Carbon and water cycles, decomposition · GCSE Biology

Cycling materials

Materials in the living world are recycled to provide the building blocks for future organisms.
Decomposers (bacteria and fungi) break down dead plants and animals. This returns carbon to the atmosphere as carbon dioxide and mineral ions to the soil.

Photosynthesis removes carbon dioxide from the atmosphere; the carbon becomes part of glucose and other compounds in plants.
Carbon passes along food chains when animals eat plants and other animals.
Respiration by plants, animals and decomposers returns carbon dioxide to the atmosphere.
Combustion of fossil fuels and wood also releases carbon dioxide. Fossil fuels formed from organisms that did not fully decay millions of years ago.

Energy from the Sun evaporates water from seas and lakes, and plants lose water by transpiration.
Water vapour cools and condenses to form clouds, then falls as precipitation (rain, snow).
Water flows through the land to rivers and the sea. This provides fresh water for plants and animals on land.

The rate of decay depends on temperature, water and the availability of oxygen, because decomposers need these to grow and respire.
Gardeners make compost by keeping dead plant material warm, moist and aerated so it decays quickly. Compost is used as natural fertiliser.
Anaerobic decay produces methane gas. Biogas generators use this to make fuel.
Required practical: the effect of temperature on the rate of decay can be measured by the fall in pH as lipase breaks down the fat in milk, using an indicator such as phenolphthalein.

Changes in the environment affect the distribution of species. They may be seasonal, geographic or caused by human activity.
Important factors are temperature, water availability and the composition of atmospheric gases.

Plants need nitrates from the soil to make proteins. Nitrogen gas makes up about 78% of the air, but it is unreactive, so plants and animals cannot use it directly.
Nitrogen-fixing bacteria, in the soil and in the root nodules of legumes such as peas, beans and clover, turn nitrogen gas into ammonium compounds. Lightning also fixes some nitrogen.
Decomposers break down proteins in dead organisms, and urea in animal waste, into ammonium compounds.
Nitrifying bacteria turn ammonium compounds into nitrites and then nitrates, which plants absorb through their roots.
Denitrifying bacteria turn nitrates back into nitrogen gas. They live in waterlogged soil, where there is little oxygen.
Farmers add nitrates with fertilisers (manure or artificial fertiliser) and by crop rotation, growing legumes in some years.

Decomposer: An organism, such as a bacterium or fungus, that breaks down dead material.
Carbon cycle: The movement of carbon between the atmosphere, living things and fossil fuels.
Water cycle: The continuous movement of water by evaporation, condensation and precipitation.
Combustion: Burning, which releases carbon dioxide.
Precipitation: Water falling from clouds as rain, sleet, hail or snow.
Compost: Decayed plant material used as a natural fertiliser.
Biogas: A fuel containing methane, made by anaerobic decay.
Anaerobic decay: Decay without oxygen, which produces methane.
Nitrogen-fixing bacteria: Bacteria that turn nitrogen gas from the air into compounds plants can use.
Nitrifying bacteria: Bacteria that turn ammonium compounds into nitrates.
Denitrifying bacteria: Bacteria that turn nitrates back into nitrogen gas.