The light reaction

The light reaction (also called the photochemical phase) includes: absorbing light, splitting water, releasing oxygen, and making the energy carriers ATP and NADPH.

Pigment molecules are grouped into light harvesting complexes (like antennae) in two photosystems: Photosystem I (PS I) and Photosystem II (PS II). They are numbered by the order of their discovery, not the order in which they work (PS II actually works first). Each has one special chlorophyll a at its reaction centre: P680 in PS II and P700 in PS I, named after the wavelength of light (in nm) they absorb best.

Follow an electron:

• In PS II, P680 absorbs light of 680 nm. Its electrons get excited, jump to an electron acceptor, and are passed down a chain of carriers (cytochromes), going "downhill" in energy, to PS I.
• In PS I, P700 is excited by light of 700 nm. Its electrons jump to another acceptor and then go downhill to NADP⁺, which becomes NADPH + H⁺.
• Drawn on a scale of energy (the redox scale), this path looks like the letter Z, so it is called the Z scheme.

PS II has given away its electrons, so how does it get new ones? By splitting water: 2H₂O → 4H⁺ + O₂ + 4e⁻. This happens on the inner side of the thylakoid membrane (facing the lumen). The water gives electrons to PS II, protons to the lumen, and oxygen, which is released into the air.