Calvin-Benson Cycle

Definition

The Calvin-Benson Cycle is a series of light-independent biochemical reactions that take place in the stroma of chloroplasts, using ATP and NADPH produced during the light-dependent reactions to convert carbon dioxide into glucose. This cycle is crucial for photosynthesis, as it allows plants to synthesize carbohydrates, which serve as energy sources for growth and metabolism.

5 Must Know Facts For Your Next Test

1. The Calvin-Benson Cycle operates through three main phases: carbon fixation, reduction phase, and regeneration of ribulose bisphosphate (RuBP).
2. Carbon dioxide enters the cycle and combines with RuBP to form a 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA).
3. ATP and NADPH from the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), which can be further processed into glucose and other carbohydrates.
4. For every three turns of the cycle, one molecule of G3P exits to contribute to carbohydrate synthesis, while five molecules are used to regenerate RuBP.
5. The efficiency of the Calvin-Benson Cycle can be influenced by factors such as temperature, light intensity, and the availability of carbon dioxide.

Review Questions

• Explain the role of ATP and NADPH in the Calvin-Benson Cycle.
  • ATP and NADPH play essential roles in the Calvin-Benson Cycle by providing the necessary energy and reducing power for converting carbon dioxide into organic molecules. During the reduction phase of the cycle, ATP is utilized to phosphorylate intermediates, while NADPH donates electrons to reduce 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P). Without these two molecules generated in the light-dependent reactions, the cycle would not be able to synthesize carbohydrates effectively.
• How does carbon fixation occur in the Calvin-Benson Cycle, and why is RuBisCO important in this process?
  • Carbon fixation in the Calvin-Benson Cycle occurs when carbon dioxide is incorporated into ribulose bisphosphate (RuBP), catalyzed by the enzyme RuBisCO. This reaction produces a 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA). RuBisCO is crucial because it is responsible for initiating this first step of carbon fixation, making it a vital enzyme in photosynthesis and determining the efficiency of converting atmospheric CO2 into organic compounds.
• Evaluate how changes in environmental conditions might affect the overall efficiency of the Calvin-Benson Cycle.
  • Changes in environmental conditions, such as increased temperature or decreased carbon dioxide availability, can significantly impact the efficiency of the Calvin-Benson Cycle. For instance, higher temperatures may lead to increased respiration rates or denaturation of enzymes like RuBisCO, reducing their activity. Similarly, lower levels of carbon dioxide could limit carbon fixation and thus decrease glucose production. Consequently, understanding these factors is crucial for optimizing photosynthesis in agricultural practices and predicting plant responses to climate change.

Related terms

Photosynthesis: The process by which green plants and some other organisms convert light energy into chemical energy in the form of glucose, using carbon dioxide and water.

Carbon Fixation: The conversion of inorganic carbon (carbon dioxide) into organic compounds by living organisms, primarily during the Calvin-Benson Cycle.

RuBisCO: Ribulose-1,5-bisphosphate carboxylase/oxygenase, an enzyme that catalyzes the first step of the Calvin-Benson Cycle, facilitating the fixation of carbon dioxide.