(a) With reference to the food chain, explain the role of the followings; (i) green plants; (ii) herbivores; (iii) carnivores; (iv) decomposers (b) With the...

(a) Roles in the food chain

• (i) Green plants: they are the producers. Using chlorophyll they trap sunlight and manufacture their own food (glucose) from carbon dioxide and water by photosynthesis. They form the first trophic level and are the source of food and energy for all other organisms in the chain.
• (ii) Herbivores: they are the primary consumers. They feed directly on green plants and convert plant material into animal tissue, passing energy on to the next level.
• (iii) Carnivores: they are the secondary and tertiary consumers. They feed on herbivores (and on other carnivores), transferring energy along the food chain and helping to control the population of the animals they eat.
• (iv) Decomposers: bacteria and fungi that break down dead plants, dead animals and waste matter, releasing simple substances and mineral nutrients back into the soil and air, so that nutrients are recycled for use by green plants.

(b) The carbon cycle

The carbon cycle is the continuous circulation of carbon between the atmosphere, living organisms and the environment. A diagram should show carbon dioxide in the atmosphere at the centre with arrows into and out of it. The main processes are:

1. Photosynthesis: green plants remove carbon dioxide (CO₂) from the atmosphere and fix it into organic food (carbohydrates).
2. Feeding: carbon passes from plants to animals when herbivores eat plants and carnivores eat herbivores.
3. Respiration: plants and animals break down food and release CO₂ back into the atmosphere.
4. Death and decay: when plants and animals die, decomposers (bacteria and fungi) break down their bodies and release CO₂ during decomposition.
5. Combustion: burning of wood and fossil fuels (coal, petroleum, natural gas) releases CO₂ into the atmosphere.
6. Fossilisation: over millions of years the remains of dead organisms form fossil fuels which store carbon.

These processes keep the amount of carbon dioxide in nature roughly in balance.