4dHuman influences on the environment

Where these gases come from

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Sulfur dioxide → acid rain

1. SO₂ is released into the air.
2. It dissolves in water droplets in clouds, forming an acidic solution (sulfuric acid).
3. It falls as acid rain, often a long way from where the gas was produced.

Biological consequences of acid rain

In lakes and rivers

• The pH of the water falls. Fish eggs and young fish die; many invertebrates cannot survive below about pH 5.
• Acid rain leaches aluminium ions out of soil into the water; these damage fish gills, causing mucus build-up so the fish cannot absorb enough oxygen.
• Fewer species survive → biodiversity falls, and food chains break down.

On land

• Damages the leaves (and conifer needles) of trees directly — yellowing and leaf loss, so less photosynthesis and reduced growth.
• Leaches mineral ions such as magnesium and calcium out of the soil. Without magnesium a plant cannot make chlorophyll, so leaves yellow and growth is stunted.
• Lichens are very sensitive to SO₂, so they are useful indicator species — few or no lichen species grow near heavily polluted areas.

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Carbon monoxide → less oxygen in the blood

• CO is colourless and odourless, so people do not notice it.
• It binds to haemoglobin in red blood cells, forming carboxyhaemoglobin. The binding is very difficult to reverse.
• That haemoglobin can no longer carry oxygen, so the oxygen-carrying capacity of the blood falls.
• Less oxygen reaches the tissues → less aerobic respiration, less energy released → headaches, dizziness, drowsiness, and at high concentrations death.

Common mistake: carbon monoxide does not cause acid rain and is not a greenhouse gas. Keep its one job clear: it stops haemoglobin carrying oxygen.

Examiner tip. Examiners repeatedly report candidates writing carbon monoxide when they mean carbon dioxide (and vice versa). Before you write either name, check which effect you are describing: carbon di­oxide = greenhouse gas; carbon mon­oxide = binds to haemoglobin. Also watch the command word — 'describe' acid rain means say what happens; 'explain' means give the reason (e.g. because aluminium ions damage the gills).

The five greenhouse gases you must know

Learn this list exactly — nothing else counts:

• water vapour
• carbon dioxide
• nitrous oxide
• methane
• CFCs (chlorofluorocarbons)

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How human activities increase each one

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Why these gases matter

All five absorb heat (long-wavelength radiation) radiated from the Earth's surface and re-radiate some of it back down. More of these gases in the atmosphere = more heat trapped (see the note on the enhanced greenhouse effect).

Common mistake: CFCs are famous for damaging the ozone layer, but on this specification they appear as greenhouse gases. Ozone depletion and the greenhouse effect are two different problems — don't drift into ozone unless the question asks.

Common mistake: carbon monoxide, sulfur dioxide and oxygen are not on the greenhouse gas list. Nitrous oxide (N₂O) is — do not muddle it with nitrogen dioxide from exhausts.

Examiner tip. Reports note that 'a significant number of candidates incorrectly wrote carbon monoxide' in questions about carbon dioxide. When naming greenhouse gases, write the full word dioxide clearly. Also, when giving human activities, name a specific activity ('burning fossil fuels in power stations', 'cattle farming') — vague answers like 'pollution' or 'factories' score nothing.

The greenhouse effect (this part is natural and necessary)

1. Radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it.
2. The Earth re-radiates this energy as longer-wavelength heat radiation.
3. Greenhouse gases in the atmosphere absorb this heat radiation and re-radiate some of it back towards the Earth.
4. Heat is therefore trapped, keeping the Earth warm enough for life.

The enhanced greenhouse effect

Human activities have increased the concentration of greenhouse gases, so more heat is absorbed and re-radiated back to the Earth and less escapes into space. This extra trapping is the enhanced greenhouse effect, and it leads to global warming — a rise in the average temperature of the Earth's atmosphere and oceans.

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Consequences of global warming

Ice, sea level and habitats

• Melting of polar ice caps and glaciers, plus expansion of warmer seawater → rising sea levels → flooding of low-lying land, loss of homes and farmland.
• Loss of habitats (e.g. polar habitats), so species that cannot move die out → reduced biodiversity / extinction.

Climate and food

• Changed rainfall patterns: some regions get drought and desertification, others get flooding → crop failure and famine.
• More extreme weather events, e.g. storms.

Living organisms

• Changes in the distribution of species — many move towards the poles or to higher ground; migration and flowering/breeding times shift.
• Insect pests and disease-carrying insects (e.g. mosquitoes) spread into new areas.

Effect on enzymes (a favourite follow-on question)

• A rise in body/environmental temperature above the optimum means enzymes become denatured.
• The active site changes shape, so the substrate no longer fits, enzyme–substrate complexes are not formed and the rate of reaction falls — metabolism is disrupted and organisms may die.

Common mistake: never write that heat 'kills enzymes'. Enzymes are denatured (they are not alive); bacteria are killed, not denatured. Examiners specifically report this confusion.

Examiner tip. Two reported failings on this material: candidates stop after saying 'enzymes are affected' instead of continuing to explain the change in enzyme structure (active site changes shape) and function (substrate no longer fits, reaction rate falls); and candidates simply repeat phrases from the passage. Always take the answer one step further into consequences for the organism.

Both sewage and leached fertiliser damage water in the same basic way: they cause bacteria to use up the dissolved oxygen. Learn the sequences as numbered chains — the marks are for the order.

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Pollution of water by sewage

Untreated sewage contains organic waste (faeces, food waste) and pathogens.

1. Oxygen depletion

1. Sewage releases organic matter into the water.
2. Aerobic decomposer bacteria feed on it and reproduce rapidly — their numbers rise steeply.
3. These bacteria respire aerobically, using up the dissolved oxygen in the water (the biochemical oxygen demand, BOD, rises).
4. Fish and other aerobic organisms suffocate and die — only tolerant species such as sludgeworms and bloodworms survive.
5. Biodiversity falls; the food web collapses.

2. Disease

• Sewage contains pathogens. If it contaminates drinking water it spreads diseases such as cholera and typhoid.

3. Extra minerals

• Sewage also contains nitrates and phosphates, so it can cause eutrophication as well.

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Eutrophication from leached fertiliser

1. Farmers spread fertiliser containing nitrates (and phosphates) on fields; more is applied than the crop can absorb.
2. Rain leaches the soluble mineral ions through the soil into rivers and lakes (also washed in as run-off).
3. The extra nitrate causes rapid growth of algae at the surface — an algal bloom.
4. The algae block the light reaching submerged water plants.
5. Those plants cannot photosynthesise, so they die.
6. Decomposer bacteria feed on the dead plants and dead algae, multiply rapidly and respire aerobically.
7. The concentration of dissolved oxygen falls.
8. Fish and aerobic invertebrates die — the water may end up almost lifeless.

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Comparing the two

Common mistake: don't say the algae 'take all the nutrients' or 'use up all the oxygen' from the plants. The mark is for blocking light, and it is the decomposer bacteria that consume the oxygen.

Examiner tip. Examiners report that weaker candidates blur the language around bacteria and enzymes — write bacteria are killed / bacteria respire aerobically and never 'bacteria are denatured'. Also write dissolved oxygen (in the water), not just 'oxygen', and make sure the chain ends with a named consequence such as 'fish die and biodiversity falls'.

Forests are cleared for timber, farmland, cattle grazing, roads and building. Four consequences are named in the specification — learn all four.

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1. Leaching

• Tree roots normally absorb mineral ions such as nitrates from the soil.
• With the trees gone, and with no canopy to intercept the rain, water passes straight down through the soil.
• Soluble mineral ions are washed (leached) out of the topsoil, below the reach of roots, and into rivers.
• The soil becomes infertile, so crops grow poorly; the minerals entering rivers can cause eutrophication.

Common mistake: decomposition does not stop. Decomposers still release mineral ions from dead material — the problem is that the minerals are leached away because no tree roots take them up.

2. Soil erosion

• Roots normally bind the soil together and the canopy intercepts rainfall, so raindrops do not hit the soil directly.
• Without trees, heavy rain washes the loose topsoil away into rivers.
• Consequences: land can no longer support crops; rivers become silted and cloudy, blocking light for aquatic plants; increased flooding.

3. Disturbance of evapotranspiration

• Trees absorb water through their roots and lose it as water vapour by transpiration; together with evaporation this is evapotranspiration.
• Fewer trees → less water vapour returned to the atmosphere → less cloud formation and less rainfall locally → drier climate, possible drought and desertification.
• Water that is not taken up by roots runs off the surface instead, increasing flooding and soil erosion.

4. The carbon cycle and the balance of atmospheric gases

• Fewer trees means less carbon dioxide is removed from the atmosphere by photosynthesis, and less oxygen is released by photosynthesis.
• Burning (slash and burn) the trees releases the carbon stored in their biomass as CO₂.
• Felled timber and roots left to rot are broken down by decomposers (microorganisms), which respire and release CO₂.
• Net result: the concentration of carbon dioxide in the atmosphere rises, contributing to the enhanced greenhouse effect and global warming.

Also: loss of habitats → loss of species / reduced biodiversity.

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Quick summary

Examiner tip. The most-reported failing here is not mentioning removal of carbon dioxide by photosynthesis — always write 'fewer trees, so less CO₂ is removed from the atmosphere by photosynthesis' as well as 'burning releases CO₂'. Two more traps: less photosynthesis means less oxygen released (candidates have written that more photosynthesis reduces oxygen), and if a diagram or data is given, refer to it — don't claim decomposition and mineral release decrease when the diagram shows otherwise.