Which of the following occurs during the Calvin cycle?
A. release of oxygen
B. reduction of NADP+
C. generation of CO₂
D. carbon fixation
The Correct Answer and Explanation is:
The correct answer is D. carbon fixation.
Explanation:
The Calvin cycle is a key part of photosynthesis that occurs in the stroma of the chloroplast in plant cells. It involves a series of chemical reactions that convert carbon dioxide and other compounds into glucose, a form of sugar that plants use for energy and growth. The Calvin cycle does not require light, so it is considered part of the light-independent reactions (or dark reactions) of photosynthesis.
During the Calvin cycle, the first major process that occurs is carbon fixation, which is answer D. This is where carbon dioxide (CO₂) from the atmosphere is “fixed” into an organic molecule. This step is carried out by an enzyme called RuBisCO (ribulose bisphosphate carboxylase/oxygenase), which catalyzes the attachment of CO₂ to a 5-carbon molecule called ribulose bisphosphate (RuBP). This results in the formation of a 6-carbon molecule that quickly breaks down into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound.
Once the carbon is fixed, the Calvin cycle proceeds through additional steps that involve the reduction of NADPH (produced in the light reactions of photosynthesis) to convert 3-PGA into glyceraldehyde-3-phosphate (G3P). G3P can later be used to synthesize glucose and other carbohydrates.
Incorrect Options:
- A. Release of oxygen: This occurs during the light-dependent reactions, not the Calvin cycle. Oxygen is produced as a byproduct of splitting water molecules (photolysis) in the light reactions.
- B. Reduction of NADP+: This occurs in the light reactions, where NADP+ is reduced to form NADPH.
- C. Generation of CO₂: CO₂ is not generated during the Calvin cycle. Instead, it is consumed as a reactant in the carbon fixation step.
Thus, the Calvin cycle is primarily responsible for fixing carbon dioxide into organic molecules, which is essential for the synthesis of sugars in plants.