Optometry Admission Test (OAT) Practice

In which cycle does RuBP capture CO2 to generate PGAL?

1. Calvin cycle

2. Light-dependent reactions

3. Krebs cycle

4. Electron transport chain

The correct answer is: Calvin cycle

The process of RuBP (ribulose bisphosphate) capturing CO2 to generate PGAL (phosphoglyceraldehyde) occurs in the Calvin cycle, which is a key component of photosynthesis in plants. During the Calvin cycle, CO2 is fixed by RuBP in a reaction catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). This reaction ultimately leads to the formation of unstable 6-carbon intermediates that quickly break down into two molecules of 3-phosphoglycerate (3-PGA). Through a series of enzymatic steps, these molecules are then converted into PGAL. The Calvin cycle operates independently of light reactions, although it relies on the energy and reducing power generated during the light-dependent reactions. Therefore, it is essential in the conversion of inorganic carbon into organic molecules that can be utilized by the plant for growth and energy. Other cycles mentioned in the options do not involve RuBP or the fixation of carbon dioxide in the same way. The light-dependent reactions of photosynthesis primarily focus on absorbing light energy to generate ATP and NADPH, while the Krebs cycle (also known as the citric acid cycle) occurs in cellular respiration and is responsible for the oxidation