1. Light-Dependent Reactions:
* Photosystem II: Light energy is captured by chlorophyll, causing electrons to become excited and jump to a higher energy level. These excited electrons are then passed along an electron transport chain. This process is an oxidation, as the chlorophyll molecule loses electrons.
* Electron Transport Chain: The electrons travel through a series of protein complexes, losing energy along the way. This energy is used to pump protons (H+) across the thylakoid membrane, creating a proton gradient.
* Photosystem I: The electrons from the electron transport chain are passed to Photosystem I, where they are re-energized by light. These energized electrons are then used to reduce NADP+ to NADPH. This is a reduction, as NADP+ gains electrons.
* Water Splitting: To replace the electrons lost by Photosystem II, water molecules are split. This process releases oxygen as a byproduct, along with protons (H+). This is also an oxidation, as water loses electrons.
2. Light-Independent Reactions (Calvin Cycle):
* Carbon Fixation: Carbon dioxide (CO2) from the atmosphere is incorporated into an organic molecule (RuBP) using the enzyme rubisco.
* Reduction: The newly formed molecules are reduced using NADPH, which acts as an electron donor. This is another reduction reaction.
* Regeneration: The molecules are then rearranged and used to regenerate RuBP, allowing the cycle to continue.
Overall:
* Photosynthesis is a reduction-oxidation (redox) process. Light energy is used to drive electrons from water molecules (oxidized) to NADP+ (reduced).
* The reducing power of NADPH is then used in the Calvin cycle to reduce carbon dioxide into sugars, storing energy in chemical bonds.
In summary, redox reactions are the backbone of photosynthesis, allowing the conversion of light energy into chemical energy in the form of sugars.
