OxfordAQA International GCSE Biology: Decay, the carbon cycle and human effects on the environment (9201) – Revision Notes

These notes condense sections 3.3.3 and 3.3.4 of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. For full explanations and worked examples, read the study guide, then test yourself with the practice questions. Course pages: Biology hub and printable checklist. Not sure where you stand? Take a free diagnostic.

3.3.3 a-d: Decay

Key points

• Organisms remove materials from the environment to grow and for other processes.
• Materials return in waste or when organisms die and decay.
• Decay happens because microorganisms digest (break down) the material.
• Decay releases substances plants need to grow.
• In a stable community, removal of materials is balanced by return, so the materials are part of a constant cycle.

Decay conditions

Condition	Effect on decay	Reason
Warm	Faster	Enzymes work faster; microorganisms grow and reproduce faster
Too hot	Stops	Enzymes denature; microorganisms die
Cold	Slow	Enzyme action and growth slow down
Moist	Faster	Water needed for enzyme action and cell growth
Dry	Slow	Microorganisms cannot grow or digest
Aerobic	Faster	Oxygen for aerobic respiration, which supplies energy
Waterlogged	Slow	Little oxygen; spaces filled with water

Detritus feeders (earthworms, woodlice, maggots) eat dead material and waste. They break it into small pieces, which gives microorganisms a larger surface area.

Method in steps: explain a decay result

1. Say what the microorganisms do: digest the material using enzymes.
2. Name the condition that changed (temperature, water or oxygen).
3. Say how it changes enzyme action or respiration.
4. Link back to the result: faster or slower mass loss.

Worked reminder. Dry mass falls from 20.0 g to 9.8 g. Mass lost = 10.2 g. Percentage lost = 10.2 ÷ 20.0 × 100 = 51.0%. Over 21 days the mean rate is 10.2 ÷ 21 = 0.49 g per day.

3.3.3 e: The carbon cycle

Step	Process	Carbon form
Carbon dioxide taken from air	Photosynthesis (green plants, algae)	CO₂ becomes carbohydrates, fats, proteins
Passes along food chain	Feeding	Fats and proteins in consumers
Returned to air	Respiration (plants, algae, animals, microorganisms)	CO₂
Dead bodies and waste	Feeding by animals and microorganisms; decay	Eventually CO₂
Returned to air	Combustion of wood and fossil fuels	CO₂

Learn the cycle as a loop in words:

Air carbon dioxide, then photosynthesis, then plant and algal compounds, then feeding, then animal compounds, then death and waste, then feeding by detritus feeders and microorganisms, then respiration, then air carbon dioxide.

Respiration arrows also run from plants, algae and animals straight to the air, and combustion adds more.

Energy note. By the time microorganisms and detritus feeders have broken down waste and dead bodies and cycled the materials as plant nutrients, all the energy originally absorbed by green plants and algae has been transferred. Carbon is recycled; energy is not.

Method in steps: apply the carbon cycle

1. List every process that removes CO₂ from the air (photosynthesis).
2. List every process that adds CO₂ (respiration by all groups, combustion).
3. Compare totals: more removed means carbon is being stored; more added means the air gains CO₂.
4. A balanced total fits a stable community.

Worked reminder. Removed 120 units, returned 55 + 48 + 7 + 4 = 114. Net removal = 6 units, stored as extra biomass.

Applying the principles to unfamiliar situations

• A sealed, lit jar with a plant and a snail can run for a long time. Photosynthesis takes in CO₂, respiration by the plant, snail and microorganisms returns it, and decay returns mineral ions. Nothing is added, so the materials are cycling.
• Fallen leaves in a dry, cold place stay for years because decay is slow. The same leaves in a warm, damp, well-aerated compost heap are digested quickly.
• A field crop is harvested and removed. The mineral ions taken up by the plants are not returned by decay, so fertiliser may be needed. This breaks the balance of a stable community.
• Burning a forest turns carbon stored in wood into CO₂ at once, far faster than decay would.

3.3.4 a-b: Waste and pollution

• Rapid population growth and a rising standard of living produce more waste.
• Waste and chemicals that are not handled properly cause pollution.

Medium	Pollutant	Consequence
Water	Sewage	Eutrophication
Water	Fertiliser	Eutrophication
Water	Toxic chemicals	Harm to aquatic organisms
Air	Smoke	Air pollution
Air	Sulfur dioxide	Contributes to acid rain
Land	Pesticides, herbicides	Toxic to organisms; can be washed into water

Eutrophication in four stages

1. Mineral ions rise, so algae and/or plants grow more.
2. Dense growth shades others; some cannot get enough light for photosynthesis and die.
3. Microorganisms feeding on the dead organisms increase greatly.
4. Their respiration depletes the oxygen; aerobic organisms die.

Key point: the oxygen is used by the microorganisms’ respiration. Dead algae do not use oxygen themselves.

Worked reminder. A lake’s dissolved oxygen falls from 10.4 to 1.2 mg per dm³. Fall = 9.2 mg per dm³. Percentage fall = 9.2 ÷ 10.4 × 100 = 88.5%. Fish and other aerobic organisms die because they cannot get enough oxygen for aerobic respiration.

Other pollution to link up

• Sulfur dioxide comes from burning fuels that contain sulfur. It dissolves in rainwater and contributes to acid rain, which harms aquatic life and plants.
• Pesticides and herbicides used on fields can be washed from land into streams and lakes, so land pollution becomes water pollution.
• Untreated sewage and fertiliser both supply mineral ions, so both can cause eutrophication.

3.3.4 c: Deforestation

• Large-scale tropical deforestation has reduced biodiversity (fewer species and fewer habitats).
• Less photosynthesis, so less CO₂ removed from the air.
• Burning releases CO₂ by combustion; decay of remains releases CO₂ by microorganisms’ respiration.

Worked reminder. Forest mean 30.5 species per plot, cleared mean 9.5. Reduction = (30.5 − 9.5) ÷ 30.5 × 100 = 68.9%.

3.3.4 d: Global warming

• Carbon dioxide and methane in the atmosphere are increasing and contribute to global warming.
• A rise of only a few degrees Celsius may cause: big changes in climate; a rise in sea level; changes in migration patterns (for example birds); changes in the distribution of species.

Worked reminder. CO₂ rises from 317 to 414 ppm. Rise = 97 ppm. Percentage = 97 ÷ 317 × 100 = 30.6%. Over 60 years the mean rise is 97 ÷ 60 = 1.6 ppm per year.

Linking effects to biology

• Rising sea level reduces the habitat of coastal organisms.
• Warmer conditions can move the range in which a species can live, so its distribution changes.
• Birds that migrate may arrive earlier or later than before, so migration patterns change.

Analysing data and evaluating methods

Term	Meaning	Improved by
Valid	Measures what it claims to measure	Controlling other variables; representative sites
Reliable	Gives similar results if repeated	Repeats, larger samples, a mean
Precise	Small differences can be detected	Suitable instruments

Method in steps: describe a trend from a table

1. Quote the first and last values with units.
2. Give the size of the change, and the percentage if useful.
3. Mention any peak, fall or change in rate.
4. Link to the biology.

Must-know distinctions

• Decay (microorganisms, digestion) versus respiration (energy release in cells): microorganisms do both.
• Carbon cycles; energy is transferred and lost to the surroundings.
• Eutrophication (too many nutrients) versus acid rain (sulfur dioxide).
• Correlation versus cause.

Quick self-test

1. Name the microorganisms mainly responsible for decay.
2. State three conditions that make decay faster.
3. Why does decay stop at a very high temperature?
4. Which process removes carbon dioxide from the air?
5. Name two processes that return carbon dioxide to the air.
6. Calculate the percentage lost when 40.0 g falls to 31.0 g.
7. Name a gas that contributes to acid rain.
8. In eutrophication, which organisms use up the oxygen?
9. Give two effects of tropical deforestation.
10. Name two gases that are increasing and contribute to global warming.
11. Give two possible effects of a few degrees of warming.
12. What does “reliable” mean?

Answers

1. Bacteria and fungi.
2. Warm, moist, aerobic.
3. Enzymes denature and the microorganisms die.
4. Photosynthesis.
5. Respiration; combustion (also decay by microorganisms, which is their respiration).
6. 9.0 ÷ 40.0 × 100 = 22.5%.
7. Sulfur dioxide.
8. The microorganisms feeding on the dead algae and plants.
9. Reduced biodiversity; less CO₂ removed from the air (or more CO₂ released by burning).
10. Carbon dioxide and methane.
11. Rise in sea level; changed species distribution (or changed migration patterns, big changes in climate).
12. Repeating the measurements gives similar results.

Where marks are usually lost

• Writing “bacteria eat the leaves” instead of describing digestion by enzymes.
• Saying decay happens fastest in hot conditions without noting that enzymes denature at very high temperature.
• Leaving out oxygen as a condition, or ignoring that waterlogging removes it.
• Forgetting respiration by plants and microorganisms in the carbon cycle.
• Drawing a carbon cycle where CO₂ goes straight from animals to plants.
• Saying the algae use up the oxygen in eutrophication.
• Calculating percentage change using the wrong starting value.
• Missing units in data answers (ppm, mg per dm³, g per day).
• Calling a poorly designed sample “reliable” instead of naming what makes it invalid.

Official syllabus

OxfordAQA International GCSE Biology (9201), Version 5.2, for exams May/June 2018 onwards, sections 3.3.3 and 3.3.4. Published by OxfordAQA.