Eduqas GCSE Biology 5 Photosynthesis Exam Questions

About Eduqas GCSE Biology 5 Photosynthesis

Photosynthesis is a short but heavily examined topic in Eduqas GCSE Biology. You need the word equation, why photosynthesis is endothermic, and how temperature, light intensity and carbon dioxide concentration affect its rate. You must be able to explain limiting factors and how they interact, and at higher tier apply the inverse square law to light intensity. The topic links to leaf structure and transport in plants (topic 2), to enzymes (topic 1) and to the carbon cycle (topic 6). Examiners often give a rate graph and ask which factor is limiting at each part, or ask you to plan or evaluate SP5, the photosynthesis rate practical.

What you need to know

• Equation Carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll in chloroplasts. It is endothermic because energy is transferred from the environment (as light) to the chloroplasts.
• Uses of glucose Glucose is used in respiration, converted to starch for storage, used to make cellulose for cell walls, and combined with nitrate ions to make amino acids and proteins.
• Limiting factors Light intensity, carbon dioxide concentration and temperature can each limit the rate. On a graph, where the rate rises as the factor increases, that factor is limiting; where the line levels off, something else is limiting.
• Temperature Photosynthesis is controlled by enzymes, so the rate rises with temperature up to an optimum, then falls as the enzymes are denatured.
• Inverse square law Light intensity is proportional to 1 / distance^2 from the lamp. Doubling the distance reduces the light intensity to one quarter.
• SP5 Investigate factors affecting the rate of photosynthesis, for example counting oxygen bubbles from pondweed at different distances from a lamp. Control temperature (for example with a water bath or heat shield) and carbon dioxide concentration (sodium hydrogencarbonate solution).

Common mistakes in 5 Photosynthesis

• Saying plants get their food from the soil. Plants make glucose by photosynthesis; the soil supplies water and mineral ions.
• Writing that the rate stops increasing because the plant has enough light. The correct answer is that another factor, such as carbon dioxide concentration or temperature, has become limiting.
• Calling photosynthesis exothermic. It is endothermic: energy is taken in from light.
• Forgetting that the lamp heats the plant. Temperature must be controlled, otherwise moving the lamp changes two variables.
• Halving the light intensity when the distance doubles. The inverse square law means it falls to a quarter.

Try three 5 Photosynthesis exam-style questions