OCR GCSE Biology B B3.1 What happens during photosynthesis? Exam Questions

About OCR GCSE Biology B B3.1 What happens during photosynthesis?

B3.1 begins chapter B3, Living Together, Food and Ecosystems, with photosynthesis, the process that supplies almost all food chains with energy. You need the word and balanced symbol equations, where photosynthesis happens, what plants do with the glucose produced, and how enzymes control the reactions. Limiting factors (light intensity, carbon dioxide concentration and temperature) are tested through graphs and the pondweed practical. Higher tier students should apply the inverse square law for light intensity. This topic links to respiration in B4.1 and to ecosystems in B3.3. Examiners often ask you to explain the shape of a rate graph.

What you need to know

• Equations carbon dioxide + water to glucose + oxygen, using light. Balanced: 6CO2 + 6H2O to C6H12O6 + 6O2. It is an endothermic reaction that takes in energy from light.
• Chloroplasts Photosynthesis happens in chloroplasts, which contain chlorophyll to absorb light. It occurs in two main stages: light is used to split water releasing oxygen, then carbon dioxide is combined with hydrogen to make glucose.
• Uses of glucose Glucose is used in respiration, converted to starch for storage, cellulose for cell walls, and combined with nitrate ions to make amino acids and proteins.
• Enzymes Enzymes are proteins that catalyse reactions. The substrate fits the active site (lock and key); high temperatures or extreme pH change the active site shape and denature the enzyme.
• Limiting factors Rate is limited by light intensity, carbon dioxide concentration or temperature, whichever is in shortest supply. On a graph, where the line levels off another factor has become limiting.
• Light intensity practical Count oxygen bubbles or measure gas volume from pondweed at different distances from a lamp, keeping temperature and carbon dioxide constant.
• Inverse square law Light intensity is proportional to 1 / distance^2, so halving the distance from the lamp makes the light four times more intense.

Common mistakes in B3.1 What happens during photosynthesis?

• Saying plants photosynthesise instead of respiring. Plants respire all the time; they photosynthesise only in the light.
• Stating that light energy is turned into glucose. Light energy is used to drive the reaction; glucose stores chemical energy.
• Saying enzymes die at high temperature. Enzymes are denatured because the active site changes shape.
• Identifying the wrong limiting factor on a graph. On the rising part, the factor on the x-axis is limiting; on the plateau something else is.
• Forgetting that a lamp also heats the water. Use a heat filter or LED so temperature is controlled.

Try three B3.1 What happens during photosynthesis? exam-style questions