# Human influences on the environment

> Edexcel International GCSE Biology · 4BI1 (2017 Issue 3)
> Source: https://www.owlsprep.com/study/edexcel-igcse-biology-s4-human-influences-on-the-environment/

This exam-focused guide covers all required content for the Human Influences on the Environment topic for Edexcel IGCSE Biology (4BI1), including air/water pollution, greenhouse gases, eutrophication, and Paper 2-only deforestation effects.

**Prerequisites:** [Understanding of the carbon cycle (Section S4_T03)](https://www.owlsprep.com/study/edexcel-igcse-biology-s4-cycles-within-ecosystems/)

## Learning objectives

- Recall biological consequences of air pollution by sulfur dioxide and carbon monoxide
- List the 5 required greenhouse gases and their human sources
- Explain the enhanced greenhouse effect and its consequences
- Describe the biological impacts of water pollution by sewage
- Outline the sequence of eutrophication from fertiliser leaching
- (Paper 2 only) State the effects of deforestation on ecosystems and the atmosphere

## Air Pollution (4.12)

Two key air pollutants are examined in this topic, each with specific named biological consequences you must memorize for the mark scheme.

**Air pollutant** — Substance released into the atmosphere by human activity that causes harm to living organisms or ecosystems

- **Sulfur dioxide**: Released from burning fossil fuels (especially coal in power stations). It dissolves in rainwater to form acid rain, which damages plant leaves, acidifies lakes killing aquatic organisms, irritates human lung tissue, and erodes stone buildings.
- **Carbon monoxide**: Released from incomplete combustion of fossil fuels (e.g. car engines, faulty gas boilers). It binds irreversibly to haemoglobin in red blood cells to form carboxyhaemoglobin, reducing the oxygen-carrying capacity of blood and leading to asphyxiation at high concentrations.

**Worked example:** State one biological consequence of sulfur dioxide pollution for aquatic ecosystems and one consequence of carbon monoxide poisoning for humans.

1. 1. Sulfur dioxide forms acid rain which lowers the pH of lakes, killing fish and aquatic invertebrates.
2. 2. Carbon monoxide binds to haemoglobin, reducing the amount of oxygen transported in blood leading to dizziness, loss of consciousness or death.

> **Exam tip**
>
> Always pair each pollutant with its specific named consequence: do not write generic answers like 'it causes harm' as these will not get marks.

*Calculator:* allowed

## Greenhouse Gases and Global Warming (4.13-4.15)

Greenhouse gases trap heat in the Earth's atmosphere naturally to keep the planet warm enough for life. Human activities have increased concentrations of these gases leading to an enhanced greenhouse effect.

**Enhanced greenhouse effect** — Increased heat trapping caused by higher than natural concentrations of greenhouse gases from human activity, leading to global warming

- **Required list of greenhouse gases (memorize all 5)**: Water vapour, carbon dioxide, nitrous oxide, methane, CFCs
- **Human sources**: Burning fossil fuels (CO₂), deforestation (reduced CO₂ removal), livestock farming/rice paddies (methane), nitrogen-based fertiliser use (nitrous oxide), old refrigerants/aerosols (CFCs)
- **Consequences of global warming**: Rising sea levels (melting ice caps/thermal expansion), more frequent extreme weather events (droughts, floods, storms), habitat loss (e.g. melting Arctic sea ice), changing species distribution as organisms move to cooler areas

**Worked example:** Name three greenhouse gases and one human activity that increases the concentration of each.

1. 1. Carbon dioxide: released from burning fossil fuels for transport and electricity generation
2. 2. Methane: released from digestive processes of cattle in livestock farming
3. 3. Nitrous oxide: released from use of artificial nitrogen fertilisers on agricultural crops

*Calculator:* allowed

## Water Pollution by Sewage (4.16)

Untreated sewage contains high levels of organic nutrients and pathogens, which cause a predictable sequence of ecosystem damage when released into rivers or lakes.

1. Excess organic nutrients from sewage cause rapid growth of algae on the water surface (algal bloom)
2. Algal bloom blocks sunlight from reaching submerged aquatic plants, which die as they cannot photosynthesize
3. Aerobic decomposer bacteria multiply to break down dead plant matter, using large amounts of dissolved oxygen for respiration
4. Biological Oxygen Demand (BOD) increases, dissolved oxygen levels drop too low to support aquatic life, so fish and invertebrates die

**Worked example:** Explain why untreated sewage released into a river causes fish to die several kilometres downstream.

1. 1. Sewage adds excess nutrients to the river, causing an algal bloom on the surface that blocks sunlight from submerged plants.
2. 2. Submerged plants die from lack of light, and are broken down by aerobic bacteria that multiply and use up dissolved oxygen in the water.
3. 3. Dissolved oxygen levels drop below the threshold needed for fish respiration, so fish die from oxygen starvation as the polluted water flows downstream.

*Calculator:* allowed

## Eutrophication from Fertiliser Leaching (4.17)

Eutrophication follows the same oxygen depletion sequence as sewage pollution, but the source of excess nutrients is inorganic nitrates and phosphates leached from agricultural fertilisers after heavy rain.

> **warning**
>
> You must not skip steps when describing eutrophication: the mark scheme explicitly awards marks for each stage of the sequence, including the light-blocking step and bacterial respiration.

**Worked example:** Describe the process of eutrophication in a lake next to a wheat farm that uses artificial fertilisers.

1. 1. Heavy rain washes excess nitrate and phosphate fertiliser off the farm fields into the lake (leaching), causing nutrient enrichment of the water.
2. 2. High nutrient levels cause an algal bloom on the lake surface, which blocks sunlight from reaching submerged aquatic plants.
3. 3. Submerged plants die, decomposer bacteria break down the dead material using dissolved oxygen, so oxygen levels drop leading to death of fish and other aquatic organisms.

*Calculator:* allowed

## Biology-only: Effects of Deforestation (4.18B)

> **Paper 2 only**
>
> This content is only tested on Paper 2 for Single Award Biology students. It is not examined on Paper 1 or for Double Award Science.

- **Soil degradation**: Loss of tree roots that bind soil leads to increased soil erosion, and leaching of minerals from the topsoil making it less fertile
- **Water cycle disruption**: Reduced evapotranspiration from tree leaves means less water is returned to the atmosphere, leading to changed rainfall patterns and increased risk of droughts or floods
- **Atmospheric gas imbalance**: Less photosynthesis from removed trees means less CO₂ is removed from the atmosphere, increasing greenhouse gas concentrations, and less oxygen is released
- **Biodiversity loss**: Destruction of forest habitats leads to extinction of plant, animal and fungal species that rely on forest ecosystems

**Worked example:** Explain two effects of large-scale deforestation on the carbon cycle.

1. 1. Removing trees reduces the amount of photosynthesis taking place, so less carbon dioxide is removed from the atmosphere to be stored as biomass in trees.
2. 2. If felled trees are burned, the carbon stored in their biomass is released as carbon dioxide back into the atmosphere, further increasing greenhouse gas concentrations.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Listing ozone as a greenhouse gas in answers
  - Why it fails: The Edexcel IGCSE Biology spec only requires you to recall 5 named greenhouse gases: ozone is not on the approved list and adding it will lose you marks.
  - Correct: Memorize the exact 5 gases: water vapour, carbon dioxide, nitrous oxide, methane, CFCs, and only list these in answers.
- **Wrong:** Skipping the light-blocking step when explaining eutrophication/sewage pollution
  - Why it fails: The mark scheme explicitly awards a mark for stating that algal blooms block sunlight leading to death of submerged plants: jumping straight to oxygen depletion will lose you this mark.
  - Correct: Always follow the full sequence: excess nutrients → algal bloom → light blocked → plant death → decomposers multiply → BOD rises → oxygen falls → aquatic life dies.
- **Wrong:** Stating that carbon monoxide reduces the number of red blood cells
  - Why it fails: Carbon monoxide binds to haemoglobin molecules inside red blood cells, it does not destroy red blood cells. This is a common misconception tested in multiple choice questions.
  - Correct: Explain that carbon monoxide forms carboxyhaemoglobin, which prevents haemoglobin from carrying oxygen, reducing the oxygen-carrying capacity of blood.
- **Wrong:** Stating that carbon dioxide causes acid rain
  - Why it fails: Carbon dioxide dissolves to form weak carbonic acid, but the acid rain examined in this topic is explicitly caused by sulfur dioxide pollution.
  - Correct: Pair sulfur dioxide with acid rain consequences, and carbon dioxide with the enhanced greenhouse effect and global warming.
- **Wrong:** Including deforestation content in Paper 1 answers
  - Why it fails: Deforestation is a Biology-only Paper 2 topic, it will never be tested on Paper 1 and including it in answers wastes time and will not get extra marks.
  - Correct: Only revise deforestation content if you are sitting Paper 2 for Single Award Biology, and do not reference it in Paper 1 responses.

## Cheatsheet

| Topic | Key Exam Facts | Mark Scheme Keywords |
| --- | --- | --- |
| Sulfur dioxide pollution | Causes acid rain, irritates lungs | Acid rain, kills plants/aquatic life, lung irritation |
| Carbon monoxide pollution | Binds irreversibly to haemoglobin | Carboxyhaemoglobin, reduced oxygen capacity, asphyxiation |
| Greenhouse gases | Exact list of 5 required | Water vapour, CO₂, nitrous oxide, methane, CFCs |
| Eutrophication/sewage | Full 6-step sequence required | Leaching, algal bloom, light block, BOD, oxygen depletion |
| Deforestation (Paper 2 only) | 4 key effect categories | Soil erosion, disrupted carbon/water cycle, biodiversity loss |

## What's next

Now that you have mastered all content for human influences on the environment, you are ready to practice exam-style questions to reinforce your understanding and memorize the mark scheme keywords for extended response questions. This topic is frequently tested in both short answer and 4-6 mark extended questions, so make sure you can write out the full eutrophication sequence and pollutant consequences without notes. You should also review the carbon cycle content to prepare for common cross-topic questions that link greenhouse gas emissions to natural carbon cycling processes. For Paper 2 students, practice deforestation extended response questions to maximize marks on Biology-only content.

- [Cycles within Ecosystems (S4_T03)](https://www.owlsprep.com/study/edexcel-igcse-biology-s4-cycles-within-ecosystems/)

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