CCEA GCSE Biology 1.2 Photosynthesis and Plants Exam Questions

About CCEA GCSE Biology 1.2 Photosynthesis and Plants

Photosynthesis and Plants is the second topic in CCEA GCSE Biology Unit 1. It explains how green plants use light energy to make glucose, what happens to that glucose and how the structure of a leaf is adapted for photosynthesis and gas exchange. You also need to understand limiting factors and how growers use them in glasshouses. The topic links to cells (chloroplasts), food tests (starch), respiration and the carbon cycle in ecology. Examiners regularly ask for the prescribed starch test on a destarched or variegated leaf, interpretation of rate graphs where a factor stops being limiting, and explanations of net gas exchange at different light intensities.

What you need to know

• Photosynthesis equations Carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll. Balanced: 6CO2 + 6H2O → C6H12O6 + 6O2. Glucose can be used for respiration or converted to starch for storage, cellulose for cell walls and proteins.
• Prescribed practical: starch test Destarch a plant by keeping it in the dark for 48 hours. Boil a leaf in water to kill it and break membranes, then heat it in ethanol in a water bath (no flame) to remove chlorophyll, rinse in water to soften, and add iodine. Blue-black shows starch; orange-brown shows none.
• Showing what is needed A variegated leaf shows chlorophyll is needed (only the green parts turn blue-black). Covering part of a leaf with foil shows light is needed. Putting a plant in a sealed container with sodium hydroxide or soda lime, which absorbs carbon dioxide, shows carbon dioxide is needed.
• Limiting factors Light intensity, carbon dioxide concentration and temperature can each limit the rate of photosynthesis. On a graph, where the line rises the factor on the x-axis is limiting; where it levels off, another factor is limiting. Growers raise light, carbon dioxide and temperature in glasshouses to increase yield.
• Gas exchange in leaves In bright light, photosynthesis is faster than respiration so there is a net uptake of carbon dioxide and release of oxygen. In the dark only respiration happens, so carbon dioxide is released. Hydrogencarbonate indicator shows these changes by changing colour.
• Leaf structure A thin, broad leaf gives a short diffusion distance and large surface area. Palisade mesophyll cells near the upper surface are packed with chloroplasts. Air spaces in the spongy mesophyll allow gases to diffuse, and stomata, opened and closed by guard cells, let carbon dioxide in and oxygen out. A waxy cuticle reduces water loss.

Common mistakes in 1.2 Photosynthesis and Plants

• Heating ethanol over a Bunsen flame. Ethanol is flammable, so it must be heated in a hot water bath with the flame turned off.
• Saying the iodine test shows that photosynthesis is happening right now. It shows starch is present, which is evidence that photosynthesis took place earlier.
• Stating that plants only respire in the dark. Plants respire all the time; in the light, photosynthesis is simply faster than respiration.
• Explaining a levelled-off graph by saying the plant has reached its maximum. Name the other limiting factor, for example carbon dioxide concentration or temperature.
• Writing that chlorophyll makes light energy. Chlorophyll absorbs light energy, which is used to make glucose.

Try three 1.2 Photosynthesis and Plants exam-style questions