Calvin Cycle

Definition

The Calvin Cycle is a series of biochemical reactions that occur in the stroma of chloroplasts in plants, where carbon dioxide is fixed and converted into glucose using ATP and NADPH generated from the light-dependent reactions. This cycle is crucial for photosynthesis, enabling plants to produce organic compounds necessary for growth and energy.

5 Must Know Facts For Your Next Test

1. The Calvin Cycle consists of three main phases: carbon fixation, reduction, and regeneration of ribulose bisphosphate.
2. In the carbon fixation phase, CO2 is attached to ribulose bisphosphate by Rubisco, forming a 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA).
3. During the reduction phase, ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), which can be used to form glucose and other carbohydrates.
4. The regeneration phase involves the conversion of G3P back into ribulose bisphosphate, allowing the cycle to continue.
5. For every three turns of the Calvin Cycle, one molecule of G3P is produced, which contributes to the formation of glucose and other carbohydrates.

Review Questions

• How does ATP and NADPH produced in the light-dependent reactions influence the Calvin Cycle?
  • ATP and NADPH are essential for the Calvin Cycle as they provide the energy and reducing power necessary for the conversion of 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P). ATP supplies energy for phosphorylation reactions, while NADPH donates electrons during the reduction phase. Without these two components generated from the light-dependent reactions, the Calvin Cycle would not be able to synthesize carbohydrates effectively.
• Evaluate the role of Rubisco in the Calvin Cycle and its significance for plant metabolism.
  • Rubisco plays a pivotal role in the Calvin Cycle as it catalyzes the first step of carbon fixation by attaching CO2 to ribulose bisphosphate. This enzyme is crucial because it directly influences the rate at which plants can convert inorganic carbon into organic compounds. Since Rubisco is one of the most abundant proteins on Earth, its efficiency significantly affects overall plant metabolism and productivity, making it a key player in global carbon cycling.
• Analyze how C4 and CAM pathways enhance carbon fixation in plants compared to the traditional Calvin Cycle, particularly under stressful environmental conditions.
  • C4 and CAM pathways provide adaptations that optimize carbon fixation by minimizing photorespiration during stressful environmental conditions such as high temperatures and low CO2 availability. C4 plants initially fix CO2 into a four-carbon compound using a different enzyme, phosphoenolpyruvate carboxylase, which reduces reliance on Rubisco. CAM plants further enhance this by fixing CO2 at night when temperatures are cooler, thus conserving water. Both adaptations allow these plants to maintain productivity even in challenging environments, demonstrating a strategic evolution beyond the traditional Calvin Cycle.

Related terms

Rubisco: Ribulose bisphosphate carboxylase/oxygenase, an enzyme that catalyzes the first step of the Calvin Cycle by fixing carbon dioxide to ribulose bisphosphate.

NADPH:

Nicotinamide adenine dinucleotide phosphate, an electron carrier that provides the reducing power needed for the reduction phase of the Calvin Cycle.

Carbon Fixation: The process of converting inorganic carbon (CO2) into organic compounds, which is the primary function of the Calvin Cycle in photosynthesis.