We now shift our attention to photosynthesis, the second key process for synthesizing ATP. In plants, photosynthesis occurs in chloroplasts, large organelles found mainly in leaf cells. During photosynthesis, chloroplasts capture the energy of sunlight, convert it into chemical energy in the form of ATP and NADPH, and then use this energy to make complex carbohydrates out of carbon dioxide and water. The principal carbohydrates produced are polymers of hexose (six-carbon) sugars: sucrose, a glucose-fructose disaccharide (see Figure 2-19), and starch, a mixture of two types of large, insoluble glucose polymers called amylose and amylopectin. Starch is the primary storage carbohydrate in plants (Figure 12-36). Starch is synthesized and stored in the chloroplast. Sucrose is synthesized in the leaf cytosol from three-carbon precursors generated in the chloroplast; it is transported to non-photosynthetic (nongreen) plant tissues (e.g., roots and seeds), which metabolize it for energy by the pathways described in the previous sections.

Photosynthesis in plants, as well as in eukaryotic single-celled algae and in several photosynthetic bacteria (e.g., the cyanobacteria and prochlorophytes), also generates oxygen. The overall reaction of oxygen-generating photosynthesis,

6 CO₂ +6 H₂O → 6 O₂ + C₆H₁₂O₆

is the reverse of the overall reaction by which carbohydrates are oxidized to CO₂ and H₂O. In effect, photosynthesis in chloroplasts produces energy-rich sugars that are broken down and harvested for energy by mitochondria using oxidative phosphorylation.

Although green and purple bacteria also carry out photosynthesis, they use a process that does not generate oxygen. As discussed in Section 12.7, detailed analysis of the photosynthetic system in these bacteria has helped elucidate the first stages in the more common process of oxygen-generating photosynthesis. In this section, we provide an overview of the stages in oxygen-generating photosynthesis and introduce the main molecular components of the process, including the chlorophylls, the principal light-absorbing pigments.