow during photosynthesis in the thylakoid membrane. The mobile electron carriers in the chain are plastoquinone (which closely resembles the ubiquinone of mitochondria), plastocyanin (a small copper-containing protein), and ferredoxin (a small (more...)

The final electron acceptor in this electron-transport chain is the second photosystem, photosystem I, which accepts an electron into the electron-deficient hole created by light in the chlorophyll molecule in its reaction center. Each electron that enters photosystem I is finally boosted to a very high-energy level that allows it to be passed to the iron-sulfur center in ferredoxin and then to NADP⁺ to generate NADPH (Figure 14-47).

Figure 14-47

Changes in redox potential during photosynthesis. The redox potential for each molecule is indicated by its position along the vertical axis. In photosystem II, the excited reaction center chlorophyll has a redox potential high enough to withdraw electrons (more...)

The scheme for photosynthesis just discussed is known as the Z scheme. By means of its two electron-energizing steps, one catalyzed by each photosystem, an electron is passed from water, which normally holds on to its electrons very tightly (redox potential = +820 mV), to NADPH, which normally holds on to its electrons loosely (redox potential = -320 mV). There is not enough energy in a single quantum of visible light to energize an electron all the way from the bottom of photosystem II to the top of photosystem I, which is presumably the energy change required to pass an electron efficiently from water to NADP⁺. The use of two separate photosystems in series means that the energy from two quanta of light is available for this purpose. In addition, there is enough energy left over to enable the electron-transport chain that links the two photosystems to pump H⁺ across the thylakoid membrane (or the plasma membrane of cyanobacteria), so that the ATP synthase can harness some of the light-derived energy for ATP production.

Chloroplasts Can Make ATP by Cyclic Photophosphorylation Without Making NADPH

In the noncyclic photophosphorylation scheme just discussed, high-energy electrons leaving photosystem II are harnessed to generate ATP and are passed on to photosystem I to drive the production of NADPH. This produces slightly more than 1 molecule of ATP for every pair of electrons that passes from H₂O to NADP⁺ to generate a molecule of NADPH. But 1.5 molecules of ATP per NADPH are needed for carbon fixation (see Figure 14-39). To produce extra ATP, the chloroplasts in some species of plants can switch photosystem I into a cyclic mode so that it produces ATP instead of NADPH. In this process, called cyclic photophosphorylation, the high-energy electrons from photosystem I are transferred to the cytochrome b₆-f complex rather than being passed on to NADP⁺. From the b₆-f complex, the electrons are passed back to photosystem I at a low energy. The only net result, besides the conversion of some light energy to heat, is that H⁺ is pumped across the thylakoid membrane by the b₆-f complex as electrons pass through it, thereby increasing the electrochemical proton gradient that drives the ATP synthase. (This is analogous to the right side of the diagram for purple nonsulfur bacteria in Figure 14-71, below.)

To summarize, cyclic photophosphorylation involves only photosystem I, and it produces ATP without the formation of either NADPH or O₂. The relative activities of cyclic and noncyclic electron flows can be regulated by the cell to determine how much light energy is converted into reducing power (NADPH) and how much into high-energy phosphate bonds (ATP).

Photosystems I and II Have Related Structures, and Also Resemble Bacterial Photosystems

The mechanisms of fundamental cell processes such as DNA replication or respiration generally turn out to be the same in eucaryotic cells and in bacteria, even though the number of protein components involved is considerably greater in eucaryotes. Eucaryotes evolved from procaryotes, and the additional proteins presumably were selected for during evolution because they provided an extra degree of efficiency and/or regulation that was useful to the cell.

Photosystems provide a clear example of this type of evolution. Photosystem II, for example, is formed from more than 25 different protein subunits, creating a large assembly in the thylakoid membrane with a mass of about 1 million daltons. The atomic structures of the eucaryotic photosystems are being revealed by a combination of electron and x-ray crystallography. The task is difficult because the complexes are large and embedded in the lipid bilayer. Nevertheless, as illustrated in Figure 14-48, the close relationship of photosystem I, photosystem II, and the photochemical reaction center of purple bacteria has been clearly demonstrated from these atomic-level analyses.

Figure 14-48

Three types of photosynthetic reaction centers compared. Pigments involved in light harvesting are colored green; those involved in the central photochemical events are colored red. (A) The photochemical reaction center of purple bacteria, whose detailed (more...)

The Proton-Motive Force Is the Same in Mitochondria and Chloroplasts

The presence of the thylakoid space separates a chloroplast into three rather than the two internal compartments of a mitochondrion. The net effect of H⁺ translocation in the two organelles is, however, similar. As illustrated in Figure 14-49, in chloroplasts, H⁺ is pumped out of the stroma (pH 8) into the thylakoid space (pH ~5), creating a gradient of 3–3.5 pH units. This represents a proton-motive force of about 200 mV across the thylakoid membrane, and it drives ATP synthesis by the ATP synthase embedded in this membrane. The force is the same as that across the inner mitochondrial membrane, but nearly all of it is contributed by the pH gradient rather than by a membrane potential, unlike the case in mitochondria.

Figure 14-49

A comparison of the flow of H⁺ and the orientation of the ATP synthase in mitochondria and chloroplasts. Those compartments with similar pH values have been colored the same. The proton-motive force across the thylakoid membrane consists almost entirely (more...)

Like the stroma, the mitochondrial matrix has a pH of about 8. This is created by pumping H⁺ out of the mitochondrion into the cytosol (pH ~7) rather than into an interior space in the organelle. Thus, the pH gradient is relatively small, and most of the proton-motive force across the inner mitochondrial membrane is instead caused by the resulting membrane potential (see Figure 14-13).

For both mitochondria and chloroplasts, the catalytic site of the ATP synthase is at a pH of about 8 and is located in a large organelle compartment (matrix or stroma) that is packed full of soluble enzymes. Consequently, it is here that all of the organelle's ATP is made (see Figure 14-49).

Carrier Proteins in the Chloroplast Inner Membrane Control Metabolite Exchange with the Cytosol

If chloroplasts are isolated in a way that leaves their inner membrane intact, this membrane can be shown to have a selective permeability, reflecting the presence of specific carrier proteins. Most notably, much of the glyceraldehyde 3-phosphate produced by CO₂ fixation in the chloroplast stroma is transported out of the chloroplast by an efficient antiport system that exchanges three-carbon sugar phosphates for an inward flux of inorganic phosphate.

Glyceraldehyde 3-phosphate normally provides the cytosol with an abundant source of carbohydrate, which is used by the cell as the starting point for many other biosyntheses—including the production of sucrose for export. But this is not all that this molecule provides. Once the glyceraldehyde 3-phosphate reaches the cytosol, it is readily converted (by part of the glycolytic pathway) to 1,3-phosphoglycerate and then 3-phosphoglycerate (see p. 97), generating one molecule of ATP and one of NADH. (A similar two-step reaction, but working in reverse, forms glyceraldehyde 3-phosphate in the carbon-fixation cycle; see Figure 14-39.) As a result, the export of glyceraldehyde 3-phosphate from the chloroplast provides not only the main source of fixed carbon to the rest of the cell, but also the reducing power and ATP needed for metabolism outside the chloroplast.

Chloroplasts Also Perform Other Crucial Biosyntheses

The chloroplast performs many biosyntheses in addition to photosynthesis. All of the cell's fatty acids and a number of amino acids, for example, are made by enzymes in the chloroplast stroma. Similarly, the reducing power of light-activated electrons drives the reduction of nitrite (NO₂^-) to ammonia (NH₃) in the chloroplast; this ammonia provides the plant with nitrogen for the synthesis of amino acids and nucleotides. The metabolic importance of the chloroplast for plants and algae therefore extends far beyond its role in photosynthesis.

Summary

Chloroplasts and photosynthetic bacteria obtain high-energy electrons by means of photosystems that capture the electrons that are excited when sunlight is absorbed by chlorophyll molecules. Photosystems are composed of an antenna complex that funnels energy to a photochemical reaction center, where a precisely ordered complex of proteins and pigments allows the energy of an excited electron in chlorophyll to be captured by electron carriers. The best-understood photochemical reaction center is that of purple photosynthetic bacteria, which contain only a single photosystem. In contrast, there are two distinct photosystems in chloroplasts and cyanobacteria. The two photosystems are normally linked in series, and they transfer electrons from water to NADP⁺ to form NADPH, with the concomitant production of a transmembrane electrochemical proton gradient. In these linked photosystems, molecular oxygen (O₂) is generated as a by-product of removing four low-energy electrons from two specifically positioned water molecules.

Compared with mitochondria, chloroplasts have an additional internal membrane (the thylakoid membrane) and a third internal space (the thylakoid space). All electron-transport processes occur in the thylakoid membrane: to make ATP, H⁺ is pumped into the thylakoid space, and a backflow of H⁺ through an ATP synthase then produces the ATP in the chloroplast stroma. This ATP is used in conjunction with the NADPH made by photosynthesis to drive a large number of biosynthetic reactions in the chloroplast stroma, including the all-important carbon-fixation cycle, which creates carbohydrate from CO₂. Along with some other important chloroplast products, this carbohydrate is exported to the cell cytosol, where—as glyceraldehyde 3-phosphate—it provides organic carbon, ATP, and reducing power to the rest of the cell.