Carbon fixation involves the addition of carbon dioxide to _____. A. Rubisco B. NADPH C. G^3P D. RuBP E. 3 -PGA
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Answer

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Step 1:
I'll solve this problem step by step, focusing on the process of carbon fixation in photosynthesis.

Step 2:
: Understand Carbon Fixation

This process converts inorganic carbon dioxide ($$CO_{2}$$) into organic compounds.
Carbon fixation is the first major step of the Calvin cycle (light-independent reactions) in photosynthesis.

Step 3:
: Identify the Key Enzyme

The enzyme responsible for carbon fixation is Rubisco (Ribulose- 1,5 -bisphosphate carboxylase/oxygenase). Rubisco catalyzes the first major carbon-fixing reaction in the Calvin cycle.

Step 4:
: Recognize the Substrate

Rubisco adds $$CO_{2}$$ to a 5-carbon sugar called ribulose bisphosphate (RuBP).
This is the critical step of carbon fixation.

Step 5:
: Analyze the Reaction

When $$CO_{2}$$ is added to RuBP, it creates an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglyceric acid (3-PGA).

Step 6:
: Evaluate the Answer Choices

Reviewing the options, we can see that RuBP (Option D) is the substrate to which $$CO_{2}$$ is added during carbon fixation.

Final Answer

RuBP (Ribulose bisphosphate)