3.4Unit 2 · Ecology & the Environment
Human Influences on the Environment — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 7 parts Lesson 5 of 5 1 quiz
Lesson summary
Describe the biological consequences of pollution of air by sulfur dioxide and carbon monoxide.The main greenhouse gases are water vapour, CO2, methane, nitrous oxide and CFCs.The Sun's short-wavelength radiation passes through the atmosphere; Earth re-emits longer-wavelength inf…
- Consequences of global warming
- Air pollution – sulfur dioxide
- Air pollution – carbon monoxide
- Greenhouse effect
- Water pollution
- Deforestation
Part 1 of 7Key ideas
1
The main greenhouse gases are water vapour, CO2, methane, nitrous oxide and CFCs.
2
The Sun's short-wavelength radiation passes through the atmosphere; Earth re-emits longer-wavelength infrared, some absorbed by greenhouse gases.
3
The greenhouse effect is natural and keeps Earth warm; the enhanced effect causes global warming.
4
Consequences of global warming
Consequences of global warming melting ice caps, sea level rise, extreme weather, changes in species distribution.
5
Untreated sewage adds nutrients and pathogens and increases biochemical oxygen demand.
6
Sulfur dioxide + water forms acidic solutions that damage leaves and release toxic aluminium ions into lakes.
7
Carbon monoxide combines with haemoglobin more readily than oxygen does.
Part 2 of 7Learning objectives
1
Describe the biological consequences of pollution of air by sulfur dioxide and carbon monoxide.
2
Name the greenhouse gases and explain the enhanced greenhouse effect.
3
Explain eutrophication caused by fertiliser and sewage.
4
Describe the effects of deforestation.
Part 3 of 7Essential knowledge
Essential knowledge
Air pollution – sulfur dioxide
Air pollution – sulfur dioxide produced when fossil fuels containing sulfur, especially coal, are burnt. It dissolves in water droplets forming acid rain, which damages leaves and kills trees, lowers the pH of lakes (killing fish and invertebrates), and leaches minerals from soil. It also irritates the lungs.
Air pollution – carbon monoxide
Air pollution – carbon monoxide released from incomplete combustion in car engines, faulty boilers and cigarettes. It binds very strongly (almost irreversibly) to haemoglobin, reducing oxygen transport, causing drowsiness and in high doses death.
Greenhouse gases
Greenhouse gases water vapour, carbon dioxide (combustion, deforestation), methane (cattle, rice paddies, landfill), nitrous oxide (fertilisers, vehicles) and CFCs (old refrigerants). They trap heat in the atmosphere.
Greenhouse effect
Greenhouse effect sunlight warms the Earth, which radiates infrared. Greenhouse gases absorb and re-radiate some infrared back to Earth. This natural effect keeps the planet warm enough for life.
Enhanced greenhouse effect
Enhanced greenhouse effect increased greenhouse gas levels from human activity trap more heat, causing global warming. Consequences include melting polar ice and rising sea levels, flooding, changes in rainfall and more extreme weather, changes in species ranges and migration, and possible extinctions and lower crop yields.
Water pollution
Water pollution fertiliser leached from fields and untreated sewage cause eutrophication: algal bloom, plant death, bacterial decomposition and oxygen depletion so fish die. Sewage also contains pathogens causing disease.
Deforestation
Deforestation clearing forests for timber, farming or building reduces photosynthesis so less CO2 is removed, and burning or decomposing wood adds CO2.
- It destroys habitats and reduces biodiversity, causes soil erosion and leaching of nutrients,
- and disrupts the water cycle.
- (less transpiration, less rain locally, more flooding)
Sewage and oxygen
Organic matter in sewage is food for aerobic bacteria, which multiply and use up dissolved oxygen. Organisms needing high oxygen die, leaving only species tolerant of low oxygen, so biodiversity falls.
More effects of global warming
Warmer oceans cause coral bleaching; habitats change or are lost; species migrate to new areas, which can spread pests and diseases; food chains are disrupted and extinction rates may rise.
Part 4 of 7Process & sequence
1
Fertiliser or sewage enters a lake or river.
2
Nitrates and phosphates cause rapid algal growth (algal bloom).
3
Algae block light, so plants below cannot photosynthesise and die.
4
Decomposer bacteria feed on dead plants and algae and multiply.
5
Bacteria use up dissolved oxygen in aerobic respiration.
6
Fish and other aerobic organisms die from lack of oxygen.
Eutrophicationwrite these steps in this order; each step is a marking point.
1
Nutrients enter water
2
Algal bloom blocks light
3
Water plants die
4
Decomposers multiply
5
Oxygen used up
6
Fish suffocate
Part 5 of 7Worked examples
1Aerobic respiration
Explain how the use of fertiliser on farmland can lead to the death of fish in a nearby river.
- Rain leaches nitrates from the fertiliser into the river.
- This causes rapid growth of algae, which blocks light so water plants die.
- Bacteria decompose the dead plants and algae and, as they multiply, use up dissolved oxygen in aerobic respiration.
- The fish die from lack of oxygen.
- This is eutrophication.
2Suggest two ways deforestation increases atmospheric carbon dioxide.
- 1) Fewer trees are present to remove CO2 by photosynthesis.
- 2) Trees are often burned, or left to be decomposed by microorganisms, releasing CO2 through combustion and respiration.
Part 6 of 7Summary
1
SO2 causes acid rain; CO reduces oxygen transport.
2
CO2 and methane are key greenhouse gases.
3
The enhanced greenhouse effect leads to global warming.
4
Fertilisers and sewage cause eutrophication.
5
Deforestation causes erosion, leaching, CO2 rise and lost biodiversity.
Part 7 of 7Compare: Compare
| Pollutant | Source | Biological consequence |
|---|---|---|
| Sulfur dioxide | Burning fossil fuels containing sulfur | Acid rain: kills trees, lowers pH of lakes, killing fish |
| Carbon monoxide | Incomplete combustion (car engines) | Binds haemoglobin, less O2 carried; can be fatal |
| Carbon dioxide | Burning fossil fuels, deforestation | Enhanced greenhouse effect |
| Methane | Cattle, rice fields, landfill | Strong greenhouse gas |
| Nitrous oxide / CFCs | Fertilisers, old aerosols/fridges | Greenhouse gases; CFCs damage ozone |
| Effect | Explanation |
|---|---|
| Less CO2 absorbed | Fewer trees photosynthesising; burning releases more CO2 |
| Soil erosion | No roots to hold soil; rain washes it away |
| Leaching of nutrients | Minerals washed out of exposed soil |
| Disturbed water cycle | Less transpiration, less rainfall, more flooding |
| Loss of biodiversity | Habitats destroyed, species become extinct |
Key terms
Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Pollution · Greenhouse gas · Enhanced greenhouse effect · Acid rain · Eutrophication · Deforestation · Carbon monoxide · Leaching · Biochemical oxygen demand
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Instant feedbackCheck yourself
Q1. Which gas is mainly responsible for acid rain?
- A. Carbon monoxide
- B. Sulfur dioxide
- C. Methane
- D. Oxygen
Show answer
B. Sulfur dioxide dissolves in water forming acidic solutions.
Q2. Which activity releases large amounts of methane?
1
A. Cattle farming
2
B. Burning coal
3
C. Photosynthesis
4
D. Planting trees
Show answer
A. Cattle produce methane during digestion; rice fields and landfill are other sources.
Q3. In eutrophication, fish die mainly because:
1
A. Algae are poisonous
2
B. The water is too warm
3
C. Nitrates are toxic
4
D. Bacteria use up dissolved oxygen
Show answer
D. Decomposing bacteria respire aerobically and deplete oxygen.
Q4. Carbon monoxide is dangerous because it:
1
A. Causes acid rain
2
B. Destroys ozone
3
C. Combines with haemoglobin
4
D. Is a greenhouse gas only
Show answer
C. CO binds to haemoglobin, reducing oxygen carried to cells.
Q5. Which is NOT a consequence of deforestation?
1
A. Soil erosion
2
B. Increased biodiversity
3
C. Increased atmospheric CO2
4
D. Leaching of nutrients
Show answer
B. Deforestation destroys habitats, reducing biodiversity.
Q6. Greenhouse gases warm the Earth by:
1
A. Absorbing infrared radiation re-emitted from Earth
2
B. Reflecting sunlight away
3
C. Destroying the ozone layer
4
D. Absorbing all visible light
Show answer
A. They absorb and re-radiate longer-wavelength infrared, trapping heat.
Last updated: