Calvin Cycle

Definition

The Calvin Cycle is a series of biochemical reactions that occur in the chloroplasts of photosynthetic organisms, primarily plants, where carbon dioxide is fixed and converted into glucose using ATP and NADPH produced during the light-dependent reactions of photosynthesis. This cycle plays a crucial role in transforming inorganic carbon into organic molecules, which are essential for energy storage and growth.

5 Must Know Facts For Your Next Test

1. The Calvin Cycle occurs in the stroma of chloroplasts and does not directly require light, although it depends on the products (ATP and NADPH) from light-dependent reactions.
2. The cycle consists of three main phases: carbon fixation, reduction phase, and regeneration of ribulose bisphosphate (RuBP).
3. Carbon fixation involves the enzyme Rubisco catalyzing the reaction between carbon dioxide and RuBP to produce 3-phosphoglycerate (3-PGA).
4. In the reduction phase, ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that can be further processed into glucose.
5. For every three turns of the Calvin Cycle, one molecule of G3P is produced, which can be used to form glucose and other carbohydrates.

Review Questions

• How does the Calvin Cycle utilize the products from light-dependent reactions in photosynthesis?
  • The Calvin Cycle relies on ATP and NADPH generated during the light-dependent reactions to drive its biochemical processes. Specifically, ATP provides the energy required for converting 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P), while NADPH supplies the reducing power needed to facilitate these transformations. This interplay between light-dependent and light-independent processes is essential for efficiently converting solar energy into chemical energy stored in carbohydrates.
• Discuss the importance of Rubisco in the Calvin Cycle and its role in carbon fixation.
  • Rubisco is a crucial enzyme in the Calvin Cycle because it catalyzes the first step of carbon fixation by combining carbon dioxide with ribulose bisphosphate (RuBP). This reaction leads to the formation of 3-phosphoglycerate (3-PGA), which is then transformed into carbohydrates. Given that Rubisco is responsible for incorporating atmospheric CO2 into organic molecules, it is a key player in determining the efficiency of photosynthesis and subsequently affects plant growth and productivity.
• Evaluate how variations in environmental conditions can impact the efficiency of the Calvin Cycle in plants.
  • Environmental conditions such as temperature, light intensity, and CO2 concentration significantly influence the efficiency of the Calvin Cycle. For instance, higher temperatures can increase enzyme activity but may also lead to photorespiration when stomata close under stress, reducing CO2 availability. Similarly, low light intensity limits ATP and NADPH production during light-dependent reactions, thereby decreasing substrate availability for the Calvin Cycle. Understanding these impacts helps explain how climate change and varying environmental factors can affect plant growth and agricultural productivity.

Related terms

Photosynthesis: The process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose using carbon dioxide and water.

Light-dependent Reactions: The first stage of photosynthesis occurring in the thylakoid membranes of chloroplasts, where sunlight is captured and converted into ATP and NADPH.

Rubisco: An enzyme that catalyzes the first major step of the Calvin Cycle, facilitating the fixation of carbon dioxide to ribulose bisphosphate (RuBP).