B4d4SD0 · Section B4: Ecology and the environment

Human influences on the environment

Key learnings

Two air pollutants — sulfur dioxide and carbon monoxide
Sulfur dioxide ( $\text{SO}_2$ ): made when the sulfur impurities in fossil fuels burn; it dissolves in rain to form acid rain, whose biological effects are killing plants/trees and killing aquatic animals (by lowering the pH of soil and water). Carbon monoxide (CO): made by incomplete combustion; it binds to haemoglobin as carboxyhaemoglobin, reducing the oxygen the blood can carry. Eroding stone buildings is a chemical, not a biological, effect.
Greenhouse gases, the enhanced effect and global warming
The two greenhouse gases named in 4SD0 are carbon dioxide ( $\text{CO}_2$ ) and methane ( $\text{CH}_4$ ). The greenhouse effect is the natural trapping of heat (infra-red radiation) by these gases. Human activity raises their levels: burning fossil fuels and deforestation raise $\text{CO}_2$ ; cattle, rice paddy fields and landfill release methane. The enhanced greenhouse effect — extra trapping from higher concentrations — causes global warming (melting ice caps, rising seas, extreme weather).
Water pollution — sewage and eutrophication
Sewage: organic-rich waste; decomposer bacteria feed on it and multiply, and as they respire aerobically they use up the dissolved oxygen. Eutrophication: excess fertiliser leaches nitrates/phosphates into water $\rightarrow$ algal bloom $\rightarrow$ the bloom blocks light so plants below die $\rightarrow$ decomposer bacteria feed on the dead material and respire, using up the dissolved oxygen $\rightarrow$ aquatic animals die. The decomposers, not the algae, remove most of the oxygen.

Common mistakes

Drawn from real examiner reports.

Sulfur dioxide — give a biological effect

A "biological" effect of sulfur dioxide means a named living-organism consequence: acid rain kills trees/plants, or kills fish and aquatic animals. Two errors lose the mark: writing something vague like "it affects organisms", and giving the erosion of stone buildings, which is a chemical effect, not biological. Name the organism AND what happens to it, linked to acid rain.

CO and SO2 are not greenhouse gases

The two greenhouse gases in 4SD0 are carbon dioxide ( $\text{CO}_2$ ) and methane ( $\text{CH}_4$ ). Carbon monoxide (CO) and sulfur dioxide ( $\text{SO}_2$ ) are air POLLUTANTS with different effects (carboxyhaemoglobin; acid rain) and are NOT greenhouse gases. Keep the two lists separate: a gas is either a named greenhouse gas (CO2, methane) or one of these pollutants (CO, SO2).

Algae do not use up the oxygen

The mark scheme rewards the full chain: excess nitrates/phosphates $\rightarrow$ algal bloom $\rightarrow$ light blocked $\rightarrow$ plants below die $\rightarrow$ decomposer bacteria respire on the dead material, using up the dissolved oxygen $\rightarrow$ animals die. The frequent slip is jumping to "the algae use up the oxygen". Name dissolved oxygen and credit the decomposers.

"Poisonous gas" is too vague for CO

Explaining why carbon monoxide is dangerous, "a poisonous gas" is too vague. CO binds to haemoglobin in red blood cells, forming carboxyhaemoglobin, so the blood carries less oxygen and cells receive less oxygen for respiration; this can cause drowsiness and death. Always name carboxyhaemoglobin and the reduced oxygen carriage.

Greenhouse effect traps heat, not light

The greenhouse effect is the trapping of heat — infra-red radiation — by greenhouse gases, NOT the "trapping of light". Also, listing which gases are greenhouse gases is not the same as describing the effect: an "explain the greenhouse effect" answer must say that the gases absorb/trap outgoing infra-red (heat), warming the Earth.

Exam tips

Write pollution answers as a chain
Sewage and eutrophication questions are marked point-by-point, so write numbered links, each causing the next: cause $\rightarrow$ algal bloom $\rightarrow$ light blocked or oxygen used $\rightarrow$ organisms die. Missing a link, especially the dissolved-oxygen step, loses that mark.
Evaluate GWP data, do not just quote it
With a table of amounts and Global Warming Potentials (GWP), do not just copy numbers. Explain why $\text{CO}_2$ is the biggest contributor: released in the largest amount and staying longest, despite a low GWP per molecule. Consider every gas, then justify with the data.
Use precise terms
Precise terminology is repeatedly decisive here: say "dissolved oxygen" not just "oxygen", name the specific pollutant (CO2, methane, SO2 or CO) rather than "pollution", and give a biological effect when one is asked. Vague wording is the commonest avoidable mark loss.

Revision notes