What happens during the carbon fixation stage of the Calvin Cycle (light-independent reactions)? A. The enzyme Rubisco attaches to RuBP, causing the formation of molecules of 3-phosphoglycerate. B. Oxygen is not produced through this process. C. The remaining G3P molecules that have exited the cycle are used to reform RuBP through a series of chemical reactions, so that the Calvin cycle can continue. D. Energy from ATP and electrons from NADPH are used to reduce 3-phosphoglycerate to form G3P molecules (which can be used to form other organic molecules such as glucose).
The Correct Answer and Explanation is:
The correct answer is A. The enzyme Rubisco attaches to RuBP, causing the formation of molecules of 3-phosphoglycerate.
During the carbon fixation stage of the Calvin cycle, the enzyme Rubisco (ribulose bisphosphate carboxylase/oxygenase) plays a central role. It catalyzes the initial step by attaching a molecule of carbon dioxide (CO₂) to ribulose bisphosphate (RuBP), a five-carbon sugar. This reaction results in the formation of an unstable six-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon molecule. This process essentially “fixes” the inorganic carbon from CO₂ into an organic form, making it available for the rest of the cycle and for the plant to build other molecules.
Here is a breakdown of what happens during carbon fixation:
- CO₂ binding: Rubisco catalyzes the reaction between CO₂ and RuBP, a 5-carbon compound, leading to the formation of an unstable intermediate compound.
- Splitting: The unstable compound breaks down into two molecules of 3-PGA, a 3-carbon compound. This is the initial product of carbon fixation.
- Energy requirements: No direct energy input (like ATP or NADPH) is required in this stage. The energy comes later in the cycle.
After carbon fixation, the Calvin cycle continues with the reduction of 3-PGA into glyceraldehyde-3-phosphate (G3P), utilizing ATP and NADPH produced during the light-dependent reactions. G3P molecules can be used for various biosynthetic pathways, including the formation of glucose. However, the carbon fixation step itself is the starting point where inorganic carbon is incorporated into the organic molecules of the plant.
Thus, the Calvin cycle ensures that carbon is made available in a form that can be used for building organic compounds, which are essential for the plant’s structure and energy needs.
