9 3 cycles within ecosystems

The water cycle is the series of processes by which water moves through an ecosystem.

Water enters the atmosphere during the water cycle by:

During the water cycle, water vapour in the atmosphere condenses to form clouds.

True. Water from the atmosphere returns to the earth as precipitation, e.g. rain or snow.

Water can be made potable, or drinkable, as follows:

Carbon is taken out of the atmosphere in the form of carbon dioxide by plants during photosynthesis.

When primary consumers eat a plant the following may happen to the stored carbon:

Carbon is returned to the atmosphere when it is released as carbon dioxide by the following processes:

Combustion releases carbon dioxide into the atmosphere. This occurs during the combustion of fossil fuels, or of other organic matter such as wood.

True. Fossil fuel use releases carbon dioxide into the atmosphere by the process of combustion. This raises the atmospheric carbon dioxide concentration.

Mass deforestation (cutting down many trees) affects the carbon cycle as follows:

Nitrogen is required by living organisms to make proteins.

False. Neither plants nor animals can absorb nitrogen gas (N2) from the air. N2 needs to be fixed to produce nitrates before nitrogen can be absorbed by plants and passed on to animals.

Plants absorb nitrogen from the soil in the form of nitrates.

Nitrogen gas can be converted into a usable form as follows:

Animals get the nitrogen they need from plant proteins or from the tissues of other animals when they eat.

True. Decomposers break down proteins in the dead and waste material, allowing nitrogen to return to the soil.

False. Denitrifying bacteria take nitrates out of the soil and convert them back into nitrogen gas, reducing soil fertility and plant growth.

Farmers often need to use fertilisers because harvesting crops can disrupt the nitrogen cycle. The nitrogen contained within the tissues of crop plants is not returned to the soil by decomposition, so fertilisers are needed to replace lost nitrates (as well as other minerals).

The purpose of crop rotation is to reduce the problem of soil depletion. Different crops have different nutrient requirements, so growing different crops each year means that the same nutrients are not constantly being removed from the soil. Farmers can also introduce crops that aid soil fertility into crop rotation, e.g. legumes with nitrogen fixing bacteria in their root nodules.

Decomposition is the break down of organic material by, e.g. bacteria and fungi.

The rate of decay is influenced by: pH can also affect the activity of decomposers.

Oxygen availability impacts the rate of decay because decomposers require oxygen for respiration. If oxygen levels become depleted decomposers will die and decomposition will slow down or stop.

True. Higher temperatures increase the rate of decay. This is because the enzyme controlled reactions required for decomposition occur faster at higher temperatures.

True. Water availability is low inside dried foods, meaning that conditions are not favourable for the growth of the microorganisms that cause decay.

Vacuum-packing aids food preservation because it removes oxygen from the storage environment. This means that decay organisms cannot respire.

Compost develops more quickly inside insulated compost bins because insulation retains heat energy, raising the temperature of the compost heap. This allows the enzyme-controlled reactions of decay to occur more quickly.

The rate of decay can be calculated using the formula: rate of decay = mass lost ÷ time taken