In Photosynthesis the light reactions were a box: light and water in, oxygen, ATP and NADPH out. Now we open the box. It is a small electrical circuit built into the thylakoid membrane, powered by light, and it uses the same trick as mitochondria: an H⁺ gradient that runs ATP synthase. Free energy explains why light is needed: water is a poor electron donor, and pulling electrons off it and lifting them to NADPH is steeply uphill.
- Photosystem II absorbs light; its reaction center (P680) passes an excited electron to the chain. It replaces that electron by splitting water: 2 H₂O → 4 H⁺ + 4 e⁻ + O₂. That is where the oxygen we breathe comes from.
- The electron travels through plastoquinone and the cytochrome complex, which uses the energy to pump H⁺ into the thylakoid space, then through plastocyanin.
- Photosystem I (P700) absorbs light again and re-energizes the electron.
- Ferredoxin and NADP⁺ reductase hand two electrons to NADP⁺, making NADPH in the stroma.
- H⁺ built up in the thylakoid space flows back to the stroma through ATP synthase, which makes ATP (chemiosmosis).
Key idea
Electrons flow water → photosystem II → electron transport chain → photosystem I → NADP⁺. The products are O₂ (from water), NADPH, and ATP made by H⁺ flowing through ATP synthase.
Why it works
Why two photosystems? One photon does not carry enough energy to lift an electron all the way from water to NADPH. Photosystem II gives the first boost, the chain spends some of that energy pumping H⁺, and photosystem I gives a second boost. Splitting the climb into two steps lets cells use two photons of visible light per electron.
Worked examples
Worked example: Counting electrons
A chloroplast releases 3 molecules of O₂. How many electrons came from water, and how many NADPH could they make?
- Each O₂ comes from 2 H₂O, which give up 4 electrons.
- $3 \times 4 = 12$ electrons.
- Each NADPH takes 2 electrons: $12 / 2 = 6$ NADPH.
Answer: 12 electrons and 6 NADPH.
Worked example: Blocking the chain
A chemical blocks the cytochrome complex. What happens to O₂, NADPH and ATP production?
- Electrons from photosystem II cannot reach photosystem I, so NADPH stops.
- The cytochrome complex no longer pumps H⁺, so the gradient fades and ATP slows.
- Photosystem II soon has nowhere to send electrons, so water splitting and O₂ release stop too.
Answer: All three fall. The chain is one line; block it and everything after, and soon before, stops.
Common mistake
Thinking photosystem I acts first because of its number. Electrons go from photosystem II to photosystem I. The numbers reflect the order in which they were discovered. A memory check: water is split at the start of the line, and that is photosystem II.
Photosystem I can also send electrons back to the cytochrome complex in a loop (cyclic electron flow). That makes extra ATP but no NADPH and no O₂, and helps balance what the Calvin cycle needs.