OCR GCSE Biology A (Gateway)
OCR GCSE Biology A B1.4 Photosynthesis Exam Questions
Free OCR GCSE Biology A exam questions for B1.4 Photosynthesis, with a mark scheme for every pack. Covers photosynthesis equation, chloroplasts, endothermic reactions, limiting factors, inverse square law for light intensity, and investigating the rate of photosynthesis.
B1.4 Photosynthesis exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About OCR GCSE Biology A B1.4 Photosynthesis
B1.4 explains how plants and algae use light energy to make glucose, making them the producers that feed almost every food chain. On the OCR Gateway Biology A course this is the final part of Topic B1, assessed in Paper 1 (Foundation) or Paper 3 (Higher). It links to respiration in B1.3, transport in plants in B2.2 and food production in B6.2. Examiners frequently test the photosynthesis equation, explanations of limiting factor graphs, the inverse square law relating light intensity to distance (Higher), and the PAG practical measuring the rate of photosynthesis using pondweed or algae.
What you need to know
- The reaction Photosynthesis takes place in chloroplasts, where chlorophyll absorbs light: carbon dioxide + water -> glucose + oxygen. Balanced: 6CO2 + 6H2O -> C6H12O6 + 6O2.
- Endothermic process Photosynthesis is endothermic: energy is transferred from the environment (light) to the chloroplasts. Plants are producers of biomass for almost all food chains.
- Uses of glucose Glucose is used for respiration, converted to starch for storage, cellulose for cell walls, and combined with nitrate ions to make amino acids for proteins. It is also used to make lipids.
- Limiting factors The rate of photosynthesis is limited by light intensity, carbon dioxide concentration and temperature. A limiting factor is the factor in shortest supply; increasing it increases the rate until another factor becomes limiting.
- Inverse square law (Higher) Light intensity is inversely proportional to the square of the distance from the light source: light intensity is proportional to 1 / d^2. Halving the distance increases light intensity four times.
- Measuring the rate Count oxygen bubbles from pondweed or measure the volume of oxygen collected in a set time at different distances from a lamp. Control temperature (for example with a water bath or heat filter) and carbon dioxide concentration.
Common mistakes in B1.4 Photosynthesis
- Saying plants get their food from the soil. Plants make glucose by photosynthesis; the soil supplies water and mineral ions.
- Describing a graph plateau as 'the plant has reached its maximum'. The plateau shows that another factor, such as carbon dioxide or temperature, is now limiting.
- Stating that high temperature denatures chlorophyll. It is the enzymes controlling photosynthesis that are denatured above the optimum.
- Forgetting the square in the inverse square law. Doubling the distance reduces light intensity to one quarter, not one half.
- Calling photosynthesis exothermic. It is endothermic because it takes in energy from light.
Try three B1.4 Photosynthesis exam-style questions
1. A lamp is moved from 20 cm to 10 cm away from some pondweed. Use the inverse square law to calculate how many times greater the light intensity is. (2 marks)
Show the mark scheme answer
Intensity is proportional to 1 / d^2; (20 / 10)^2 = 4, so light intensity is 4 times greater.
2. Explain why the rate of photosynthesis stops increasing at high light intensity. (2 marks)
Show the mark scheme answer
Light is no longer the limiting factor; another factor such as carbon dioxide concentration or temperature is now limiting the rate.
3. Describe how a student could investigate the effect of light intensity on the rate of photosynthesis. (4 marks)
Show the mark scheme answer
Place pondweed in a test tube of water (with sodium hydrogencarbonate); put a lamp at a measured distance; count bubbles or collect oxygen volume over a fixed time; repeat at different distances; keep temperature constant (heat shield or water bath) and use the same piece of pondweed; repeat and calculate means.
B1.4 Photosynthesis FAQs
What does OCR GCSE Biology A B1.4 Photosynthesis cover?
It covers the photosynthesis reaction in chloroplasts, why it is endothermic, how glucose is used, limiting factors, the inverse square law and practical work measuring photosynthesis rate.
Which paper is B1.4 Photosynthesis examined in?
It is part of Topic B1, assessed in Paper 1 (Foundation) or Paper 3 (Higher).
How do I answer limiting factor graph questions for B1.4?
Describe the trend with values, then explain which factor is limiting on the rising part of the curve and that a different factor limits the rate where the curve levels off.
