Photosynthesis — Cambridge IGCSE Combined Science

Revision notes

5 short notes, in the order of the specification. Each one in short:

1. Photosynthesis and chlorophyll

   Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. The word equation is carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll. Chlorophyll is a green pigment in chloroplasts that absorbs light and transfers energy from light into energy in chemicals.

2. Investigating the need for chlorophyll, light and carbon dioxide

   Practical

   Testing a leaf for starch shows whether it has photosynthesised, because glucose made in photosynthesis is converted to starch. The plant is destarched in the dark for 24 to 48 hours first. The leaf is boiled, then heated in ethanol to remove the chlorophyll, and iodine turns from orange-brown to blue-black where starch is present.

3. Rate of photosynthesis and limiting factors

   Supplement

   A limiting factor is the factor in shortest supply, which stops the rate of photosynthesis from increasing. Light intensity, carbon dioxide concentration and temperature can each be limiting. Increasing a limiting factor raises the rate; increasing a factor that is not limiting makes no difference. On a graph, the factor is limiting where the line is still rising.

4. Investigating light, carbon dioxide and temperature

   PracticalSupplement

   The rate of photosynthesis can be measured by counting the oxygen bubbles released by pondweed such as Elodea per minute, or by collecting and measuring the gas. Change light intensity by moving a lamp, carbon dioxide with sodium hydrogencarbonate, or temperature with a water bath, keeping the other factors constant. Collecting the gas is more accurate.

5. Gas exchange in light and dark: hydrogencarbonate indicator

   PracticalSupplement

   Hydrogencarbonate indicator is a solution that changes colour with carbon dioxide concentration: yellow when high, red at atmospheric level and purple when low. An aquatic plant in bright light turns it purple because photosynthesis uses carbon dioxide faster than respiration releases it. In the dark the plant only respires, so the indicator turns yellow.