1. Light Absorption: Light energy is absorbed by chlorophyll and other pigments in the photosystems (PSI and PSII) within the thylakoid membranes of chloroplasts.
2. Excitation of Electrons: The absorbed light energy excites electrons in the chlorophyll molecules, boosting them to a higher energy level.
3. Electron Transport Chain: The energized electrons are passed along an electron transport chain within the thylakoid membrane. This chain consists of various protein complexes that accept and donate electrons, releasing energy with each transfer.
4. Proton Gradient Formation: As electrons move along the electron transport chain, the energy released is used to pump protons (H+) from the stroma into the thylakoid lumen, creating a proton gradient across the membrane.
5. ATP Synthesis: The proton gradient created across the thylakoid membrane drives ATP synthase, an enzyme that uses the potential energy of the gradient to generate ATP from ADP and inorganic phosphate (Pi).
6. Water Splitting: In PSII, the energy from light is used to split water molecules into oxygen, protons (H+), and electrons. The oxygen is released as a byproduct, while the protons contribute to the proton gradient and the electrons replace those lost by chlorophyll in PSII.
7. NADPH Formation: In PSI, the excited electrons are used to reduce NADP+ to NADPH. NADPH is an electron carrier that will be used in the light-independent reactions to power the synthesis of sugars.
Summary:
The light-dependent reactions harness light energy to generate ATP and NADPH, which are essential for the light-independent reactions (Calvin cycle) to produce sugars. This process also releases oxygen as a byproduct.
