Which of the following best explains how the Calvin cycle and the Krebs cycle differ?

• A. Both the Calvin cycle and the Krebs cycle occur in the mitochondria of plant cells.
• B. The Calvin cycle generates ATP, whereas the Krebs cycle consumes ATP.
• C. The Calvin cycle requires oxygen, while the Krebs cycle does not.
• D. The Calvin cycle fixes carbon dioxide to produce glucose, while the Krebs cycle breaks down glucose to release carbon dioxide.

Answer: (D)

Verified step by step guidance

1. Step 1: Begin by understanding the Calvin cycle. The Calvin cycle is part of photosynthesis and occurs in the stroma of chloroplasts in plant cells. Its primary function is to fix carbon dioxide (CO₂) into organic molecules, ultimately producing glucose.
2. Step 2: Next, understand the Krebs cycle (also known as the citric acid cycle). The Krebs cycle is part of cellular respiration and occurs in the matrix of mitochondria. Its primary function is to break down glucose-derived molecules (like pyruvate) to release energy in the form of ATP, NADH, and FADH₂, while also releasing CO₂ as a byproduct.
3. Step 3: Compare the locations of the two cycles. The Calvin cycle occurs in chloroplasts, while the Krebs cycle occurs in mitochondria. This is a key distinction between the two processes.
4. Step 4: Compare the inputs and outputs of the two cycles. The Calvin cycle uses ATP and NADPH (produced during the light-dependent reactions of photosynthesis) to fix CO₂ and produce glucose. In contrast, the Krebs cycle consumes glucose-derived molecules and releases CO₂ while generating ATP, NADH, and FADH₂.
5. Step 5: Summarize the key difference: The Calvin cycle is anabolic, building glucose from CO₂, while the Krebs cycle is catabolic, breaking down glucose to release CO₂ and energy. This explains why the correct answer is: 'The Calvin cycle fixes carbon dioxide to produce glucose, while the Krebs cycle breaks down glucose to release carbon dioxide.'