Interaction of living and non-living things

Water cycle refers to how water circulates in the environment. Movement of water in the environment occurs as shown in the figure below:

1. Ground water and run-off (water from rain) flow into streams and rivers,
2. The streams and rivers flow into lakes and oceans
3. Water evaporates into the atmosphere from water bodies such as oceans and lakes and from plants through transpiration
4. The evaporated water precipitates to form water vapor. Water vapor condenses to form clouds
5. Wind causes clouds to move, for example from above the ocean to above the
6. Rain falls and is absorbed by plants or forms groundwater and run-off. The cycle thus begins

Forests act as water catchment areas and prevent excess loss of water from the land. Wetlands, such as swamps and marshes, help to control flooding and are also important for water purification.

Carbon cycle refers to a biochemical cycle in the environment where by carbon dioxide is taken up from the atmosphere and incorporated into the plant tissues during photosynthesis.

Carbon cycle

Carbon dioxide from the atmosphere is absorbed by plants and used for photosynthesis. These plants serve as food for herbivores, which are in turn eaten by carnivores.

When plants and animals die, microorganisms cause decomposition and carbon dioxide is released into the atmosphere.

The remains of plants and animals after millions of years result in the formation of fossil fuels such as coal, natural gas and oil. When these fuels are burnt, they release carbon dioxide into the atmosphere.

Respiration of living things also releases carbon dioxide into the atmosphere. The carbon dioxide is absorbed by plants and then the cycle starts again.

Nitrogen cycle refers to a biochemical cycle in the environment whereby nitrates in the soil are taken up by plant roots and may pass along food chains into animals absorb it in this form. It must first be converted into either nitrates or ammonium compound. Figure below shows the nitrogen cycle.

1. Nitrogen-fixing bacteria in the root nodules of legumes plant carry out fixation by converting atmospheric nitrogen to nitrates.
2. Lightning converts atmospheric nitrogen into nitrates. The nitrates get into the soil and are later absorbed by plants.

Plants use nitrogen compounds to produce plant proteins. Plants are eaten by animals. Animals use the nitrogen to produce animal proteins. When plants and animals die, decomposers such as bacteria and fungi feed on them. The decomposers release ammonia gas (NH3) which contains nitrogen. The ammonia is converted into nitrites and then nitrates by bacteria. Denitrifying bacteria release nitrogen from nitrates back into the atmosphere.