The term “cycle” is used to describe the Calvin cycle because:
Group of answer choices
the same reactions occur at each step in the pathyway.
the metabolic pathway begins and ends with RUBP.
glucose is synthesized.
the process depends on ATP.
The correct answer and explanation is:
The correct answer is: the metabolic pathway begins and ends with RUBP.
Explanation:
The Calvin cycle is called a “cycle” because it starts and ends with the same molecule, ribulose bisphosphate (RUBP), which is a five-carbon sugar. The cycle itself consists of a series of chemical reactions that fix carbon dioxide (CO₂) into organic molecules. It takes place in the stroma of the chloroplast and is a key part of photosynthesis in plants and other photosynthetic organisms.
Here’s a breakdown of the cycle:
- Carbon Fixation: CO₂ is attached to RUBP by the enzyme RuBisCO, forming an unstable six-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
- Reduction Phase: ATP and NADPH, which were produced in the light reactions of photosynthesis, are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
- Regeneration of RUBP: Some of the G3P molecules are used to regenerate RUBP, allowing the cycle to continue. This process requires ATP.
Since the Calvin cycle regenerates RUBP at the end of the cycle, it essentially “recycles” this molecule through each round of the pathway. This repeating nature is why the term “cycle” is applied.
While glucose is indeed synthesized as part of the broader process of photosynthesis, the Calvin cycle itself doesn’t directly produce glucose. Instead, it produces G3P, which can be used to form glucose and other carbohydrates later. The process does rely on ATP (and NADPH), but these molecules are consumed and produced during the cycle, which is a separate concept from the naming of the “cycle” itself.