Dark Reaction (Light-Independent Reaction) = Calvin Cycle
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Goal: to fix CO2
- Take a boring, uninteresting molecule, CO2, and create an energy-rich molecule, glucose, C6H12O6.
- It is a cycle, requiring six turns to make one glucose molecule.
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There must be an energy input to drive the cycle, so the energy-rich molecules created in the light reaction are used:
- Can occur in dark or light
Calvin Cycle
Overview
1. Carbon Fixation
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CO2 is attached to a five-carbon sugar named ribulose bisphosphate (RuBP).
- Catalyzed by the enzyme RuBP carboxylase, or rubisco (the most abundant protein on Earth!).
- Product: a six-carbon intermediate that immediately splits in half to form two molecules of 3-phosphoglycerate
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Quick summary
- Begin with: 1 CO2 + 1 RuBP (5-carbon)
- End with: 2 3-phosphoglycerate (3-carbon)
2. Reduction
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3-phosphoglycerate is converted to the energy-rich sugar glyceraldehyde 3-phosphate (G3P).
- This requires 6 ATP and 6 NADPH.
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Two steps:
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Six 3-phosphoglycerates becomes six 1,3-bisphosphoglycerates.
- Six 1,3-bisphosphoglycerate are converted into six glyceraldehyde 3-phosphates.
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Result:
- One glyceraldehyde 3-phosphate leaves the cycle.
- Five glyceraldehyde 3-phosphates continue around the cycle.
Molecule Count Thus Far…
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Phase 1: fixation
- 3 CO2 + 3 RuBP (5-C) → 6 3-phosphoglycerate (3-C)
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Phase 2: reduction
- 6 3-phosphoglycerate (3-C) → 6 glyceraldehyde 3-phosphate (3-C)
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Going into phase 3
- 1 G3P (3-C) leaves the cycle.
- 5 G3P (3-C) continue.
3. Regeneration of CO2 Acceptor (RuBP)
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5 glyceraldehyde 3-phosphates (3-C) are rearranged into 3 ribulose biphosphates (5-C), or RuBPs.
- RuBP was the molecule that began the cycle by combining with CO2 in phase 1: carbon fixation.
Total Energy Cost
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To make 1 glyceraldehyde 3-phosphate, a three-carbon sugar:
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To make 1 glucose, a six-carbon sugar:
Photosynthesis Review
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Light reactions
- Occur in the thylakoids.
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Light energy is transformed into ATP and NADPH.
- Noncyclic electron flow
- Cyclic electron flow
- Water contributes its electrons to the process.
- Oxygen is a byproduct.
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Calvin cycle (dark reactions/light-independent reactions)
- Occur in the stroma.
- Light is not directly involved.
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Glucose and other sugars are constructed out of CO2.
- ATP and NADPH from the light reaction are the energy source driving this process.
- Overall reaction: light + 6CO2 + 6H2O → C6H12O6 + 6O2