Key Concepts of Section 12.7

• In the single photosystem of purple bacteria, cyclic electron flow from light-excited special-pair chlorophyll a molecules in the reaction center generates a proton-motive force, which is used mainly to power ATP synthesis by the F₀F₁ complex in the plasma membrane (see Figure 12-43).

• Plants contain two photosystems, PSI and PSII, which have different functions and are physically separated in the thylakoid membrane. PSII converts H₂O into O₂, and PSI reduces NADP⁺ to NADPH. Cyanobacteria have two analogous photosystems.

• In chloroplasts, light energy absorbed by light-harvesting complexes (LHCs) is transferred to chlorophyll a molecules in the reaction centers (P₆₈₀ in PSII and P₇₀₀ in PSI).

• Electrons flow through PSII via the same carriers that are present in the bacterial photosystem. In contrast to the bacterial system, photochemically oxidized P₆₈₀⁺ in PSII is regenerated to P₆₈₀ by electrons derived from the formation of O₂ from H₂O (see Figure 12-44, left).

• In linear electron flow, photochemically oxidized P₇₀₀⁺ in PSI is reduced, regenerating P₇₀₀, by electrons transferred from PSII via the cytochrome bf complex and soluble plastocyanin. Electrons released from P₇₀₀ following excitation of PSI are transported via several carriers ultimately to NADP⁺, generating NADPH (see Figure 12-44, right).

• The absorption of light by pigments in the chloroplast can generate reactive oxygen species (ROS), including singlet oxygen, ¹O₂, and hydrogen peroxide, H₂O₂. In small amounts, they are used as intracellular signaling molecules; in larger amounts, they can be toxic. Small-molecule scavengers, such as carotenoids, and antioxidant enzymes help to protect against ROS-induced damage; however, singlet oxygen damage to the D1 subunit of PSII still occurs, causing photoinhibition. An HSP70 chaperone helps PSII recover from the damage.

• In contrast to linear electron flow, which requires both PSII and PSI, cyclic electron flow in plants involves only PSI. In this pathway, neither NADPH nor O₂ is formed, although a proton-motive force is generated.

• Reversible phosphorylation and dephosphorylation of the light-harvesting complex II (LHCII) control the functional organization of the photosynthetic apparatus in thylakoid membranes. State 1 favors linear electron flow, whereas state 2 favors cyclic electron flow (see Figure 12-46). Very large super-supercomplexes appear to be involved in cyclic electron flow.