OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What is a limiting factor in the photosynthesis process?

1. An element that promotes growth

2. A resource that enhances sunlight absorption

3. A reactant that is in short supply

4. A nutrient that increases plant size

The correct answer is: A reactant that is in short supply

A limiting factor in the photosynthesis process is a reactant that is in short supply. In photosynthesis, plants convert carbon dioxide and water into glucose and oxygen using sunlight. For this process to proceed efficiently, certain key resources—namely sunlight, carbon dioxide, and water—must be available in sufficient quantities. If any of these reactants are limited, the overall rate of photosynthesis will decrease, regardless of the amounts of the other reactants. For example, if carbon dioxide levels are low, plants cannot perform photosynthesis at their maximum potential, since carbon dioxide is crucial for producing glucose. This concept clearly demonstrates that the availability of reactants directly influences the photosynthetic process, establishing the idea of limiting factors in ecosystems. Thus, the correct understanding hinges on the recognition that a reactant in short supply restricts the process's efficiency and capacity.