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8.1 Oxidation of Glucose and Fatty Acids to CO2

8.2 Electron Transport and Generation of the Proton-Motive Force

8.3 Harnessing the Proton-Motive Force for Energy-Requiring Processes

8.4 Photosynthetic Stages and Light-Absorbing Pigments

8.5 Molecular Analysis of Photosystems

8.6 CO2 Metabolism During Photosynthesis plants and eukaryotic single-celled algae, photosynthesis occurs in chloroplasts. Several prokaryotes also carry out photosynthesis on their plasma membrane or its invaginations by a mechanism similar to that in chloroplasts. The oxygen generated during photosynthesis is the source of virtually all the oxygen in the air, and the carbohydrates produced are the ultimate source of energy for virtually all nonphotosynthetic organisms. Bacteria living in deep ocean vents, where there is no sunlight, disprove the popular view that sunlight is the ultimate source of energy for all organisms on earth. These bacteria obtain energy for converting carbon dioxide into carbohydrates and other cellular constituents by oxidation of reduced inorganic compounds in dissolved vent gas.

At first glance, photosynthesis and aerobic oxidation appear to have little in common. However, a revolutionary discovery in cell biology is that bacteria, mitochondria, and chloroplasts all use the same basic mechanism, called chemiosmosis (or chemiosmotic coupling), to generate ATP from ADP and P(. In chemiosmosis, a proton (H+) concentration gradient and an electric potential (voltage gradient) across the membrane, collectively termed the proton-motive force, drive an energy-requiring process such as ATP synthesis (Figure 8-1, bottom).

Chemiosmosis can occur only in sealed, membrane-limited compartments that are impermeable to H + . The proton-motive force is generated by the stepwise movement of electrons from higher to lower energy states via membrane-bound electron carriers. In mitochondria and nonphotosyn-thetic bacterial cells, electrons from NADH (produced during the metabolism of sugars, fatty acids, and other substances) are transferred to O2, the ultimate electron acceptor. In the thylakoid membrane of chloroplasts, energy absorbed from light strips electrons from water (forming O2) and pow ers their movement to other electron carriers, particularly NADP+; eventually these electrons are donated to CO2 to synthesize carbohydrates. All these systems, however, contain some similar carriers that couple electron transport to the pumping of protons across the membrane—always from the cytosolic face to the exoplasmic face of the membrane— thereby generating the proton-motive force (Figure 8-1, top). Invariably, the cytosolic face has a negative electric potential relative to the exoplasmic face.

Moreover, mitochondria, chloroplasts, and bacteria utilize essentially the same kind of membrane protein, the F0F1 complex, to synthesize ATP. The F0F1 complex, now commonly called ATP synthase, is a member of the F class of ATP-powered proton pumps (see Figure 7-6). In all cases, ATP synthase is positioned with the globular F1 domain, which catalyzes ATP synthesis, on the cytosolic face of the membrane, so ATP is always formed on the cytosolic face of the membrane (Figure 8-2). Protons always flow through ATP synthase from the exoplasmic to the cytosolic face of the membrane, driven by a combination of the proton concentration gradient ([H+]exoplasmic > [H+]cytosolic) and the membrane electric potential (exoplasmic face positive with respect to the cytosolic face).

These commonalities between mitochondria, chloro-plasts, and bacteria undoubtedly have an evolutionary origin. In bacteria both photosynthesis and oxidative phos-phorylation occur on the plasma membrane. Analysis of the sequences and transcription of mitochondrial and chloroplast DNAs (Chapters 10 and 11) has given rise to the popular hypothesis that these organelles arose early in the evolution of eukaryotic cells by endocytosis of bacteria capable of oxidative phosphorylation or photosynthesis, respectively (Figure 8-3). According to this endosymbiont

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