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  1. Describe the carbon cycle, limited to photosynthesis, respiration, feeding, decomposition, fossilisation and combustion

For this learning objective, all you really have to do is study the diagram, but for your sake, I’m writing down all the important points:

Carbon moves into and out of the atmosphere, mainly in the form of Carbon dioxide.

Plants take carbon dioxide out of the air by photosynthesis, and convert it into organic materials (carbohydrates, fats and proteins).

Herbivores eat plants, obtaining carbon compounds in the process.

Carnivores gain carbon compounds by eating other animals.

Animals and plants release carbon back into the air, in the form of carbon dioxide, through respiration.

When organisms die, they usually decompose. Decomposers breakdown the organic molecules through the process of respiration to gain energy, releasing carbon dioxide back into the atmosphere.

If a dead organism does not decompose, the carbon is trapped in its body. It becomes a fossil fuel over time.

Combustion of fossil fuels release carbon dioxide back into the air.


  1. Discuss the effects of the combustion of fossil fuels and the cutting down of forests on the oxygen and carbon dioxide concentrations in the atmosphere

Carbon dioxide is a greenhouse gas. This means it absorbs heat.

The greenhouse effect is natural, and good, but in excess, can have disastrous effects such as the melting of ice bergs (destroying ecosystems near the poles), the rise of sea levels (causing the flooding of many coastal areas), heat strokes which can lead to death in many countries in the tropics, etc. So when there is an excess of CO2 in the atmosphere, global warming accelerates, which, to put it lightly, is bad.

Photosynthesis takes CO2 out of the air and replaces it with O2. Respiration takes O2 out of the air and replaces it with CO2. Therefore, photosynthesis and respiration mostly cancel each other out, so they have little effect on the balance of CO2 in the air.

When fossil fuels are burnt, the carbon in the fuels combine with the oxygen in the air, and forms carbon dioxide. This process is called combustion. This is thought to increase the carbon dioxide levels in the atmosphere.

Cutting down trees reduces the amount of photosynthesis taking place, so less CO2 is being taken out of the air. This means atmospheric carbon dioxide levels rise and atmospheric oxygen levels fall.

Therefore, the combustion of fossil fuels and the cutting down of trees have a negative effect on the atmosphere.


  1. List the undesirable effects of deforestation as an example of habitat destruction, to include extinction, loss of soil, flooding and increase of carbon dioxide in the atmosphere

Deforestation is the act of clearing a wide area of trees. It involves cutting down many trees, and therefore destroying the habitats of many forms of life.

The negative effects of deforestation include species extinction, loss of soil, flooding, and the increase of carbon dioxide in the atmosphere.


  1. Explain the undesirable effects of deforestation on the environment
  • Extinction through habitat loss – the destruction of habitats and/ or food sources for animals results in fewer resources for animals and plants to survive. It should also be noted that the destruction of forest habitats also reduces the diversity of plants and animals, thus disrupting several food chains. The combined effect is the reduction in population size of many forms of life, sometimes leading to extinction.
  • Loss of soil – Less trees and flora in general mean that there are less roots to hold the soil. This means that each time it rains, a thin layer of soil is washed away. This causes soil erosion and the leaching of minerals (leaching is when a soluble chemical or mineral is washed away from the soil by rainwater). The eventual result is that the land becomes a desert.
  • Flooding – soil erosion is washed into rivers, causing them to fill up or become blocked. This causes flooding. The loss of flora also means that there are no plant roots to take up rainwater, which means more rainwater washes into nearby streams and rivers. This makes flooding easier.
  • Carbon dioxide build-up – Forests have high rates of photosynthesis, which means a great deal of carbon dioxide is removed from the atmosphere by the flora in forests. Therefore, deforestation means that a lot less carbon dioxide will be removed from the atmosphere, causing an increase in the CO2 levels in the atmosphere.


  1. State the sources and effects of pollution of water (rivers, lakes and the sea) by chemical waste, discarded rubbish, untreated sewage and fertilisers

As the population increases, the volume of waste and pollution increases too.

Chemical waste dumped into water bodies, like rivers or the sea, can make living conditions toxic to the local aquatic life. This could result in a huge amount of death, and/ or the migration of local aquatic species to elsewhere. This would negatively impact the biodiversity of the area.

Discarded rubbish in water bodies presents a major hazard to the local aquatic life. Aquatic animals can accidently ingest rubbish, get rubbish trapped in their airways and choke on it, get trapped in rubbish, become strangled by rubbish, etc. A lot of rubbish can come from companies and businesses dumping their waste into water bodies. It also comes from individuals, like you and me, littering. Whether or not we leave rubbish in the water itself, heavy rains can wash it into nearby water bodies.

Untreated sewage entering water provides a source of food for decomposing bacteria. As a result, the population of these bacteria rapidly increase, and they use up the dissolved oxygen in the water by aerobic respiration. This leaves much less dissolved oxygen in the water, making it very difficult for other aquatic organisms, like fish and insects, to survive. When sewage is treated before entering water bodies, lots of oxygen is provided by stirring the waste or injecting jets of compressed air. This allows microorganism to completely break down the waste before it enters any body of water.

When farmers use too much fertiliser, especially chemical fertilisers, they create an environmental hazard for nearby water bodies. During heavy rains, this fertiliser can be washed off the ground and towards nearby water, like a pond. When lots of fertiliser reaches the pond, eutrophication occurs. The fertiliser results in excess growth of plants. When they die, there is lots of decomposition by decomposers, the decomposers use up the oxygen in the pond, causing other aquatic life to suffocate.


  1. Explain the process of eutrophication of water in terms of:
    • Increased availability of nitrate and other ions
    • Increased growth of producers
    • Increased decomposition after death of producers
    • Increased aerobic respiration by decomposers
    • Reduction in dissolved oxygen
    • Death of organisms requiring dissolved oxygen in water

This has already been explained, however, I’ll do it again here:

Heavy rains can wash excess chemical fertilisers out off the ground and into nearby bodies of water. This process is called leaching.

When fertilisers are washed into a water body (e.g. a pond), there are a lot more ions available for plant growth, e.g. nitrate ions.

This causes an increased growth in producers, like aquatic plants and algae.

Eventually, these producers will die. This leaves a large amount of food for decomposers, so the decomposer population increases and there is increased decomposition. This means the decomposers increase the amount of aerobic respiration happening in the pond, and use up the dissolved oxygen.

This causes the death of organisms that require dissolved oxygen in the water, like fish and insects.

Note: the above explanation is what your syllabus wants you to know, however, some papers may require the following knowledge:

the excessive fertiliser also causes algal bloom (lots and lots of algae grow on the surface of the water, forming a sheet of algae). This can prevent sunlight from reaching plants at the bottom of the water, causing those plants to die. This provides more food for decomposers. The large population of algae may also use  up oxygen in respiration.




Notes submitted by Sarah.

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