bio

The Calvin-Benson cycle

The loop of chemistry that assembles CO₂ into sugar, running on the energy currency the light-requiring half of photosynthesis just earned. Rubisco attaches each CO₂ to a five-carbon acceptor molecule; the product is rearranged over several steps, some leaving as sugar and the rest regenerating that original acceptor so the loop can continue. Every three CO₂ entering costs nine units of ATP and six of reducing power. Worth remembering: this cycle does not need light, only what light buys — so it keeps turning for a while after dark, as long as there is currency in the till.