Here's how it works:
1. Light Absorption: Chlorophyll and other pigments in the photosystems of chloroplasts absorb light energy.
2. Electron Excitation: This absorbed light energy excites electrons within the pigment molecules, raising them to a higher energy level.
3. Electron Transport Chain: These energized electrons are passed along an electron transport chain, losing energy as they move.
4. Energy Conversion: Some of this energy is used to pump protons (H+) across the thylakoid membrane, creating a proton gradient. This gradient drives the production of ATP, a molecule that stores chemical energy.
5. NADPH Formation: Another part of the energy from the electron transport chain is used to reduce NADP+ to NADPH. This reaction requires the enzyme NADP reductase.
So, the light energy initially absorbed by chlorophyll is used to create a proton gradient, which powers ATP production, and to ultimately reduce NADP+ to NADPH.
NADPH is a crucial molecule in the Calvin cycle (dark reactions), where it provides the reducing power necessary to convert carbon dioxide into sugar.
