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

Which of the following factors does NOT affect the rate of photosynthesis?

1. Light intensity

2. Soil pH

3. Carbon dioxide concentration

4. Temperature

The correct answer is: Soil pH

The factor that does not affect the rate of photosynthesis is soil pH. Photosynthesis primarily occurs in the leaves of plants, where chloroplasts convert light energy into chemical energy using carbon dioxide and water. The essential variables that influence this process are light intensity, carbon dioxide concentration, and temperature, as they directly impact the chlorophyll's ability to capture light energy and the overall metabolic rates of the plant. Light intensity affects the amount of energy available for the photosynthesis reaction. As light intensity increases, the rate of photosynthesis typically increases, up to a point. Carbon dioxide concentration is crucial because it is one of the raw materials needed for photosynthesis. Higher concentrations generally lead to an increase in the rate of photosynthesis until other factors become limiting. Temperature influences the enzymes involved in the photosynthesis process. As temperature rises, the rate of photosynthesis generally increases, but extreme temperatures may denature enzymes and decrease the rate. Soil pH, while important for overall plant health and nutrient uptake, does not directly influence the rate of photosynthesis itself. It may affect how well a plant can absorb nutrients, which can indirectly influence growth and health, but it is not a direct factor in the photosynthesis process.