By David Chandler Updated Mar 24, 2022
During the light phase of photosynthesis, water is split to release free oxygen atoms that pair to form oxygen gas.
The light reactions harvest solar energy and convert it into chemical energy for the dark reactions. Sunlight excites electrons in chlorophyll, which travel through an electron transport chain, creating a proton gradient across the thylakoid membrane. ATP synthase harnesses this gradient to produce ATP, while NADP⁺ is reduced to NADPH. These molecules power carbohydrate synthesis in the Calvin cycle.
Photophosphorylation can occur in two modes. In cyclic photophosphorylation, the electron returns to the photosystem after energizing the chain, producing ATP without forming NADPH. In noncyclic (linear) photophosphorylation, the electron is ultimately transferred to NADP⁺, generating NADPH and requiring a new electron from water. This turnover drives oxygen evolution.
In eukaryotic photosynthetic organisms, the entire process occurs inside chloroplasts. These organelles contain thylakoid membranes that form stacks called grana. The thylakoid membranes house the photosystems and establish the proton gradient necessary for ATP synthesis. While all photosynthetic organisms possess thylakoid membranes, only eukaryotes encapsulate them within chloroplasts.
Photosystems are pigment‑protein complexes embedded in the thylakoid membrane. Chlorophyll a sits at the core of each photosystem and captures light energy, energizing electrons. The chlorophyll also hosts a water‑splitting complex that replenishes the lost electron by oxidizing water, releasing oxygen as a byproduct.
When the water‑splitting complex oxidizes water, it splits a molecule of H₂O into two protons and one electron. Two such electrons combine, and the liberated oxygen atoms from two water molecules form one molecule of O₂. Four electrons, therefore, must be transferred to generate a single O₂ molecule. The resulting protons contribute to the proton gradient across the thylakoid membrane, while ATP and NADPH are produced for the Calvin cycle.
