(a) Describe the water cycle using an annotated diagram (s) only. (b) What is the importance of water tc living organisms?

(a) The water cycle

The water cycle is the continuous circulation of water between the earth's surface and the atmosphere. The main stages are:

1. Evaporation: heat from the sun evaporates water from seas, rivers, lakes and the soil into water vapour. Plants add water vapour by transpiration and animals by respiration and sweating.
2. Condensation: the rising water vapour cools at higher altitudes and condenses into tiny droplets that form clouds.
3. Precipitation: when the droplets become large and heavy they fall as rain, hail or snow.
4. Collection / run-off: the water flows over the ground as run-off into rivers, lakes and seas, or sinks into the soil as ground water, and the cycle repeats.

(An annotated diagram should show the sun, the sea, arrows for evaporation and transpiration rising to a cloud, condensation in the cloud, precipitation as rain falling, and run-off returning to the sea.)

(b) Importance of water to living organisms

1. It is the medium in which all metabolic reactions and enzyme actions take place inside cells.
2. It is a universal solvent for transporting dissolved food, minerals, gases and waste products.
3. It is a raw material for photosynthesis in green plants.
4. It helps to regulate body temperature (through sweating and transpiration).
5. It maintains turgidity in plant cells and gives them support.
6. It aids digestion, absorption and the removal of excretory waste, and forms a large part of protoplasm.