Photosynthesis: the light reactions

Photosynthesis turns light energy into chemical energy: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O. In plants it happens in chloroplasts: the light reactions in the thylakoid membranes, and carbon fixation in the stroma.

There are two photosystems. PSII absorbs best at 680 nm (its reaction centre is P680) and has about 30 chlorophylls and four manganese ions. PSI absorbs best at 700 nm (P700) and has about 60 chlorophylls and three iron-sulphur clusters.

Non-cyclic electron flow, step by step:

• Light excites an electron in PSII, which passes to pheophytin, then plastoquinone, then cytochrome b₆f, then plastocyanin, and so to PSI.
• Light excites it again in PSI; it passes via ferredoxin to NADP⁺, which ferredoxin-NADP reductase turns into NADPH.
• PSII gets back its lost electrons from water, split by the oxygen-evolving complex: 2H₂O → 4H⁺ + 4 electrons + O₂. That is where the oxygen we breathe comes from.

Protons pile up inside the thylakoid (the lumen) and flow out through ATP synthase (its CF₀ part is the channel, its CF₁ head makes ATP). This is non-cyclic photophosphorylation: it makes ATP, NADPH and O₂.

In cyclic flow, only PSI works (in the stroma lamellae). The excited electron goes back to PSI through phylloquinone, cytochrome b₆f and plastocyanin, pumping protons on the way. This makes only ATP, with no NADPH and no oxygen.