The primary reaction in photosynthesis is the light-dependent reaction. This is where light energy is captured and converted into chemical energy in the form of ATP and NADPH.

Here's a breakdown:

1. Light Absorption: Chlorophyll and other pigments in the thylakoid membrane of chloroplasts absorb light energy.

2. Electron Excitation: This light energy excites electrons within the chlorophyll molecules.

3. Electron Transport Chain: The excited electrons are passed along an electron transport chain, releasing energy.

4. ATP Production: This energy is used to generate ATP (adenosine triphosphate), the energy currency of cells.

5. NADPH Production: The electrons also contribute to the reduction of NADP+ to NADPH, a reducing agent used in the next stage of photosynthesis.

In essence, the light-dependent reaction is the process of converting light energy into chemical energy that can be used by the plant.