The carbon cycle
Carbon is an essential element for life on Earth and parts of each of the cells in our bodies are made from it. The The processes and events involved in recycling carbon in the environment. shows how atoms of this element can exist within different compounds at different times.
All cells - whether animal, plant or bacteria - contain carbon, because they all contain proteins, fats and carbohydrates. Plant cell walls, for example, are made of cellulose - a carbohydrate.
Carbon is passed from the atmosphere, as carbon dioxide, to living things, passed from one organism to the next in complex molecules, and returned to the atmosphere as carbon dioxide again. This is known as the carbon cycle.
Removing carbon dioxide from the atmosphere
Green plants remove carbon dioxide from the atmosphere by A chemical process used by plants to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis. Algae subsumed within plants and some bacteria are also photosynthetic.. The carbon becomes part of complex molecules such as proteins, fats and carbohydrates in the plants.
Returning carbon dioxide to the atmosphere
Organisms return carbon dioxide to the atmosphere by The chemical change that takes place inside living cells, which uses glucose and oxygen to release the energy that organisms need to live. Carbon dioxide is a by-product of respiration.. It is not just animals that respire. Plants and microorganisms do, too. Carbon dioxide is also released by combustion. The burning of fossil fuels releases large quantities into the atmosphere.
Passing carbon from one organism to the next
When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal. Decomposers and some animals, called Animals that obtain nutrients by consuming detritus (decomposing plant and animal tissue as well as waste e.g. faeces)., feed on waste material from animals, and the remains of dead animals and plants. The carbon then becomes part of these organisms.
Image caption, Stage one of the carbon cycle
Carbon enters the atmosphere as carbon dioxide from respiration and combustion.
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The three key processes and how they convert carbon are shown in the table below:
| Process | Carbon starts as | Carbon ends as |
| Photosynthesis | Carbon dioxide | Glucose |
| Respiration | Glucose | Carbon dioxide |
| Combustion (burning) | Fuel (eg methane or wood) | Carbon dioxide |
| Process | Photosynthesis |
|---|---|
| Carbon starts as | Carbon dioxide |
| Carbon ends as | Glucose |
| Process | Respiration |
|---|---|
| Carbon starts as | Glucose |
| Carbon ends as | Carbon dioxide |
| Process | Combustion (burning) |
|---|---|
| Carbon starts as | Fuel (eg methane or wood) |
| Carbon ends as | Carbon dioxide |