Question 1
Why is carbon monoxide harmful to humans?
- It dissolves in blood plasma and makes it acidic
- It combines with haemoglobin so less oxygen is carried
- It destroys white blood cells
- It blocks the alveoli with mucus
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Answer: B – carbon monoxide binds to haemoglobin, reducing oxygen transport (1)
Question 2
Explain how burning fuels that contain sulfur can kill fish in a lake.
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- sulfur dioxide is released (1)
- it dissolves in rain water to form acid rain (1)
- acid rain lowers the pH of the lake so fish cannot survive (1)
- allow acidic / acidity for acid
Question 3
Name two greenhouse gases other than carbon dioxide. For each one, describe a human activity that increases the amount of it in the atmosphere.
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- first gas named: methane / nitrous oxide / CFCs / water vapour (1)
- a human activity that matches the first gas (1)
- second gas named (1)
- a human activity that matches the second gas (1)
- activities: methane – cattle / rice paddy fields / landfill; nitrous oxide – (nitrogen) fertilisers / vehicle exhausts; CFCs – old refrigerators / aerosols / air conditioners; water vapour – burning fuels
- reject carbon dioxide; the activity must match the gas named; ignore 'pollution' or 'factories' alone
Question 4
Explain how an increase in carbon dioxide in the atmosphere may lead to global warming.
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- the Earth's surface gives out infrared radiation (1)
- greenhouse gases / carbon dioxide absorb infrared radiation (1)
- and radiate it back towards the Earth (1)
- more carbon dioxide means more heat is trapped, so the average temperature rises (1)
- allow heat for infrared radiation; reject answers based on the ozone layer
Question 5
Untreated sewage is released into a river. Explain why the number of fish in the river falls.
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- sewage contains organic matter / is food for decomposers, so decomposers / bacteria increase (1)
- they use up oxygen in respiration (1)
- fish die from lack of oxygen (1)
- ignore 'sewage poisons the fish'
Question 6
Fertiliser is spread on a field next to a lake. After heavy rain, many fish in the lake die. Explain how the fertiliser could have caused the death of the fish.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear, logical and complete sequence from leaching of minerals to the death of fish, including algal bloom, loss of light, death of plants, increase in decomposers and loss of oxygen. |
| 2 | 3–4 | A sequence with most stages, but with one or two missing or unclear links, for example no mention of decomposers or of light being blocked. |
| 1 | 1–2 | One or two correct but isolated points, such as that fertiliser makes algae grow, without explaining how fish are affected. |
Indicative content
- heavy rain leaches nitrates / minerals from the fertiliser into the lake
- extra nitrates cause rapid growth of algae (algal bloom) on the surface
- the algae block light from plants lower down
- plants cannot photosynthesise and die
- decomposers / bacteria feed on the dead plants and algae and increase in number
- decomposers use up oxygen in aerobic respiration
- low oxygen concentration so fish die / suffocate
- biodiversity falls