Photosynthesis: Light and Carbon Fixation

Objective: Trace electrons from water through photosystems and carriers.

In oxygenic photosynthesis, light excites electrons in photosystems embedded in thylakoid membranes. At photosystem II, water is oxidized to replace electrons, releasing O₂ and contributing protons. Electrons pass through carriers to photosystem I, where another light input raises their energy before NADP⁺ is reduced to NADPH. The electron pathway helps build a proton gradient used for ATP synthesis. ATP and NADPH are products that supply energy and reducing power to carbon fixation. Light is captured by pigments and transferred through specific molecular interactions; electrons are not made from sunlight.

Objective: Explain how a gradient and ATP synthase connect electron flow to ATP formation.

As light-driven electron transport proceeds in the thylakoid membrane, protons accumulate in the thylakoid lumen relative to the stroma. Water oxidation adds protons to the lumen and electron-transport steps contribute to the gradient. Protons moving back toward the stroma through ATP synthase help drive phosphorylation of ADP to ATP. This chemiosmosis converts energy stored in an electrochemical gradient into chemical energy in ATP. ATP synthase does not create energy, and an intact membrane is needed to maintain the gradient. The exact contribution of different steps varies with pathway and conditions.

Objective: Describe carbon fixation and the roles of ATP and NADPH.

The Calvin cycle in the chloroplast stroma fixes CO₂ into organic carbon. Rubisco catalyzes attachment of CO₂ to RuBP; subsequent reactions use ATP and NADPH to reduce intermediates, producing some G3P that can contribute to sugars and other molecules. Most G3P is used to regenerate RuBP so the cycle can continue. A common bookkeeping summary is that three CO₂ molecules entering the cycle can yield one net G3P while consuming nine ATP and six NADPH, though this is a simplified stoichiometric model. The cycle does not directly use photons, but it depends on products of light reactions and does not automatically run indefinitely in darkness.