Multiple Choice
2. At the end of the Calvin cycle, which molecule is primarily produced and can be used to synthesize glucose and other carbohydrates?
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9. Photosynthesis
Calvin Cycle
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What provides the carbon atoms that are incorporated into sugar molecules during the Calvin cycle?
A
Water ($\mathrm{H_2O}$)
B
Oxygen ($\mathrm{O_2}$)
C
Glucose ($\mathrm{C_6H_{12}O_6}$)
D
Carbon dioxide ($\mathrm{CO_2}$)
Verified step by step guidance1
Understand the Calvin cycle: The Calvin cycle is a series of biochemical reactions that occur in the stroma of chloroplasts during photosynthesis. Its primary function is to synthesize sugar molecules using carbon atoms from carbon dioxide ($\mathrm{CO_2}$).
Identify the source of carbon atoms: During the Calvin cycle, carbon dioxide ($\mathrm{CO_2}$) is the molecule that provides the carbon atoms. These carbon atoms are incorporated into organic molecules through a process called carbon fixation.
Examine the role of carbon fixation: In the first step of the Calvin cycle, an enzyme called RuBisCO catalyzes the reaction between carbon dioxide ($\mathrm{CO_2}$) and ribulose-1,5-bisphosphate (RuBP), forming two molecules of 3-phosphoglycerate (3-PGA). This is the step where carbon atoms from $\mathrm{CO_2}$ are integrated into the cycle.
Understand why other options are incorrect: Water ($\mathrm{H_2O}$) provides electrons during the light-dependent reactions, not carbon atoms. Oxygen ($\mathrm{O_2}$) is a byproduct of the light-dependent reactions and does not contribute carbon. Glucose ($\mathrm{C_6H_{12}O_6}$) is the product of photosynthesis, not the source of carbon atoms.
Conclude the correct answer: Carbon dioxide ($\mathrm{CO_2}$) is the correct answer because it is the molecule that supplies the carbon atoms used to build sugar molecules during the Calvin cycle.
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