Photosynthesis ETC

Compare linear and cyclic electron flow. Green dots indicate electron transport, pink dots indicate proton return through ATP synthase. Select PSII, cytochrome b₆f, PSI or ATP synthase in the diagram to inspect its role; keyboard users can Tab and press Enter or Space. Start/Pause, Light and animation speed control the illustration. Reset rewinds and pauses it, keeping your selected settings.

Linear flow: water → PSII → plastoquinone (PQ) → cytochrome b₆f → plastocyanin (PC) → PSI → ferredoxin (Fd) → FNR → NADPH. PSII and PSI both need light. Water oxidation releases O₂ and lumen protons; the b₆f Q cycle transfers further protons from the stroma to the lumen. ATP synthase uses their return to the stroma, not electron transport.

Cyclic flow follows the simplified ferredoxin-to-PQ route through b₆f and PSI. It supports proton motive force and ATP formation without net NADPH or O₂. PSII is shown inactive. The alternative NDH route, with additional proton pumping, is not modeled.

The optional ideal balance uses four electrons: linear flow consumes two H₂O and eight absorbed photons, forms one O₂ and two NADPH, and adds twelve H⁺ to the lumen (four from water, eight via the Q cycle). This cyclic route uses four photons and adds eight H⁺. ATP equivalents use the chloroplast c₁₄-ring assumption: fourteen translocated H⁺ per three ATP, giving about 2.57 and 1.71 ATP respectively. These are stoichiometric averages, not fractional individual molecules or measured yields.

This is an educational pathway diagram, not a molecular dynamics or kinetic simulation. Dots, timing and spatial positions are illustrative, not molecule counts, redox energies, rates or pH. Light off freezes the illustration; it does not simulate dark relaxation of an existing gradient. Regulation, losses, ion coupling, carbon fixation and actual light intensity are omitted. No data leave your browser or persist after leaving the tool.