Photosynthesis is a biochemical process that allows plants, algae, and certain bacteria to convert light energy into chemical energy stored in glucose. This process involves two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
1. Light-Dependent Reactions
Location: These reactions take place in the thylakoid membranes of the chloroplasts.
Process:
- Absorption of Light: Chlorophyll and other pigments absorb sunlight, which excites electrons to a higher energy state.
- Water Splitting: This energy is used to split water molecules (H₂O) into oxygen (O₂), protons (H⁺), and electrons. This process releases oxygen as a byproduct.
- Electron Transport Chain (ETC): The excited electrons are transferred through a series of proteins in the thylakoid membrane known as the electron transport chain. As they move through the ETC, they lose energy.
- ATP and NADPH Formation: The energy lost by electrons is used to pump protons into the thylakoid lumen, creating a proton gradient. Protons flow back into the stroma through ATP synthase, generating ATP (adenosine triphosphate). Additionally, the final electron acceptor reduces NADP⁺ to form NADPH (nicotinamide adenine dinucleotide phosphate), a carrier molecule that stores energy and reducing power.
2. Light-Independent Reactions (Calvin Cycle)
Location: These reactions occur in the stroma of chloroplasts.
Process:
- Carbon Fixation: The Calvin cycle begins when carbon dioxide (CO₂) from the atmosphere is fixed into an organic molecule. The enzyme RuBisCO catalyzes the reaction between CO₂ and ribulose bisphosphate (RuBP), producing 3-phosphoglycerate (3-PGA).
- Reduction Phase: ATP and NADPH produced during the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. This phase incorporates energy and reducing power into the carbon compounds.
- Regeneration of RuBP: Some of the G3P molecules go on to form glucose and other carbohydrates, while others are used to regenerate RuBP. This process requires ATP and allows the cycle to continue by enabling the fixation of new CO₂ molecules.
Summary
In summary, photosynthesis consists of light-dependent reactions that harness sunlight to produce ATP and NADPH, and light-independent reactions (Calvin cycle) that use these energy carriers to convert carbon dioxide into glucose. Together, these processes are essential for energy transformation in the biosphere and contribute to the oxygen supply in the atmosphere.