9.2 Calvin cycle and carbon fixation

The Calvin cycle is the engine of photosynthesis, turning CO2 into sugar. It's a three-step dance: carbon fixation, reduction, and RuBP regeneration. These steps use energy from light reactions to build glucose, the fuel of life.

RuBisCO, the star enzyme, grabs CO2 and starts the cycle. Most of the products go back into the cycle, keeping it spinning. This process is vital for plants and, by extension, all life on Earth.

The Calvin Cycle

Steps of the Calvin cycle

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1. Carbon fixation
   • RuBisCO catalyzes the addition of CO2 to ribulose bisphosphate (RuBP) forming a 6-carbon compound
   • 6-carbon compound is unstable splits into two molecules of 3-phosphoglycerate (3-PGA)
2. Reduction
   • ATP and NADPH from light-dependent reactions convert 3-PGA into glyceraldehyde 3-phosphate (G3P)
   • Phosphoglycerate kinase catalyzes phosphorylation of 3-PGA to 1,3-bisphosphoglycerate using ATP
   • G3P dehydrogenase reduces 1,3-bisphosphoglycerate to G3P using NADPH
3. Regeneration of RuBP
   • Most G3P used to regenerate RuBP through series of reactions catalyzed by various enzymes (transketolase, aldolase)
   • Transketolase and aldolase rearrange carbon skeletons of sugar phosphates
   • Phosphatases remove phosphate groups
   • Phosphoribulokinase phosphorylates ribulose 5-phosphate to regenerate RuBP

Carbon fixation and RuBisCO

• Carbon fixation converts inorganic carbon (CO2) into organic compounds
• RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) primary enzyme responsible for carbon fixation
• RuBisCO catalyzes addition of CO2 to RuBP forming 6-carbon compound splits into two molecules of 3-PGA
• Most abundant protein on Earth due to crucial role in carbon fixation
• Efficiency limited by slow catalytic rate and ability to catalyze competing oxygenation reaction (photorespiration)
• Essential for photosynthesis in plants, algae, and cyanobacteria
• Plays a key role in the global carbon cycle by removing CO2 from the atmosphere

Regeneration of RuBP

• Regeneration of RuBP essential for continuous operation of Calvin cycle
• Most G3P produced in reduction phase used to regenerate RuBP
• Series of reactions rearrange carbon skeletons of sugar phosphates using enzymes (transketolase, aldolase)
• Phosphatases remove phosphate groups from sugar phosphates
• Phosphoribulokinase catalyzes final step phosphorylating ribulose 5-phosphate to regenerate RuBP
• Regenerated RuBP participates in another round of carbon fixation
• Ensures a constant supply of RuBP for carbon fixation

Significance of light-independent reactions

• Light-independent reactions (Calvin cycle) crucial for converting inorganic carbon (CO2) into organic compounds (glucose)
• Uses ATP and NADPH generated during light-dependent reactions to reduce CO2 and synthesize glucose
• Glucose produced serves as primary energy source for plant and organisms that consume the plant
• Main pathway for carbon fixation in C3 plants (majority of plant species)
• Essential part of global carbon cycle removes CO2 from atmosphere and incorporates it into biomass
• Provides the foundation for the food chain by producing organic compounds from inorganic carbon
• Plays a crucial role in maintaining the Earth's atmosphere and climate by regulating CO2 levels