Chapter 11: Phototrophic Metabolism: Photosynthesis

Photosynthesis is divided into two primary phases: energy transduction and carbon assimilation. The initial phase, energy transduction, involves light harvesting within the thylakoid membranes of eukaryotic chloroplasts, where pigments such as chlorophyll and various accessory pigments absorb photons and pass the energy via resonance energy transfer to the reaction centers of Photosystem II (P680) and Photosystem I (P700). Photosystem II uses absorbed light energy to oxidize water (a process called water photolysis), which releases molecular oxygen and supplies electrons to the electron transport system (ETS). These high-energy electrons flow exergonically through carriers like the cytochrome b6/f complex before reaching Photosystem I. This electron movement is coupled to the unidirectional pumping of protons from the stroma into the thylakoid lumen, generating an electrochemical proton gradient or proton motive force, which is predominantly driven by the pH difference across the membrane. The ATP synthase complex (CFoCF1) utilizes the energy stored in this gradient to synthesize ATP in the stroma via photophosphorylation. Photosystem I re-excites the electrons, ultimately leading to the reduction of NADP+ to NADPH, the primary source of reducing power for anabolic pathways. To meet varying cellular energy demands, particularly when extra ATP is needed, organisms can employ cyclic photophosphorylation, diverting electrons from ferredoxin back to the cytochrome complex instead of reducing NADP+. The second phase, carbon assimilation, occurs in the stroma and involves the Calvin cycle, which uses the energy (ATP) and reducing power (NADPH) to fix atmospheric carbon dioxide onto the acceptor molecule ribulose-1,5-bisphosphate (RuBP), a reaction catalyzed by the highly abundant enzyme rubisco, producing triose phosphates such as glyceraldehyde-3-phosphate (G3P). G3P molecules are then used for carbohydrate synthesis, with starch serving as the primary storage form in the stroma, and sucrose, the major transport carbohydrate, being synthesized in the cytosol. A major challenge to photosynthetic efficiency arises because rubisco can also act as an oxygenase, especially in hot, dry conditions, initiating photorespiration. This wasteful process diverts carbon from the Calvin cycle and involves a salvage pathway spanning the chloroplast, peroxisome, and mitochondrion. To minimize photorespiration, specialized plants have developed adaptive strategies: C4 plants concentrate carbon dioxide in specialized bundle sheath cells using the Hatch-Slack cycle, and CAM plants temporally separate CO2 fixation (at night) and release (during the day) to conserve water.