Form 5 · Worked answers

Environmental Sustainability, worked answers

Fully worked answers for Environmental Sustainability, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Linking a specific human activity to its specific effect on the environment.
Explaining the greenhouse effect and distinguishing it from global warming.
Interpreting BOD data to compare the pollution level of different water samples.
Explaining how CFCs lead to ozone layer depletion and its effects on living things.
Describing the stages of eutrophication from nutrient runoff to a drop in dissolved oxygen.
Evaluating a named renewable energy source or green technology as a solution to a specific environmental threat.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

A factory burns coal to generate electricity. Explain TWO separate environmental effects this has, one on global warming and one on air quality.

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Burning coal releases carbon dioxide, a greenhouse gas that traps extra heat in the atmosphere, contributing to global warming as one effect. Separately, burning coal also releases sulfur dioxide and particulate matter, which lower air quality: sulfur dioxide dissolves in rainwater to form acid rain, and particulate matter creates haze that irritates the respiratory system. These are two distinct effects caused by two different pollutants, so a complete answer must explain each mechanism separately rather than describing 'pollution' as a single effect.

carbon dioxidegreenhouse gassulfur dioxideacid raintwo separate effects

A river's BOD is 2 mg/L 100 m upstream of a factory's discharge pipe and 18 mg/L 100 m downstream of it. (a) State which sample is more polluted with organic matter. (b) Explain your answer.

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(a) The downstream sample is more polluted with organic matter. (b) BOD measures the amount of oxygen microorganisms use to decompose the organic matter in a water sample; a higher BOD value means more organic matter is present for microorganisms to break down, so more oxygen is consumed. Since the downstream value of 18 mg/L is far higher than the upstream value of 2 mg/L, the factory's discharge is the most likely source of the extra organic pollution entering the river.

BODorganic matterdownstreamoxygen consumptiondischarge point

Distinguish between preservation and conservation, using ONE named example of each.

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Preservation keeps a natural area completely free from exploitation, such as a strict virgin forest reserve where logging and hunting are totally banned. Conservation instead manages a resource's use so it is not exhausted, such as a fishing zone with a closed season and a minimum catch size that allows continued, regulated harvesting. Preservation therefore avoids use entirely, while conservation permits controlled, sustainable use.

preservationconservationvirgin forest reserveclosed seasoncontrolled use

Explain the sequence of events in eutrophication, from the entry of fertiliser runoff into a lake to the death of fish.

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Excess nutrients such as nitrate and phosphate from fertiliser runoff enter the lake, providing a rich nutrient supply for algae and other aquatic plants. These organisms grow and multiply rapidly, forming a dense surface bloom that blocks sunlight from reaching submerged plants, causing large numbers of them to die. Decomposers then break down the large amount of dead plant and algal material, consuming large amounts of dissolved oxygen in the process. The resulting sharp drop in dissolved oxygen leaves too little oxygen for fish and other aquatic animals, which suffocate and die in large numbers.

nutrient runoffalgal bloomblock sunlightdecomposersdissolved oxygenfish death

Explain how CFCs released from old refrigerators lead to a higher risk of skin cancer in humans.

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CFCs released into the atmosphere are stable enough to drift up into the stratosphere, where ultraviolet radiation breaks them down and releases reactive chlorine atoms. These chlorine atoms react with and break apart ozone molecules, and because each chlorine atom can destroy large numbers of ozone molecules before being removed, the ozone layer becomes thinner over time. Since the ozone layer normally absorbs most of the sun's harmful ultraviolet radiation, a thinner layer allows more UV radiation to reach the Earth's surface, and greater exposure to this radiation raises the risk of skin damage and skin cancer in humans.

CFCstratospherechlorineozone layerultraviolet radiationskin cancer

A power company is deciding between building a new coal-fired plant and a solar farm of similar output. Evaluate the solar farm as the more sustainable choice.

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A solar farm converts sunlight directly into electricity using photovoltaic cells and produces no carbon dioxide, sulfur dioxide or particulate emissions while generating power, unlike a coal-fired plant, which releases these pollutants continuously as it burns fuel. This means a solar farm does not contribute to global warming or acid rain during operation and requires no ongoing fuel extraction. Its output does depend on sunlight availability, and it needs a large area of land, so it is not without limitation. On balance, given the coal plant's continuous emissions, the solar farm is the more environmentally sustainable choice, though its cost and land requirement should also be weighed.

photovoltaic cellsno emissions during operationglobal warmingland requirementsustainable choice

Explain why an Environmental Impact Assessment is carried out before, rather than after, a highway project through a forested area is approved.

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An Environmental Impact Assessment identifies a project's likely effects on the surrounding environment, such as habitat loss or disrupted water flow, while the project is still at the planning stage. Carrying it out before approval allows mitigation measures, such as wildlife crossings or a protected buffer zone, to be built into the project's design or attached as approval conditions. If it were done only after construction began, existing damage such as cleared habitat could not be undone, so assessing the impact beforehand allows harm to be reduced rather than merely recorded.

Environmental Impact Assessmentplanning stagemitigation measuresbefore approvalhabitat loss

Using the idea of sustainable development, explain why a palm oil company replanting one tree for every mature tree it fells is considered more sustainable than clearing land without replanting.

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Sustainable development meets today's needs, such as the economic demand for palm oil, without reducing the ability of future generations to meet their own needs from the same land. Replanting one tree for every tree felled maintains the forest's long-term timber and ecological resource base, allowing continued harvesting in future cycles, whereas clearing land without replanting permanently removes the forest's capacity to regenerate. The replanting practice therefore balances present economic use with the environment's ability to keep supporting that use in the future.

sustainable developmentreplantingfuture generationsforest resource basebalance

Phrasing that earns marks

Threats to the environment: Human activities such as burning fossil fuels, deforestation and improper waste disposal cause air, water and land pollution, alongside a rise in greenhouse gases that drives global warming. Each threat has a distinct cause, so an exam answer should name a specific activity and its specific effect rather than describing pollution in general terms. Open burning and vehicle exhaust, for example, are two separate sources of air pollution with different pollutants and different control measures.
Greenhouse effect and global warming: Greenhouse gases such as carbon dioxide and methane trap heat in the atmosphere that would otherwise escape into space, a natural effect that keeps Earth warm enough for life. Human activities release extra greenhouse gases, trapping more heat and raising the average global temperature, which is global warming.
Biochemical oxygen demand (BOD): BOD measures how much oxygen microorganisms use to break down the organic matter in a water sample over a set time. A high BOD value means the water contains a lot of organic pollution, since more oxygen is used up, leaving less available for fish and other aquatic organisms. Comparing BOD values upstream and downstream of a discharge point is a common way to judge how much a specific source has polluted a river.
Preservation, conservation and restoration: Preservation protects an area of nature and keeps it untouched by human use, such as a strict nature reserve. Conservation manages the wise, sustainable use of a resource so it is not exhausted, such as controlled fishing. Restoration actively repairs an ecosystem that has already been damaged, such as replanting a cleared forest.
Green technology: Green technology reduces environmental harm compared with older methods, including renewable energy, recycling processes and cleaner production methods that release less pollution. Choosing a green technology usually involves weighing its environmental benefit against its cost and how practical it is to adopt widely. An electric vehicle, for example, produces no exhaust emissions itself, though the source of the electricity used to charge it also matters.
Sustainable development: Sustainable development meets the needs of people today without reducing the ability of future generations to meet their own needs. It balances economic growth, social wellbeing and environmental protection together rather than treating environmental protection as separate from development. Planting a fast-growing replacement crop for every tree logged in a plantation forest is one practical example of this balance.
Ozone layer depletion: The ozone layer in the upper atmosphere absorbs most of the sun's harmful ultraviolet (UV) radiation. Chemicals such as chlorofluorocarbons (CFCs), once common in aerosols and refrigerants, break down ozone molecules, thinning this layer and allowing more UV radiation to reach the Earth's surface, which raises the risk of skin damage and harm to some crops.
Eutrophication: Eutrophication begins when excess nutrients, often from fertiliser runoff or untreated sewage, enter a body of water and cause algae to grow rapidly, forming a dense bloom. When the algae die, decomposers use up large amounts of dissolved oxygen breaking them down, which can leave too little oxygen for fish and other aquatic life.

Frequently asked questions

What is the difference between preservation and conservation?

Preservation means protecting a natural area and keeping it untouched by human use, for example a strict nature reserve. Conservation means using and managing natural resources carefully and sustainably so they are not used up, for example controlled fishing or selective logging. Preservation avoids use; conservation allows wise use.

What does biochemical oxygen demand (BOD) tell us?

Biochemical oxygen demand is the amount of oxygen that microorganisms use to break down the organic matter in a sample of water over a set time. A high BOD means there is a lot of organic pollution, because the microorganisms use a lot of oxygen, which can leave too little for fish and other aquatic life. Cleaner water has a low BOD.

How does the greenhouse effect cause global warming?

Greenhouse gases such as carbon dioxide and methane in the atmosphere trap some of the heat that would otherwise escape to space, keeping the Earth warm enough for life. When human activities release extra greenhouse gases, more heat is trapped, and the average global temperature rises. This is global warming, which changes climates and sea levels.