Calvin Cycle

Definition

The Calvin Cycle is a series of biochemical reactions that take place in the chloroplasts of photosynthetic organisms, converting carbon dioxide and other compounds into glucose. This cycle is essential for photosynthesis, as it uses ATP and NADPH generated in the light-dependent reactions to drive the fixation of carbon dioxide into organic molecules, ultimately producing sugars that serve as energy sources for the organism.

5 Must Know Facts For Your Next Test

1. The Calvin Cycle occurs in the stroma of chloroplasts and is sometimes called the light-independent reactions or dark reactions.
2. The cycle starts with carbon fixation, where carbon dioxide is incorporated into a 5-carbon sugar called ribulose bisphosphate (RuBP) by the enzyme RuBisCO.
3. For every three molecules of carbon dioxide that enter the cycle, one molecule of glyceraldehyde-3-phosphate (G3P), a precursor to glucose, is produced.
4. The cycle requires a total of nine ATP and six NADPH molecules to produce one G3P molecule, emphasizing the importance of light-dependent reactions in providing energy.
5. Regeneration of RuBP is crucial for the cycle to continue, as it allows the system to restart and keep processing carbon dioxide.

Review Questions

• How does the Calvin Cycle utilize products from light-dependent reactions to produce glucose?
  • The Calvin Cycle relies on ATP and NADPH generated during the light-dependent reactions of photosynthesis to drive the conversion of carbon dioxide into glucose. ATP provides the necessary energy for various steps in the cycle, while NADPH supplies high-energy electrons needed for reducing 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P). This process transforms inorganic carbon into an organic form that can ultimately be used to create glucose.
• Evaluate the importance of RuBisCO in the Calvin Cycle and its role in carbon fixation.
  • RuBisCO is a crucial enzyme in the Calvin Cycle, facilitating the first step of carbon fixation by catalyzing the reaction between carbon dioxide and ribulose bisphosphate (RuBP). Its activity directly influences the efficiency of photosynthesis, as it determines how effectively plants can incorporate atmospheric CO2 into organic compounds. Any limitations or inefficiencies in RuBisCO's function can significantly impact a plant's overall ability to produce sugars.
• Synthesize your understanding of how environmental factors can affect the efficiency of the Calvin Cycle and its implications for plant growth.
  • The efficiency of the Calvin Cycle can be influenced by various environmental factors, including light intensity, temperature, and CO2 concentration. For instance, higher temperatures may increase enzymatic activity up to an optimal point but could also lead to increased photorespiration if conditions become too hot or dry. Additionally, limited sunlight can hinder ATP and NADPH production in light-dependent reactions, reducing overall glucose output. These factors ultimately impact plant growth and productivity by affecting how effectively plants can convert carbon dioxide into energy-rich compounds.

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.

Chloroplasts:

Organelles found in plant cells and algae that are responsible for photosynthesis, containing chlorophyll which captures light energy.

Light-dependent Reactions: The first stage of photosynthesis that occurs in the thylakoid membranes of chloroplasts, where sunlight is absorbed to produce ATP and NADPH.