Photosynthesis: Light & Calvin Cycle

Capturing sunlight to build sugars

Photosynthesis occurs in two stages within chloroplasts. Light Reactions (thylakoid membranes): Photosystem II absorbs light at 680 nm and splits water (photolysis), releasing O₂ and electrons. Electrons flow through the ETC (plastoquinone, cytochrome b6f, plastocyanin) to Photosystem I (700 nm), reducing NADP⁺ to NADPH. The proton gradient drives ATP synthase (photophosphorylation). The Calvin Cycle (stroma): RuBisCO fixes CO₂ onto RuBP (5C) to form 3-PGA (3C). ATP and NADPH from light reactions reduce 3-PGA to G3P. Every 3 turns fix 3 CO₂, producing 1 net G3P. Two G3P form glucose. Limiting factors: light intensity, CO₂ concentration, and temperature each limit the rate.