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

What are two of the three main factors that can limit the rate of photosynthesis?

1. Water availability and root depth

2. Light intensity and carbon dioxide concentration

3. Nutrient content and stem size

4. Pest levels and leaf size

The correct answer is: Light intensity and carbon dioxide concentration

The correct choice identifies two of the three main factors that can limit the rate of photosynthesis: light intensity and carbon dioxide concentration. In the process of photosynthesis, plants require light energy (usually from the sun) to convert carbon dioxide and water into glucose and oxygen. Light intensity is crucial because it affects the rate at which these reactions occur; with more light, plants can produce more energy. However, if the light intensity is too low, photosynthesis will be limited regardless of other conditions being optimal. Carbon dioxide concentration plays an equally important role because it is one of the raw materials needed for photosynthesis. A sufficient level of carbon dioxide is necessary to ensure that the plant can produce the glucose required for energy and growth. If carbon dioxide levels are inadequate, photosynthesis cannot proceed at its maximum rate. The other choices encompass factors that may influence plant health or growth, but they do not directly limit the core biochemical process of photosynthesis in the same fundamental way as light intensity and carbon dioxide do.