Chapter Summary

• In the process of photosynthesis, the energy of sunlight is captured and used to convert CO₂ into more complex carbon-containing compounds. See Animation 10.1

• Isotope-labeling experiments demonstrated that water is the source of the oxygen atoms in the O₂ produced by photosynthesis. Review Investigating Life: What Is the Chemistry of Photosynthesis, and How Will Increasing CO₂ in the Atmosphere Affect It?

• Plants, algae, and cyanobacteria live in aerobic environments and carry out oxygenic photosynthesis: the conversion of CO₂ and H₂O into carbohydrates and release of O₂.

• Some bacteria that live in anaerobic environments carry out anoxygenic photosynthesis, in which energy from the sun is used to fix CO₂ without the use of H₂O and the production of O₂.

• The light reactions of photosynthesis convert light energy into chemical energy in the form of ATP and NADPH. Review Focus: Key Figure 10.2

• The light-independent reactions do not use light directly but instead use ATP and NADPH to reduce CO₂, forming carbohydrates.

• Light is a form of electromagnetic radiation. It is emitted in particle-like packets called photons but has wavelike properties.

• Molecules that absorb light in the visible spectrum are called pigments. Photosynthetic organisms have several pigments, most notably chlorophylls.

• Absorption of a photon puts an electron of a pigment molecule in an excited state that has more energy than its ground state.

• Each pigment has a characteristic absorption spectrum. An action spectrum reflects the rate of photosynthesis carried out by a photosynthetic organism at a given wavelength of light. Review Figure 10.4

• The pigments in photosynthetic organisms are arranged into light-harvesting complexes that absorb energy from light and funnel this energy to chlorophyll a molecules in the reaction center of the photosystem. Chlorophyll can act as a reducing agent, transferring excited electrons to other molecules. Review Figure 10.6

• Noncyclic electron transport uses photosystems I and II to produce ATP, NADPH, and O₂. Cyclic electron transport uses only photosystem I and produces only ATP. Both systems generate ATP via the electron transport system. Review Figures 10.7, 10.8

• Chemiosmosis is the mechanism of ATP production in photophosphorylation. Review Figure 10.9, Animation 10.2

• The Calvin cycle makes carbohydrates from CO₂. The cycle consists of three processes: fixation of CO₂, reduction and carbohydrate production, and regeneration of RuBP. See Animation 10.3

• RuBP is the initial CO₂ acceptor, and 3PG is the first stable product of CO₂ fixation. The enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction of CO₂ and RuBP to form 3PG. Review Figures 10.10, 10.11

• ATP and NADPH formed by the light reactions are used in the reduction of 3PG to form glyceraldehyde 3-phosphate (G3P). Review Figure 10.12, Activity 10.1

• Light stimulates enzymes in the Calvin cycle, further integrating the light-dependent and light-independent pathways. Review Figure 10.13

• Rubisco can catalyze a reaction between O₂ and RuBP in addition to the reaction between CO₂ and RuBP. At high temperatures and low CO₂ concentrations, the oxygenase function of rubisco is favored over its carboxylase function.

• The oxygenase reaction catalyzed by rubisco significantly reduces the efficiency of photosynthesis. The subsequent reactions of photorespiration recover some of the fixed carbon that otherwise would be lost. Review Figure 10.14

• In C4 plants, CO₂ reacts with phosphoenolpyruvate (PEP) to form a four-carbon intermediate in mesophyll cells. The four-carbon product releases its CO₂ to rubisco in the bundle sheath cells in the interior of the leaf. Review Figures 10.15, 10.16, Activity 10.2

• Crassulacean acid metabolism (CAM) plants operate much like C₄ plants, but their initial CO₂ fixation by PEP carboxylase is temporally separated from the Calvin cycle, rather than spatially separated as in C₄ plants.

• Photosynthesis and cellular respiration are linked through the Calvin cycle, the citric acid cycle, and glycolysis. Review Figure 10.17

• To survive, a plant must photosynthesize more than it respires.

• Photosynthesis uses only a small portion of the energy of sunlight. Review Figure 10.18