Light Reaction

In the light reaction, solar energy trapped by chlorophyll is stored chemically as ATP and as the reducing agent NADPH2, with oxygen released as water is split. When a chlorophyll molecule absorbs a photon carrying at least the minimum quantum of energy needed, one of its electrons is boosted to a higher energy level, putting the molecule into an energy-rich excited state; once the light source is removed, that electron rapidly falls back to its normal low-energy orbital and the molecule returns to its stable ground state.

The light-absorbing pigments sit in the thylakoid membrane arranged in clusters, each cluster built around a special pair of chlorophyll molecules called P680 or P700 (P for Pigment, and 680/700 for the wavelength, in nanometres, at which each absorbs maximally); these special pairs form the reaction centres, or photocentres, where the actual photochemical reaction takes place. Surrounding each reaction centre are many accessory pigment and chlorophyll molecules, called light-harvesting or antenna molecules, whose job is simply to absorb light energy over a wide area and funnel it, at very high speed, to the reaction centre.

The thylakoid membrane contains two distinct photosystems built this way. Any photon absorbed anywhere in the antenna zone around a P680 centre can hand its energy on to P680 itself; together with all the pigments that funnel energy to it (those absorbing at or below 680 nm), P680 forms Photosystem II (PS-II). In the same way, P700 and its associated pigments (absorbing at or below 700 nm) form Photosystem I (PS-I). According to Arnon, this whole process converts light energy into the chemical energy stored in ATP, while NADPH2 is generated as the hydrogen-carrying reduced product; because it produces ATP, this light-driven process is called photophosphorylation.

Photosystem II carries out the photolysis of water and the release of oxygen. When PS-II absorbs light, electrons are ejected and P680 becomes oxidised; these electrons are ultimately captured by P700 (PS-I). The oxidised P680 regains its lost electrons by splitting water:

4H₂O → 4H⁺ + 4OH⁻

4OH⁻ → 4(OH) + 4e⁻

4OH → 2H₂O + O₂

so that, overall,

4H₂O → 2H₂O + O₂↑ + 4H⁺ + 4e⁻ …