Decay and the carbon cycleOxford AQA IGCSE Biology: Revision notes

Living organisms take materials from the environment for growth and other processes. These materials are returned to the environment either in waste, or when organisms die and decay.

Materials decay because they are broken down (digested) by microorganisms. Microorganisms are more active, and digest materials faster, in conditions that are:

• Warm — enzyme-controlled reactions inside the microorganisms speed up
• Moist — microorganisms need water to carry out life processes
• Aerobic — with plenty of oxygen available

The decay process releases substances, such as mineral ions, that plants need in order to grow.

In a stable community, the processes that remove materials from the environment are balanced by processes that return materials to it — the materials are part of a constant cycle. Plants and animals take up nutrients as they grow, and decay returns those same nutrients to the soil, air and water for other organisms to use. Without decay, nutrients would stay locked inside dead bodies and waste, and would not be available for new growth.

The constant cycling of carbon through the environment and living organisms is called the carbon cycle.

• Photosynthesis: green plants and algae remove carbon dioxide from the atmosphere during photosynthesis. The carbon is used to make carbohydrates, fats and proteins, which make up the bodies of the plants and algae
• Respiration: when green plants and algae respire, some of this carbon becomes carbon dioxide again and is released back into the atmosphere
• Feeding: when green plants and algae are eaten by animals, and those animals are eaten by other animals, some of the carbon becomes part of the fats and proteins in the bodies of the consumers
• Animal respiration: when animals respire, some of this carbon becomes carbon dioxide and is released into the atmosphere

At every stage, carbon moves either into an organism's body (through photosynthesis or feeding) or back out into the atmosphere as carbon dioxide (through respiration).

When plants, algae and animals die, some animals and microorganisms feed on their bodies. As these decomposers and detritus feeders respire, carbon is released into the atmosphere as carbon dioxide.

By the time microorganisms and detritus feeders have broken down all the waste products and dead bodies in an ecosystem, and cycled the materials as plant nutrients, all of the energy originally absorbed by green plants and algae during photosynthesis has been transferred (used up in life processes and lost, mostly as heat, to the surroundings). Carbon, unlike energy, is not lost from the system — it stays in the cycle, ready to be taken up again by plants during photosynthesis.

Carbon dioxide is not only released by respiration and decay. The combustion (burning) of wood and fossil fuels also releases carbon dioxide into the atmosphere. Fossil fuels formed from the remains of organisms that died millions of years ago, so burning them releases carbon that has been locked away for a very long time, adding it back into the short-term carbon cycle.

You should be able to trace a given carbon atom through the cycle and explain each step, for example: a carbon atom in atmospheric carbon dioxide is taken up by a plant during photosynthesis and built into a carbohydrate in a leaf; a rabbit eats the leaf, and the carbon becomes part of a protein in the rabbit's body; the rabbit respires, releasing some of the carbon as carbon dioxide; when the rabbit dies, microorganisms decay its body, respiring and releasing more carbon as carbon dioxide, which can then be taken up by a plant again.

When answering questions on this topic, always name the specific process involved (photosynthesis, respiration, feeding, or decay by microorganisms/combustion) rather than saying vaguely that carbon "moves" or "is released".