USA Biology Olympiad (USABO) Practice Exam

Which of the following compounds is a product of the Calvin cycle?

1. ATP

2. PGA

3. Glucose

4. Oxygen

The correct answer is: Glucose

The Calvin cycle, also known as the dark reactions or light-independent reactions of photosynthesis, primarily takes place in the stroma of chloroplasts. This cycle is vital for converting carbon dioxide and other compounds into glucose, which serves as a fundamental energy source for plants. During the Calvin cycle, the enzyme RuBisCO catalyzes the fixation of carbon dioxide, and the resulting 3-carbon molecule is 3-phosphoglycerate (PGA). However, while PGA is an intermediate in the cycle, it is not the final product. Instead, through a series of reduction and regeneration steps, ATP and NADPH (produced in the light-dependent reactions) drive the conversion of these intermediates into glucose, a crucial carbohydrate that plants can store and utilize for energy. Glucose is ultimately produced from the carbon fixation occurring in the Calvin cycle, making it the correct answer to the question. Other incorrect options include ATP, which is involved as an energy source but is not a product of the cycle; PGA, which is an intermediate formed during the process but not the end product; and oxygen, which is a byproduct of the light-dependent reactions rather than the Calvin cycle.