USA Biology Olympiad (USABO) Practice Exam

What is the main function of the dark reaction (Calvin cycle) of photosynthesis?

1. Generate ATP

2. Fix Carbon Dioxide into Glucose

3. Utilize Water

4. Produce Oxygen

The correct answer is: Fix Carbon Dioxide into Glucose

The primary function of the dark reactions, or Calvin cycle, of photosynthesis is to fix carbon dioxide into glucose. This process occurs in the stroma of chloroplasts and does not require light, hence the term "dark reaction," although it can happen in the presence of light as well. During the Calvin cycle, carbon dioxide from the atmosphere is captured and incorporated into organic molecules through a series of enzymatic reactions. The enzyme RuBisCO facilitates the conversion of carbon dioxide into a 3-carbon compound that is then transformed into glucose and other carbohydrates. This pathway is crucial for the synthesis of sugars, which serve as a primary source of energy and carbon skeletons for plants, indirectly supporting energy needs for heterotrophic organisms. The other options represent processes or products that do not align with the core function of the Calvin cycle. Generating ATP is primarily associated with the light-dependent reactions, where solar energy is converted into chemical energy. While water is utilized in photosynthesis, it’s during the light reactions that water is split, releasing oxygen. The production of oxygen primarily occurs in these light reactions through the photolysis of water. Therefore, the correct understanding of the Calvin cycle emphasizes its role in carbon fixation and glucose production, establishing its importance in