Ch 3 · Pollution · IGCSE Biology 0610
Topic 3 of 4 in Human Influences on Ecosystems — IGCSE Biology 0610 — 48 lessons.
Introduction
Pollution is the addition of harmful substances to the environment by human activity. This lesson covers the four types named in the syllabus: nutrients from sewage and fertiliser in water, non-biodegradable plastics, greenhouse gases and pesticides. The most heavily examined is eutrophication, a chain of six linked steps that examiners mark as a sequence.
Key ideas — Sewage, fertiliser and eutrophication
Untreated sewage contains organic matter, pathogens and mineral ions. Excess fertiliser is not all taken up by crops; rain dissolves nitrate and other ions and washes them into rivers and lakes (leaching). Both add nutrients to water, causing an explosion of algal growth, a fall in dissolved oxygen and the death of fish. Sewage also spreads diseases such as cholera.
🧑🏫 Teacher's explanation
Imagine a crowded party where someone accidentally spills a massive bag of sugar on the floor. Suddenly, everyone (the bacteria) starts eating the sugar and gets incredibly hyper and energetic. Because they are so busy eating, they start breathing heavily and using up all the fresh air in the room. Eventually, there is no air left for anyone else to breathe, and people start collapsing.
📖 Definitions
🔍 Worked example — Easy
A student observes an algal bloom in a pond. List the first two steps of the eutrophication process that led to this bloom.
🔍 Worked example — Standard
Explain why the death of fish occurs during eutrophication, specifically addressing the role of bacteria.
🔍 Worked example — Exam-style
Calculate the percentage decrease in dissolved oxygen if the concentration drops from 8.5 mg/L to 2.1 mg/L.
⚠️ Common errors
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🎯 Exam question
Describe the sequence of events that leads to the death of fish in the lake. [4 marks] [4 marks]
Key ideas — Non-biodegradable plastics
Most plastics are non-biodegradable: decomposers lack enzymes to break them down, so they persist for centuries.
🧑🏫 Teacher's explanation
Imagine you are cleaning your room and you find a pile of candy wrappers. If you throw them in the bin, they eventually rot away. But if you throw a pile of plastic bottles in the bin, they stay there forever, taking up space and never disappearing. This is because biological organisms (decomposers) are like specialized tools; they have specific 'keys' (enzymes) to unlock and break down natural materials like wood or leaves. Most plastics are like locks for which no key exists, so they just sit there, accumulating.
📖 Definitions
🔍 Worked example — Easy
State the biological reason why plastic is classified as non-biodegradable.
🔍 Worked example — Standard
A marine biologist observes that a species of filter-feeding mollusc has high concentrations of microplastics in its tissues. Explain how these plastics might reach the mollusc.
🔍 Worked example — Exam-style
Compare the impact of plastic pollution in an aquatic ecosystem versus a terrestrial ecosystem.
⚠️ Common errors
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🎯 Exam question
(a) Describe two ways in which plastic pollution can cause the death of marine animals. [2] (b) Explain why microplastics pose a threat to organisms higher up the food chain. [2] [4 marks]
Key ideas — Methane and carbon dioxide: the enhanced greenhouse effect
The greenhouse effect is natural and keeps Earth warm: gases in the atmosphere absorb heat radiated from the surface. Human activity is increasing the concentration of these gases, producing the enhanced greenhouse effect and climate change.
🧑🏫 Teacher's explanation
Imagine Earth is wearing a light sweater. This sweater is the natural greenhouse effect; it traps just enough heat from the sun to keep the planet at a comfortable temperature for life. Without it, Earth would be a frozen wasteland.
📖 Definitions
🔍 Worked example — Easy
Identify two distinct human activities that increase the concentration of carbon dioxide (CO_2) in the atmosphere.
🔍 Worked example — Standard
A student is asked to explain why methane is considered a greenhouse gas. Describe the source of methane from agricultural practices.
🔍 Worked example — Exam-style
Predict the environmental consequences of a 2^°C increase in global average temperature due to the enhanced greenhouse effect.
⚠️ Common errors
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🎯 Exam question
Key ideas — Insecticides and herbicides
Sources: sprayed on crops and gardens, they drift in the air, are washed by rain into soil and water, and remain on food.
🧑🏫 Teacher's explanation
Imagine you are trying to clear a garden of weeds using a powerful spray. You intend to kill only the weeds, but the wind carries the mist to your neighbor's flowers, and the rain washes the chemical into a nearby pond. Even though you only meant to target the weeds, the chemicals are now affecting the entire neighborhood.
📖 Definitions
🔍 Worked example — Easy
A farmer sprays a field with a persistent insecticide. The insecticide enters the food chain via phytoplankton arrow small fish arrow large fish arrow osprey. Explain why the osprey is at the highest risk of toxin poisoning.
🔍 Worked example — Standard
A pesticide has a concentration of 0.01 ppm in water. If a small fish eats enough plankton to reach 0.1 ppm and a large fish eats many small fish to reach 1.0 ppm, calculate the magnification factor from water to large fish.
🔍 Worked example — Exam-style
Compare the effects of insecticides and herbicides on an ecosystem.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Explain why this insecticide might lead to a decrease in the population of birds of prey in the area. [3] (b) State one effect of herbicides on an ecosystem. [1] [4 marks]
Worked example
Question. (Supplement) Heavy rain washed fertiliser from a field into a small lake. Two weeks later, dead fish were found floating on the surface. Explain how the fertiliser led to the death of the fish. [5]
🔍 Worked example — Easy
Identify the primary nutrient responsible for causing eutrophication in freshwater ecosystems.
🔍 Worked example — Standard
A lake shows a sudden increase in algae (an algal bloom). Explain why this might lead to a decrease in light intensity reaching submerged plants.
🔍 Worked example — Exam-style
(Supplement) Heavy rain washed fertiliser from a field into a small lake. Two weeks later, dead fish were found floating on the surface. Explain how the fertiliser led to the death of the fish. [5]
⚠️ Common errors
Common mistakes
Practice
Answers