3.3 Calvin cycle and carbon fixation

The Calvin cycle is the powerhouse of carbon fixation in plants. It's where CO2 gets transformed into sugar, fueling plant growth. This process happens in the chloroplast stroma, using energy from light reactions to make glucose.

RuBisCO, the star enzyme, kicks off the cycle by fixing carbon. It combines CO2 with RuBP, creating 3-PGA. This 3-carbon molecule then goes through a series of reactions, using ATP and NADPH from light reactions to eventually form glucose.

Carbon Fixation and Reduction

RuBisCO and Carbon Fixation

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• RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is the primary enzyme responsible for carbon fixation, the process of converting atmospheric CO2 into organic compounds
  • It is the most abundant protein on Earth and is found in the chloroplast stroma
  • RuBisCO catalyzes the first major step of carbon fixation by combining CO2 with ribulose 1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (3-PGA)
• Carbon fixation occurs during the light-independent reactions of photosynthesis (Calvin cycle) and is crucial for the synthesis of organic compounds
  • The process of carbon fixation converts inorganic carbon (CO2) into organic molecules that can be used by plants for growth and energy storage (glucose, starch)
  • Carbon fixation is the primary pathway for the incorporation of carbon into the biosphere and is essential for sustaining life on Earth
• Ribulose 1,5-bisphosphate (RuBP) is a 5-carbon sugar that serves as the initial acceptor molecule for CO2 during carbon fixation
  • RuBP is regenerated at the end of the Calvin cycle to ensure a continuous supply for carbon fixation
  • The enzyme phosphoribulokinase catalyzes the phosphorylation of ribulose 5-phosphate to form RuBP, using ATP as the phosphate donor

Products of Carbon Fixation and Their Fate

• 3-Phosphoglycerate (3-PGA) is the immediate product of carbon fixation, formed when RuBisCO adds CO2 to RuBP
  • Each molecule of RuBP that undergoes carboxylation produces two molecules of 3-PGA
  • 3-PGA is then reduced to form glyceraldehyde 3-phosphate (G3P) using ATP and NADPH generated from the light reactions
• ATP and NADPH, products of the light reactions, are essential for the reduction phase of the Calvin cycle
  • ATP provides the energy needed to drive the endergonic reactions of the Calvin cycle, such as the phosphorylation of 3-PGA to form 1,3-bisphosphoglycerate and the regeneration of RuBP
  • NADPH serves as the reducing agent, donating electrons to convert 1,3-bisphosphoglycerate into glyceraldehyde 3-phosphate (G3P)
  • The use of ATP and NADPH in the Calvin cycle represents the coupling of the light reactions with the light-independent reactions of photosynthesis

Regeneration of RuBP

The Regeneration Phase

• The regeneration phase of the Calvin cycle involves a series of reactions that convert glyceraldehyde 3-phosphate (G3P) back into ribulose 1,5-bisphosphate (RuBP) to maintain the continuous operation of the cycle
  • A portion of the G3P produced during the reduction phase is used to regenerate RuBP, while the remaining G3P is used for the synthesis of glucose and other organic compounds
  • The regeneration phase involves a complex series of reactions, including isomerization, transketolation, and phosphorylation steps
• Glyceraldehyde 3-phosphate (G3P) is a key intermediate in the regeneration phase of the Calvin cycle
  • G3P is a 3-carbon sugar that is produced during the reduction phase of the Calvin cycle by the reduction of 3-PGA
  • A portion of the G3P is diverted away from the Calvin cycle to be used for the synthesis of glucose and other organic compounds, while the remaining G3P enters the regeneration phase
• Ribulose 1,5-bisphosphate (RuBP) is regenerated from G3P through a series of reactions in the regeneration phase
  • The regeneration of RuBP is essential for the continuous operation of the Calvin cycle, as it ensures a constant supply of the CO2 acceptor molecule
  • The enzyme phosphoribulokinase catalyzes the final step of RuBP regeneration, phosphorylating ribulose 5-phosphate using ATP to form RuBP

Energy Requirements for RuBP Regeneration

• ATP is required during the regeneration phase of the Calvin cycle to phosphorylate ribulose 5-phosphate, forming ribulose 1,5-bisphosphate (RuBP)
  • The phosphorylation of ribulose 5-phosphate is catalyzed by the enzyme phosphoribulokinase and is an energy-consuming step
  • For every three molecules of CO2 that enter the Calvin cycle, nine molecules of ATP are required for the regeneration of three molecules of RuBP
• The regeneration phase of the Calvin cycle is an energy-demanding process, requiring a significant portion of the ATP generated during the light reactions
  • The continuous regeneration of RuBP is essential for maintaining the high efficiency of carbon fixation and the overall productivity of photosynthesis
  • The energy requirements for RuBP regeneration highlight the importance of the light reactions in providing the necessary ATP and NADPH for the Calvin cycle to function optimally