Photosynthesis: Light-Dependent and Light-Independent Reactions
Photosynthesis is a crucial process that allows autotrophs to convert light energy into chemical energy in the form of sugars. This anabolic reaction combines small molecules (CO2 and H2O) to form larger molecules (sugars). The process takes place in chloroplasts, specialized organelles found in plant cells.
Vocabulary: Autotrophs are organisms that can produce their own food using light or chemical energy.
Chloroplast Structure and Function
Chloroplasts contain thylakoids, which are membrane segments that form disc-shaped structures called grana (singular: granum). The light-dependent reactions occur in the intermembrane space of thylakoids.
Highlight: The thylakoid membrane's folding results in a large surface area, maximizing light absorption by chlorophyll.
Light-Dependent Reactions
The light-dependent reactions involve several key processes:
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Photolysis: This is the splitting of water molecules using light energy, generating hydrogen ions, electrons, and oxygen.
Definition: Photolysis is represented by the equation: 2H2O + photons → 4e- + 4H+ + O2
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Electron Transport: The electrons generated from photolysis are used to produce ATP through cyclic and non-cyclic electron transport.
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ATP and NADPH Production: The light-dependent reactions produce ATP and NADPH, which are used in the Calvin cycle.
Example: The products of the light-dependent reactions (O2, NADPH, ATP) are formed from the reactants H2O, light, NADP+, and ADP.
Light-Independent Reactions (Calvin Cycle)
The Calvin cycle, also known as the light-independent reactions, occurs in the stroma of the chloroplast. It consists of three main stages:
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Fixation: CO2 is fixed to ribulose bisphosphate (RuBP) by the enzyme RuBisCO, forming a 6-carbon molecule that splits into two 3-carbon molecules of glycerate 3-phosphate (GP).
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Reduction: GP is reduced using NADPH to form glyceraldehyde 3-phosphate (G3P or triose phosphate).
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Regeneration: Some G3P molecules are used to regenerate RuBP, completing the cycle.
Highlight: Six turns of the Calvin cycle are required to produce one glucose molecule.
The Calvin cycle uses the ATP and NADPH produced in the light-dependent reactions to fix carbon dioxide and produce glucose. After the cycle, NADP+ and ADP return to the light-dependent reactions to be recycled.
Quote: "Remaining TP molecule used for conversion to hexose molecule (glucose)"
This intricate process of photosynthesis demonstrates the remarkable ability of plants to harness light energy and convert it into the chemical energy that sustains life on Earth.