Photosynthesis is the process by which green plants and certain other organisms use sunlight to synthesize nutrients from carbon dioxide and water, generating oxygen as a by-product. It is the primary source of energy for nearly all life on Earth.
Photosynthesis occurs in the chloroplasts of plant cells, primarily in the mesophyll tissue of leaves. The process is broadly divided into two stages: the light reactions (which occur in the thylakoid membranes) and the Calvin cycle or dark reactions (which occur in the stroma).
During the light reactions, chlorophyll and other photosynthetic pigments absorb light energy, which is used to split water molecules (photolysis), releasing oxygen. This energy drives the electron transport chain, producing ATP and NADPH — the energy currencies used in the next stage.
In the Calvin cycle, CO₂ is fixed into organic molecules through a series of enzyme-catalyzed reactions. The key enzyme here is RuBisCO, which catalyzes the fixation of CO₂ into a 3-carbon compound (3-phosphoglyceric acid or 3-PGA) in C3 plants. C4 plants like sugarcane and maize have evolved an additional pathway (Hatch-Slack pathway) to concentrate CO₂ and minimize photorespiration.
The overall equation of photosynthesis is: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O. This equation, while simple, represents dozens of individual biochemical reactions.
Factors affecting the rate of photosynthesis include light intensity, CO₂ concentration, temperature, and water availability. Blackman's Law of Limiting Factors states that the rate of photosynthesis is determined by the factor that is nearest to its minimum value.
Photorespiration is a wasteful process that occurs in C3 plants when RuBisCO fixes O₂ instead of CO₂. This is why C4 plants, which have Kranz anatomy and can concentrate CO₂ around RuBisCO, are more efficient in tropical conditions.
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