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Pollution

Paper 1Paper 2Paper 3Paper 4

This topic is examined in Paper 1, Paper 2, Paper 3, and Paper 4.

Overview of Pollution Types
Pollution is the introduction of harmful substances or energy into the environment. In this topic, we focus on three main types: water pollution (sewage and fertilisers), solid waste pollution (plastics), and air pollution (greenhouse gases). Understanding these requires linking biological processes (like respiration and photosynthesis) with environmental chemistry.
Effects of Untreated Sewage on Aquatic Ecosystems
Untreated sewage affects aquatic ecosystems through two distinct mechanisms: biological and chemical. It is crucial to distinguish these from the effects of fertilisers.
Effect on Ecosystem
These pathogens spread waterborne diseases to aquatic animals and humans who use the water.
Decomposing bacteria perform aerobic respiration, consuming large amounts of dissolved oxygen. This leads to hypoxia (low oxygen), causing fish and other aerobic organisms to die.
Note: While sewage can contribute nutrients, its primary unique danger compared to fertilisers is the direct introduction of pathogens and the immediate biological oxygen demand from decomposing organic matter. Fertilisers primarily cause eutrophication via nutrient overload.
Effects of Excess Fertiliser on Aquatic Ecosystems
Excess fertilisers (containing nitrates and phosphates) enter water bodies via run-off from agricultural fields. This leads to eutrophication. While LO1 asks for the effects, LO4 details the process. The key effect is the disruption of the food web due to oxygen depletion.
Eutrophication
Eutrophication is the process where water bodies become overly enriched with minerals and nutrients, leading to excessive growth of algae and aquatic plants, followed by oxygen depletion.
The Process of Eutrophication (LO4)
You must be able to explain this chain of events step-by-step. Each step causes the next.
Consequence
These ions act as limiting factors for plant growth are removed.
An 'algal bloom' forms on the surface.
Rooted plants cannot photosynthesise and die.
Decomposers multiply rapidly.
Dissolved oxygen levels drop sharply.
Death of organisms requiring dissolved oxygen (fish, invertebrates) occurs. Biodiversity decreases.
Why this matters: The death of fish is not directly caused by the fertiliser itself, but by the lack of oxygen resulting from bacterial respiration. This is a common point of confusion.
⚠︎ Confusing Sewage and Fertiliser Effects
Error: Stating that sewage causes eutrophication primarily through algal blooms caused by nitrates.

Correction: While sewage contains some nutrients, its primary distinct effect is the introduction of pathogens and organic matter that decomposes directly. Fertilisers are the primary cause of nutrient-driven eutrophication (algal blooms). Do not attribute algal blooms solely to sewage in a comparison question.

⚠︎ Misusing 'Biomagnification' for Plastics
Error: Stating that non-biodegradable plastics biomagnify up the food chain.

Correction: Biomagnification refers specifically to the increase in concentration of a chemical substance (like DDT or heavy metals) as it moves up trophic levels. Plastics are physical waste. They do not biomagnify. Instead, they accumulate in tissues or are transferred up the food chain when one organism eats another. If plastics adsorb toxins, those toxins may biomagnify, but the plastic itself does not.

Effects of Non-Biodegradable Plastics
Non-biodegradable plastics do not break down naturally. They persist in the environment for centuries.
Mechanism
Animals mistake plastic for food (ingestion) or get trapped in nets/rings (entanglement). This can cause starvation, internal injury, or suffocation.
Land animals may ingest plastic waste, leading to blockages in the digestive system. Plastics also litter habitats, disrupting nesting and breeding grounds.
Microplastics: Small plastic particles can be ingested by plankton (producers), transferring them up the food chain to consumers. This affects biodiversity and potentially human health.
Air Pollution: Greenhouse Gases (LO3)
The two main greenhouse gases discussed are carbon dioxide (CO_2) and methane (CH_4). You must know their sources and their specific effect: the enhanced greenhouse effect leading to climate change.
Effect
Traps infrared radiation in the atmosphere, causing global warming.
Traps infrared radiation. Methane is more potent per molecule than CO_2, but less abundant.
Enhanced Greenhouse Effect: The natural greenhouse effect keeps Earth warm. Human activities increase the concentration of these gases, trapping more heat, which leads to climate change (rising global temperatures, melting ice caps, extreme weather).
⚠︎ Confusing Greenhouse Gases and Ozone Depletion
Error: Stating that carbon dioxide or methane causes ozone layer depletion.

Correction: CO_2 and CH_4 cause the enhanced greenhouse effect. Ozone depletion is caused by chlorofluorocarbons (CFCs). These are two separate environmental issues. Do not mix them up.

Describing Eutrophication Steps
When to use: When asked to 'explain the process of eutrophication' or 'describe how fertilisers cause fish death'.

Why examiners accept this: Examiners look for the causal chain. You must explicitly mention aerobic respiration by decomposers as the direct cause of oxygen depletion. Simply saying 'bacteria eat the algae' is insufficient because it doesn't link to oxygen consumption.

Correct Usage Example: 'The death of algae leads to increased decomposition by bacteria. These bacteria perform aerobic respiration, which consumes dissolved oxygen in the water, causing fish to suffocate.'

Describing Plastics and Food Chains
When to use: When asked 'how do microplastics end up in consumers?' or 'explain the effect of plastics on biodiversity'.

Why examiners accept this: You must show the transfer mechanism. Using the term 'biomagnification' for the plastic itself is incorrect. Use 'accumulation' or 'transfer'.

Correct Usage Example: 'Plankton ingest microplastics. Small fish eat the plankton, and large birds (consumers) eat the fish. The plastics accumulate in the tissues of the consumers up the food chain.'

Describing Greenhouse Gas Sources
When to use: When asked to 'describe sources of methane' or 'explain the enhanced greenhouse effect'.

Why examiners accept this: Specificity is key. 'Farming' is too vague. 'Digestion in ruminants' or 'rice paddies' is precise.

Correct Usage Example: 'Methane is produced by the anaerobic decomposition of organic matter in landfills and by the digestion of grass in cows.'

Eutrophication Process (Paper 4 Style)
Q:
Explain how the use of fertilisers on farmland can lead to the death of fish in a nearby lake. [6 marks]
A:
  1. Fertilisers contain nitrates (and other ions) which are washed into the lake (run-off). [1]
  2. This increases the availability of nutrients for producers (algae/plants). [1]
  3. This causes rapid growth of algae (algal bloom). [1]
  4. Algae block sunlight, so submerged plants die. [1]
  5. Decomposers (bacteria) decompose the dead algae and plants. [1]
  6. Decomposers perform aerobic respiration, consuming dissolved oxygen. [1]
  7. Dissolved oxygen levels drop, causing fish to suffocate/die. [1]
Plastics and Biodiversity (Paper 3 Style)
Q:
Describe two ways in which non-biodegradable plastics harm animals in the ocean. [2 marks]
A:
  1. Animals mistake plastic for food and eat it, leading to starvation or blockage. [1]
  2. Animals become entangled (trapped) in plastic waste (e.g., nets), preventing movement or causing drowning. [1]
Greenhouse Gases (Paper 4 Style)
Q:
Describe the sources and effects of pollution by methane. [3 marks]
A:
Sources: Digestion in ruminant animals (e.g., cows); decomposition of waste in landfills; rice cultivation. [1]
Effect: Methane is a greenhouse gas that traps infrared radiation in the atmosphere, contributing to the enhanced greenhouse effect and climate change (global warming). [2]
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