Concept preview

What you will be able to do

1. 01Describe where the light-dependent reactions occur inside the chloroplast and why that location matters.
2. 02Explain how chlorophyll and the two photosystems capture light energy and excite electrons.
3. 03Explain the splitting of water (photolysis) and why oxygen is released.
4. 04Describe how the electron transport chain makes ATP (photophosphorylation) and how NADPH is formed.

Photosynthesis is the process by which green plants, algae and some bacteria capture the energy in sunlight and lock it away in the chemical bonds of sugar. The whole reaction is often summarised as $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$, but that tidy equation hides two very different stages happening in two different places inside the chloroplast. The first stage, the light-dependent reactions, takes place across the thylakoid membranes and can only run while light is falling on the leaf. The second stage, the Calvin cycle (the light-independent reactions), takes place in the stroma and uses the products of the first stage to build sugar. This topic follows the light-dependent reactions step by step: how light energy is captured, how water is split, and how the cell banks that energy as ATP and NADPH.