B1.6 Waste materials from plants and animals

Decay is the process that occurs when dead animals, dead plants and waste materials such as sewage (human waste) and plant waste (compost) are broken down (decomposed) by microorganisms found in soil. Useful nutrients stored in the dead tissue are then released in to the soil and used by producers (plants) to help them grow.

Food can be preserved by removing the conditions needed for decay to happen. Freezing or canning are commonly used food preservation techniques.

Candidates should use their skills, knowledge and understanding to evaluate the necessity and effectiveness of schemes
for recycling organic kitchen or garden waste.

Decay Processes

Detritus feeders (e.g. maggots and earthworms) and decomposers (bacteria and fungi) are the main microorganisms that carry out the decay process.

Microorganisms are more active and decompose faster in warm, moist, aerobic conditions.

Microorganisms carry out respiration, as part of the decay process, so release heat and carbon dioxide gas into the atmosphere. This explains why the temperature inside compost bins can be warm (up to 60ºC).

The decay process can be replicated inside a compost bin to produce compost that can be used to help plants grow.

In a stable community, the processes that remove materials are balanced by processes that return materials.
The materials are constantly cycled.

Some practice questions

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Video clips

BBC What happens in a compost heap

BBC Decomposers and Decay (compost heap)

Complete decomposition of a baby rabbit in 4 days

BBC Bang Goes the Theory What makes food rot

My GCSE Science (decay)

The Carbon Cycle

This occurs because carbon dioxide gas in the atmosphere is chemically converted to glucose and other forms of carbon (such as fossil fuels) by a series of chemical processes. Some processes will also also convert glucose and other forms of carbon back to carbon dioxide gas. This occurs in a continuous manner, hence, there is a constant cycling of carbon known as the carbon cycle.

When green plants and algae are eaten by animals and these animals are eaten by other animals, some of the carbon becomes part of the fats and proteins that make up their bodies.

In terms of chemical equations, there are three main chemical reactions occurring in the carbon cycle:

PHOTOSYNTHESIS (only in plants/algae) Carbon Dioxide + Water + Sunlight Energy → Glucose + Oxygen
RESPIRATION (in ALL living cells) Glucose + Oxygen → Carbon Dioxide + Water + Heat Energy
COMBUSTION (BURNING) of fossil fuels Fossil Fuel + Oxygen → Carbon Dioxide + Water + Heat Energy

These equations show how the four main processes of the carbon cycle change the amount of carbon dioxide in the atmosphere.

Process	Effect on carbon dioxide (CO₂) gas in the atmosphere
1. Photosynthesis		CO₂removed from atmosphere
2. Respiration by animals and plants	CO₂ added to atmosphere
3. Decay, which involves respiration of microorganisms	CO₂ added to atmosphere
4. Combustion (burning) of fossil fuels	CO₂ added to atmosphere

Examining each process in greater detail:

1. PHOTOSYNTHESIS Carbon is ‘removed’ from the atmosphere
Carbon dioxide is removed from the environment by green plants and algae for photosynthesis.

Carbon dioxide gas is absorbed by plant leaves, water is absorbed by plant roots, and these are chemically combined together using sunlight energy, to make glucose and oxygen gas.

The carbon from the carbon dioxide is used to make carbohydrates, fats and proteins, which make up the body of plants and algae.

2. RESPIRATION Carbon is ‘added’ to the atmosphere by animals, plants and algae

Respiration is a chemical reaction that is carried out by ALL cells and provides the cell with essential energy.
During respiration, animal and plant cells convert some of their stored glucose into carbon dioxide and releasing it into the atmosphere.

3. RESPIRATION Carbon is ‘added’ to the atmosphere by respiration of microorganisms carrying out decay
Some animals (detritivores) and microorganisms (bacteria and fungi) feed on dead animals, dead plants and waste (e.g. faeces) resulting in decomposition (break down) and decay of the dead material. The microorganisms that carry out the decay process need energy to perform the decay, and they obtain this through cell respiration.

Hence, the decay of dead animal and plant tissues will release carbon dioxide into the atmosphere as carbon dioxide is being made by the respiration of the detritivores and microorganisms.

At the end of the decay process, chemical compounds are made and absorbed by the soil. These compounds are used by plants as nutrients and help them to grow.

4. COMBUSTION/BURNING Carbon is ‘added’ to the atmosphere by combustion of wood and fossil fuels
Combustion of wood and fossil fuels releases carbon dioxide into the atmosphere.
Fossil fuels (coal, petrol and methane gas) are made from dead animals and plants that have been subjected to very high underground pressure and compaction (a type of squeezing) over millions of years.