Cycles, food security and climate | Pearson Edexcel GCSE Biology Notes

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The key idea

Materials cycle through ecosystems. Human activity can reduce biodiversity, affect food security and alter the climate.

Cycles, food security and climateCarbon cycling links ecosystems, climate and reliable food supplies.

Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

The carbon cycle

The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide to be used again by plants in photosynthesis.Plants remove carbon dioxide from the air by photosynthesis and convert it into carbohydrates, fats and proteins.Carbon is passed along food chains when animals eat plants.Carbon is returned to the atmosphere as carbon dioxide through respiration of plants, animals and decomposers, and by the combustion of fossil fuels.

The water cycle

The water cycle provides fresh water for plants and animals on land before draining into the seas.Energy from the Sun causes evaporation of water from the surface of seas, lakes and land.The water vapour rises and cools, condensing to form clouds.The water then falls as precipitation such as rain or snow, providing fresh water before it runs back to the sea.

Decomposition

Decomposers such as bacteria and fungi break down dead organisms and waste, recycling materials back into the environment.The rate of decay is faster when it is warm, moist and there is plenty of oxygen, because these conditions increase the activity and reproduction of microorganisms.Gardeners and farmers provide ideal conditions in compost heaps to recycle nutrients quickly.

Food security and climate change

Food security is having enough food to feed a population.Threats include increasing human population, changing diets, new pests and pathogens, the cost of farming and environmental changes due to climate change.Increasing carbon dioxide and methane levels cause global warming, which can lead to rising sea levels, loss of habitats, changes in the distribution of species and reduced biodiversity.

Key terms

Definitions to learn

Carbon cycle

The recycling of carbon between organisms, the atmosphere and the environment.

Water cycle

The recycling of water through evaporation, condensation and precipitation.

Decomposer

A microorganism such as bacteria or fungi that breaks down dead material.

Combustion

The burning of fuels that releases carbon dioxide into the atmosphere.

Food security

Having enough food to feed a population.

Global warming

The rise in average global temperature caused by increasing greenhouse gases.

Worked example

Explain one way deforestation can increase atmospheric carbon dioxide.

Trees remove carbon dioxide by photosynthesis.

Fewer trees means less carbon dioxide is removed.

Burning or decay can also release carbon dioxide.

Final answer

Deforestation can raise carbon dioxide by reducing uptake and releasing stored carbon.

Exam habit

Separate the carbon cycle from the water cycle in answers.For deforestation, give two separate effects: reduced photosynthesis uptake AND release from burning or decay.Distinguish climate (long-term pattern) from weather (short-term conditions).

Watch out

Separate climate from weather: climate describes long-term patterns.

Examiner tips

How to score full marks

1In the carbon cycle, name the processes: photosynthesis removes carbon dioxide, while respiration, decomposition and combustion return it.
2For decomposition rate, link warmth, moisture and oxygen to increased microorganism activity, not just say it decays faster.
3When discussing climate change, give specific consequences such as rising sea levels, loss of habitats or reduced biodiversity.

Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Why are decomposers important in the carbon cycle?

Mark scheme

1.Link decay to carbon compounds.

They break down dead material and respire, returning carbon dioxide to the atmosphere.

2Give two factors that can threaten food security.

Mark scheme

1.Think environment, population and farming.

Any two from: population growth, changing diets, climate change, pests, pathogens, conflict or rising costs.

3How can maintaining biodiversity benefit ecosystems?

Mark scheme

1.Link diversity to stability.

Greater biodiversity can make ecosystems more stable and preserve resources for the future.

4Name the process by which plants remove carbon dioxide from the atmosphere.[1 mark]

Mark scheme

1.Recall how plants take in carbon.

Photosynthesis (1)

5Name two types of microorganism that act as decomposers.[1 mark]

Mark scheme

1.Recall the main decomposer groups.

Bacteria and fungi (1)

6State three conditions that increase the rate of decay.[3 marks]

Mark scheme

1.Think about what microorganisms need.

Warmth or higher temperature (1); moisture or water (1); plenty of oxygen (1)

7Describe how carbon is returned to the atmosphere as carbon dioxide.[3 marks]

Mark scheme

1.Name the processes that release carbon dioxide.

Respiration of plants, animals and microorganisms releases carbon dioxide (1); decomposition or decay of dead organisms by microorganisms releases carbon dioxide (1); combustion or burning of fossil fuels releases carbon dioxide (1)

8Explain how the increased combustion of fossil fuels can lead to reduced biodiversity through climate change.[4 marks]

Mark scheme

1.Link combustion to greenhouse gases.
2.Link warming to its consequences for species.

Burning fossil fuels releases carbon dioxide which is a greenhouse gas (1); higher levels of greenhouse gases cause global warming or climate change (1); this can cause rising sea levels and loss of habitats (1); and changes in the distribution of species, reducing biodiversity (1)

9Explain why a gardener turns their compost heap regularly and adds water to it.[4 marks]

Mark scheme

1.Link turning to oxygen supply and water to moisture.
2.Connect each to microorganism activity.

Turning the compost heap introduces oxygen (1); aerobic decomposer microorganisms need oxygen to respire and break down organic matter (1); adding water maintains moisture (1); decomposers are most active in moist conditions, so decay is faster and compost is produced more quickly (1).

10Carbon is found in organisms in the form of organic molecules. Describe the pathway by which carbon in a grass plant reaches the atmosphere as carbon dioxide.[4 marks]

Mark scheme

1.Trace carbon through the food chain to decomposers or respiration.

Carbon is in organic molecules such as glucose in the grass (1); a herbivore eats the grass and the carbon is incorporated into the animal's tissues (1); the animal and grass respire, releasing carbon dioxide to the atmosphere (1); when organisms die, decomposers break down their bodies and release carbon dioxide through respiration (1).

11Describe two threats to food security and suggest a technological solution for each.[4 marks]

Mark scheme

1.Name two threats.
2.Suggest one practical solution for each.

Threat 1: increasing human population means more food is needed (1); solution: developing GM crops with higher yields to produce more food from the same area (1). Threat 2: changing climate affects where crops can be grown / new pests and pathogens damage crops (1); solution: breeding or engineering drought-resistant or pest-resistant crop varieties (1).

12The oceans absorb large amounts of carbon dioxide from the atmosphere. Explain why scientists are concerned about the consequences of this for marine ecosystems.[5 marks]

Mark scheme

1.Link dissolved CO2 to ocean acidification.
2.State the effect on marine organisms.

Carbon dioxide dissolves in sea water to form carbonic acid (1); this lowers the pH of the oceans (ocean acidification) (1); many marine organisms such as corals and shellfish use calcium carbonate to make their shells and skeletons (1); lower pH dissolves calcium carbonate, making it harder for these organisms to build structures (1); damage to coral reefs reduces habitat for many species and reduces biodiversity (1).

13Evaluate the claim that switching from fossil fuels to renewable energy sources would solve the problem of climate change.[6 marks]

Mark scheme

1.Give ways in which the switch would help.
2.Give reasons why it might not fully solve the problem.
3.Draw a conclusion.

Burning fewer fossil fuels would reduce the amount of carbon dioxide and other greenhouse gases released (1); this would slow the rate of global warming and reduce the risk of rising sea levels and habitat loss (1); however carbon dioxide already in the atmosphere would remain for many decades (1); deforestation and agricultural methane also contribute to greenhouse gas levels and are not addressed by switching energy sources alone (1); additionally switching entirely to renewables would require massive infrastructure changes and time (1); overall the switch is necessary but not sufficient on its own — it must be combined with other measures such as reforestation and reducing deforestation (1).

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