The light reaction is how sunlight is converted into chemical energy. It involves several protein complexes.

Plants and algae contain two separate photosynthesizing complexes: Photosystem I and II. The two photosystems contain different pigment molecules that absorb specific wavelengths of light. These pigment molecules act like a funnel; the pigments absorb sunlight and transfer the excitation energy to the neighboring pigment molecule until it reaches the bottom of the funnel, the reaction center.

The reaction center consists of several molecules that enable the conversion of the excitation energy into an electron flow. Electrons are propelled out of one of the reaction center chlorophyll molecules and are quickly absorbed by nearby molecules.

In photosystem II, this electron is used to reduce a carrier molecule called plastoquinone. The plastoquinone takes up two electrons and two protons and transfers the electrons to the next complex in the electron transport chain.

Electron transport chain presented as four molecule structures aligned in one row, embedded inside the chloroplast membrane. The first molecule on the left - Photosystem II - has two reaction centers inside. Next molecule in a row - plastoquinone - has cytochrome b6f enzyme inside, and plastocyanin attached to its lower right part. Third molecule in a row is named photosystem I and the last one ATP synthase. On both photosystem I and II, two yellow beams are falling from the top of the image. The space above the row of molecules is called ‘chloroplast stroma’, and the lower part is called ‘thylakoid lumen’. Two small red spheres are broken down in the thylakoid lumen and one sphere, called oxygen, is transported inside the photosystem II. The electrons are shown as very bright blue spheres, which appear inside the photosystem II, plastoquinone and photosystem I. Above the ATP synthase molecule, two reactions are written. One shows the conversion of ADP and phosphate group into ATP, the other shows the conversion of NADP plus with hydrogen atom and two electrons into one NADPH molecule.

Figure 1: Light reaction.

The oxygen-evolving complex is located right next to the two reaction center chlorophylls of photosystem II. This structure catalyzes the water-splitting reaction. The water-splitting reaction is essential for replacing the electron of the chlorophyll and keeps the electron transport chain running. The oxygen-evolving complex breaks water into molecular oxygen, protons and electrons. The protons diffuse into the thylakoid lumen, where they accumulate. For every two water molecules that are split, one molecule of molecular oxygen is produced. Oxygen is basically the waste product of photosynthesis.

This is the chemical equation for the light reaction:

2 H2O + 2 NADP+ + nADP + nPi → O2 + 2 NADPH + 2 H+ + nATP