Select the correct statement about the Calvin cycle.
- The Calvin cycle takes place primarily in the dark.
- The Calvin cycle has three phases: carbon fixation, reduction, and regeneration of RuBP.
- The basic function of the Calvin cycle is the conversion of solar energy to chemical energy.
The Correct Answer and Explanation is:
The correct statement is:
2. The Calvin cycle has three phases: carbon fixation, reduction, and regeneration of RuBP.
Explanation:
The Calvin cycle, also known as the light-independent reactions or dark reactions of photosynthesis, is a crucial part of the process by which plants, algae, and some bacteria convert carbon dioxide into glucose. The Calvin cycle occurs in the stroma of chloroplasts in plant cells and does not directly require light, though it depends on the products of the light-dependent reactions (ATP and NADPH).
1. The Calvin cycle takes place primarily in the dark.
This statement is incorrect because, although the Calvin cycle does not directly require light to occur, it still relies on the ATP and NADPH generated during the light-dependent reactions. These light reactions occur in the presence of light, so the Calvin cycle is indirectly connected to light. The term “dark” reactions refers to the fact that these steps do not require light, but they cannot occur in complete darkness without the products from the light-dependent reactions.
2. The Calvin cycle has three phases: carbon fixation, reduction, and regeneration of RuBP.
This statement is correct. The Calvin cycle consists of three main phases:
- Carbon fixation: The enzyme RuBisCO fixes carbon dioxide by attaching it to a 5-carbon molecule called ribulose bisphosphate (RuBP). This forms an unstable 6-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA).
- Reduction: ATP and NADPH (produced in the light-dependent reactions) are used to reduce 3-PGA to glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar. Some of this G3P can be used to form glucose.
- Regeneration of RuBP: The remaining G3P molecules are used to regenerate RuBP, the molecule that accepts carbon dioxide at the start of the cycle. This process requires ATP.
3. The basic function of the Calvin cycle is the conversion of solar energy to chemical energy.
This statement is incorrect because the primary function of the Calvin cycle is the fixation of carbon (converting CO₂ into an organic molecule), rather than directly converting solar energy into chemical energy. Solar energy is first captured during the light-dependent reactions, which create the ATP and NADPH used in the Calvin cycle to convert carbon dioxide into glucose. The actual conversion of light energy to chemical energy occurs during the light-dependent reactions, not the Calvin cycle.
In summary, the Calvin cycle plays a critical role in converting carbon dioxide into organic molecules, and it consists of three main stages: carbon fixation, reduction, and regeneration of RuBP.