Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. This process mainly takes place in the chloroplasts of plant cells and involves two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
1. Light-Dependent Reactions
Location: Thylakoid membranes of chloroplasts.
Key Events:
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Photon Absorption: Chlorophyll and other pigments absorb light energy, exciting electrons.
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Water Splitting (Photolysis): Light energy splits water (H₂O) into oxygen (O₂), protons (H⁺), and electrons (e⁻): \[ 2 H_2O \rightarrow 4 H^+ + 4 e^- + O_2 \]
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Electron Transport Chain: Excited electrons move through a series of proteins in the thylakoid membrane (electron transport chain). As they move, they lose energy, which is used to pump H⁺ ions into the thylakoid lumen, creating a proton gradient.
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ATP and NADPH Production: The energy from the proton gradient is used by ATP synthase to convert ADP and inorganic phosphate into ATP. Electrons eventually reduce NADP⁺ to NADPH: \[ NADP^+ + 2 e^- + 2 H^+ \rightarrow NADPH + H^+ \]
2. Light-Independent Reactions (Calvin Cycle)
Location: Stroma of chloroplasts.
Key Events:
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Carbon Fixation: CO₂ from the atmosphere is fixed into a 5-carbon sugar (ribulose bisphosphate or RuBP) through the enzyme RuBisCO, producing a 6-carbon compound that quickly splits into two 3-carbon molecules (3-phosphoglycerate or 3-PGA): \[ CO_2 + RuBP \rightarrow 2 (3-PGA) \]
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Reduction Phase: ATP and NADPH produced during the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), another 3-carbon sugar: \[ 3-PGA + ATP + NADPH \rightarrow G3P + ADP + NADP^+ + P_i \]
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Regeneration of RuBP: Some G3P molecules exit the cycle to be used in forming glucose and other carbohydrates. The rest are used to regenerate RuBP so the cycle can continue: \[ G3P \text{ uses ATP to regenerate RuBP} \]
Overall Photosynthesis Reaction
The overall balanced equation for photosynthesis can be summarized as follows: \[ 6 CO_2 + 6 H_2O + light \ energy \rightarrow C_6H_{12}O_6 + 6 O_2 \]
This equation indicates that carbon dioxide and water, in the presence of light energy, are converted into glucose and oxygen, which is released as a byproduct.
In summary, photosynthesis involves a complex series of reactions transforming light energy into chemical energy stored in glucose, with oxygen produced as a byproduct.