Chapter 8: Photosynthesis

Photosynthesis occurs within chloroplasts and involves two interconnected stages: the light-dependent reactions occurring in thylakoid membranes and the light-independent Calvin cycle reactions in the stroma. During light reactions, photons are absorbed by pigment molecules such as chlorophyll a, chlorophyll b, and carotenoids, which funnel absorbed energy to specialized protein complexes called photosystem II and photosystem I. These photosystems contain reaction centers that excite electrons to high energy states, initiating a cascade of electron transfers through an electron transport chain. This electron flow drives the accumulation of protons within the thylakoid space, creating a concentration gradient that powers ATP synthase to generate ATP through chemiosmosis while electrons reduce NADP+ to form NADPH, a high-energy electron carrier. Simultaneously, water molecules are split to replenish electrons and release oxygen as a byproduct. The Calvin cycle, operating in the stroma, uses the ATP and NADPH generated by light reactions to fix carbon dioxide into organic molecules through a series of enzymatic reactions. The enzyme rubisco catalyzes the initial carbon fixation step, converting RuBP and CO₂ into three-phosphoglycerate, which is subsequently reduced to glyceraldehyde-3-phosphate and used to synthesize glucose and regenerate RuBP. The chapter examines evolutionary adaptations to environmental constraints, distinguishing between C₃ plants that perform standard Calvin cycle carbon fixation but lose efficiency through photorespiration, C₄ plants that spatially separate initial carbon fixation from the Calvin cycle to concentrate CO₂ around rubisco, and CAM plants that temporally separate these processes by fixing CO₂ at night to minimize water loss during the day. Together, these processes demonstrate how photosynthesis channels solar radiation into the chemical bonds of carbohydrates that form the foundation of nearly all food webs.