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Chapter 12

Photosynthesis

 

Photosynthesis produces organic compounds from inorganic carbon by using the energy of sunlight. These processes are carried out in plants, algae, and various bacteria. In all cases, the photosynthetic reactions may conveniently be divided into two phases, the light reactions and the carbon-fixing reactions.

In the case of eukaryotic organisms, photosynthesis takes place in the chloroplast. This organelle is surrounded by a double membrane and contains a complex internal membrane system, the thylakoid membranes. The two phases of photosynthesis take place in different regions of the chloroplast, with the light reactions being localized to the thylakoid membranes and the carbon-fixing reactions occurring in the stroma.

The light reactions of photosynthesis involve the photosynthetic pigments, the photosynthetic electron transport chain, and the ATP synthesis machinery. Light is absorbed by pigments that are localized into pigment– protein complexes within the thylakoid membrane. This light energy can be transferred from so-called antenna pigments to special pigment–protein complexes, known as reaction centers, where the light energy is converted into chemical products (photo-chemistry). Within the reaction center, a special chlorophyll undergoes oxidation, and an electron is transferred to an electron acceptor during the primary reaction of photosynthesis. Oxygenic photosynthetic organisms contain two reaction centers and two photo-systems, PSII and PSI. The two photosystems are spatially separated: PSII is localized in appressed thylakoids, and PSI is localized in stroma-exposed thylakoids. During noncyclic electron transfer, the two photosystems cooperate in the transfer of electrons from water to NADP+ in a series of redox reactions mediated by both mobile and integral membrane components of the photosynthetic electron transfer chain, located primarily in the thylakoid membranes. During this series of reactions, PSII oxidizes water, producing molecular oxygen. This reaction provides almost all of the oxygen required for aerobic life on our planet.

In addition to O2 and NADPH, the noncyclic electron transfer reactions are coupled to the synthesis of ATP. ATP synthesis is driven by a proton gradient established across the thylakoid membrane. During electron transport, the interior space of the thylakoid membrane, the lumen, becomes acidified as protons are translocated from the stroma during electron transport or released by water oxidation in the thylakoid lumen. Use of this electrochemical gradient as an energy source requires a large protein complex located in the membrane, the ATP synthase. This enzyme is composed of two portions—one intrinsic to the membrane and involved in transporting protons through the membrane, and the second, which is extrinsic, involved in the actual conversion of ADP and Pi into ATP. Estimates of the stoichiometry for ATP synthesis have indicated that four H+ are required per ATP molecule. The actual synthesis of ATP is believed to involve complicated conformational changes in ATP synthase, driven by the movement of protons through the enzyme.

In addition to the noncyclic pathway for ATP synthesis, chloroplasts also synthesize ATP via a cyclic pathway that involves only PSI. This pathway generates a proton gradient that can also be used for the synthesis of ATP, but does not yield O2 or NADPH.

The reduction of CO2 to carbohydrates requires NADPH and ATP, which are synthesized during the light reactions of photosynthesis. Plants employ the C3 photosynthetic pathway (Calvin cycle) to fix CO2, using the key enzyme Rubisco to convert CO2 and RuBP into the C3 product, 3-PGA. The multienzyme Calvin cycle involves three phases—carboxylation, reduction, and regeneration— and requires three ATP and two NADPH molecules per molecule of CO2 fixed. All Calvin cycle reactions are catalyzed by soluble enzymes localized in the chloroplast stroma. Regulation of the Calvin cycle occurs by multiple mechanisms, including changes in ionic strength and pH as well as protein-mediated reactions.

Variants of C3 photosynthesis exist in many plants. In one variation, known as the C4 pathway, plants fix CO2 into a C4 acid in mesophyll cells, and transport this fixed CO2 to anatomically distinct bundle sheath cells, where the CO2 is released and refixed by Rubisco. This sequence of reactions provides a higher concentration of CO2 for Rubisco in the bundle sheath cell and aids in inhibiting the oxygenase activity of Rubisco, a wasteful side-reaction that reduces the efficiency of CO2 fixation in the C3 chloroplasts, where O2 and CO2 compete for the Rubisco active site. In the case of CAM plants, another variant, CO2 is fixed at night into organic acids that are subsequently decarboxylated during the day to provide CO2 for Rubisco. CAM photosynthesis aids in the retention of water in arid environments. Key photosynthetic enzymes in C4 and CAM plants are regulated to ensure the efficient interaction of these CO2-concentrating mechanisms and the Calvin cycle for which they provide substrate.


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