The carbon cycle and decomposition - (CCEA)

Decomposition is the process where bacteria and saprophytic fungi break down dead organisms into simple compounds.

Steps

1. Bacteria/fungi secreting enzymesout onto the dead organisms.

2. Enzymes digest the organic matter outside their cells. This is known as extracellular digestion as it happens outside the cells.

3. The products of digestion are absorbed by the bacteria and fungi.

This process returns nutrients like carbon and nitrogen to the soil, air, and water (abiotic components) so plants can use them. Plants then support animals (biotic components). The remaining material forms humus in soil, which improves soil fertility.

How do you investigate the key features of the decay process?

Procedure

1. Place bread samples in aerobic (with oxygen) and anaerobic (without oxygen) setups.
2. Spray half the samples with water; leave the rest dry.
3. Store at -18°C (freezer), 4°C (fridge), and 20°C (room) for two weeks.
4. Observe and record signs of decay.

Results

Key: ✓ = signs of decay x = no signs of decay

Only 20°C is a suitable temperature, as the other two temperatures are too cold and the other columns have one or more key condition needed for decay to be absent.

Conclusion

Decomposition occurs quickly when:

• the temperature is warm
• water is present
• oxygen is available

In ecosystems, plants absorb inorganic nutrients like carbon dioxide (from the carbon cycle) and nitrates, (from the nitrogen cycle) to build complex molecules.

These molecules pass through food chains and, when organisms die, decomposers break them down, returning carbon and nitrogen to the atmosphere and soil for reuse.

A balanced carbon cycle is essential.

Carbon is a major component in carbohydrates, fats and proteins.

The carbon cycle involves the exchange of carbon between living organisms (biotic) and their atmosphere (abiotic).

In the carbon cycle, carbon is constantly removed from, and returned to, the environment keeping it in balance.

Processes involved in the carbon cycle are:

Photosynthesis

Plants absorb carbon dioxide from the atmosphere use it to form sugars and starch. This is the only process that reduces carbon dioxide levels in the air.

Feeding

Carbon in biological molecules moves through the food chain as animals eat plants and other animals.

Respiration

All living organisms (plants, animals, and decomposers) release carbon dioxide into the atmosphere when they respire. This is a form of excretion.

Fossilisation

If conditions are not favourable for the process of decomposition, dead organisms decay slowly or not at all. These organisms build up and, if compressed over millions of years, can form fossil fuels (coal, oil or gas).

Combustion

Burning fossil fuels releases stored carbon back into the atmosphere as carbon dioxide.

Excretion

Waste materials (eg urine) are released by organisms. Decomposers break these down, releasing carbon dioxide to the air.

Egestion

Animals remove undigested food as faeces, which contain carbon. Decomposers break faeces down, releasing carbon dioxide into the air.

Decomposition

Bacteria and saprophytic fungi break down complex carbon compounds in dead organisms, faeces and urine into simpler carbon compounds, returning them to the atmosphere or soil.