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

This guide covers 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: decay and the carbon cycle, then humans and their effects on the environment. The qualification is untiered, so everything here is for every candidate. Use it with the revision notes and the practice questions. Course pages: Biology hub and the printable checklist. To find your weak spots first, try the free diagnostics.

What the unit covers

Section	You must be able to
3.3.3 a	Explain how materials removed from the environment are returned in waste or when organisms die and decay
3.3.3 b	Explain why microorganisms digest materials faster when it is warm, moist and aerobic
3.3.3 c	State that decay releases substances plants need
3.3.3 d	Describe a stable community as a balance of removal and return
3.3.3 e	Describe the carbon cycle and apply its principles
3.3.4 a	Link population growth and standard of living to waste and pollution
3.3.4 b	Describe water, air and land pollution and the stages of eutrophication
3.3.4 c	Describe the effects of tropical deforestation, including lower biodiversity
3.3.4 d	Explain how rising carbon dioxide and methane contribute to global warming and what a few degrees of warming may cause
Throughout	Analyse environmental data and evaluate collection methods

3.3.3 a-d: Decay and the cycling of materials

Living organisms take materials from the environment to grow and carry out life processes. Plants take in mineral ions and carbon dioxide. Animals take in food. These materials go back to the environment in two ways: in waste materials (faeces, urine, shed leaves) and when organisms die and decay.

What decay is

Decay is the breakdown of dead material and waste by digestion. The organisms responsible are microorganisms, mainly bacteria and fungi, which release enzymes onto the material and absorb the soluble products. Detritus feeders such as earthworms, woodlice and maggots eat dead remains and waste. This breaks it into smaller pieces with a larger surface area, so the microorganisms can work faster.

Why warm, moist and aerobic conditions speed it up

Condition	Why decay is faster
Warm	Enzymes work faster as temperature rises, so digestion is quicker. Microorganisms also grow and reproduce faster. Very high temperatures denature enzymes and kill the microorganisms, so decay slows or stops.
Moist	Microorganisms need water for their enzymes to work and for their cells to grow. Dry material decays very slowly.
Aerobic	Oxygen allows aerobic respiration, which supplies the energy microorganisms need to grow and make enzymes. In waterlogged or sealed conditions oxygen is scarce and decay is slow.

Waterlogging shows that moist is not the same as soaked. Too much water fills the spaces between particles, so oxygen cannot reach the microorganisms.

Nutrients and the stable community

Decay releases substances that plants need, such as mineral ions, back into the soil. Plants absorb them and use them to grow. In a stable community, the processes that remove materials are balanced by processes that return them. The materials are part of a constant cycle, so a mature woodland can keep growing for centuries without anyone adding fertiliser.

Worked example 1. A student puts 20.0 g of dry leaf litter into each of five mesh bags and buries one bag at each temperature for 21 days. The dry mass remaining is shown.

Temperature (°C)	5	15	25	35	60
Mass remaining (g)	18.6	15.4	9.8	6.1	19.2

(a) Calculate the percentage of the mass lost at 25 °C. Mass lost = 20.0 − 9.8 = 10.2 g. Percentage lost = 10.2 ÷ 20.0 × 100 = 51.0%.

(b) Calculate the mean rate of mass loss at 35 °C. Mass lost = 20.0 − 6.1 = 13.9 g. Rate = 13.9 ÷ 21 = 0.66 g per day.

(c) Explain the result at 60 °C. Only 0.8 g (4.0%) was lost. The enzymes of the microorganisms denature at this temperature and the microorganisms die, so there is almost no digestion.

(d) Suggest why the bags were made of mesh. Mesh lets microorganisms and air in while keeping the leaf material in, so the mass can be measured at the end.

3.3.3 e: The carbon cycle

Carbon moves between the atmosphere, living organisms and dead material. Work through the cycle in this order, because the specification lists these steps:

1. Green plants and algae remove carbon dioxide from the environment by photosynthesis.
2. They use the carbon to make carbohydrates, fats and proteins, which build their bodies.
3. When plants and algae respire, some carbon is released as carbon dioxide.
4. When animals eat plants or algae, and are eaten in turn, some carbon becomes part of the fats and proteins in the consumers.
5. Animals respire, so some carbon returns as carbon dioxide.
6. When organisms die, animals and microorganisms feed on the bodies.
7. Microorganisms respire, releasing carbon dioxide.
8. Burning wood and fossil fuels (combustion) releases carbon dioxide.

Respiration appears at every feeding level, so carbon dioxide is returned by plants, animals and microorganisms alike. By the time microorganisms and detritus feeders have broken down the waste and dead bodies and cycled the materials as plant nutrients, all the energy originally absorbed by green plants and algae has been transferred. Energy, unlike carbon, is not recycled: it ends up in the surroundings.

Applying the principles

“Apply” means you are given an unfamiliar situation and must use the same rules. Ask three questions: what takes carbon dioxide in, what puts it back, and which way is the carbon moving in this scenario?

Worked example 2. In a woodland each year, plants take 120 units of carbon from the air. Plant respiration returns 55 units, decomposer respiration 48 units, animal respiration 7 units and wood fires 4 units.

(a) Calculate the total carbon returned to the air: 55 + 48 + 7 + 4 = 114 units. (b) Calculate the net change in carbon in the air: carbon in = 120 (taken out of the air); returned = 114. The air loses 6 units each year, and the 6 units are stored as extra biomass, such as growing trees. (c) Is the woodland a stable community? Almost, but input is greater than output, so the store of carbon in living and dead material is growing. A perfectly balanced community would have equal input and output.

3.3.4 a and b: Waste and pollution

The human population has grown rapidly and standards of living have risen. More people, each using more goods and energy, means more waste. If waste and chemicals are not handled properly, pollution results.

Medium	Pollutants named in the specification	Effect
Water	Sewage, fertiliser, toxic chemicals	Eutrophication (sewage and fertiliser); poisoning of aquatic organisms
Air	Smoke and gases such as sulfur dioxide	Sulfur dioxide contributes to acid rain
Land	Toxic chemicals such as pesticides and herbicides	Harm to organisms on land; can be washed from land into water

Eutrophication

Learn this as a sequence. Fertiliser or sewage runs into a pond or river.

1. The concentration of mineral ions in the water rises, which stimulates the growth of algae and/or plants.
2. The growth of algae and plants means some cannot get enough light for photosynthesis, and they die.
3. The population of microorganisms that feed on the dead organisms increases greatly.
4. The microorganisms’ respiration uses up the oxygen in the water, so the concentration falls and aerobic organisms such as fish die.

Note that the oxygen is used up by the microorganisms, not by the algae. Blaming the algae directly leaves the explanation incomplete.

Worked example 3. A pond beside a golf course is tested weekly after fertiliser washes in.

Week	0	1	2	3	4
Nitrate (mg per dm³)	2.0	6.5	9.0	8.4	7.9
Dissolved oxygen (mg per dm³)	8.8	9.6	10.1	4.2	1.9

(a) Describe the change in nitrate by week 2. It rises from 2.0 to 9.0 mg per dm³, an increase of 7.0, which is 350%, or 4.5 times the starting value. (b) Calculate the percentage fall in oxygen from its peak to week 4: (10.1 − 1.9) ÷ 10.1 × 100 = 81.2%. (c) Explain why oxygen rose to week 2 then fell. Extra nitrate let algae grow and photosynthesise, adding oxygen. Then crowded algae shaded each other, died, and the microorganisms feeding on them respired and used oxygen faster than it was replaced.

3.3.4 c: Deforestation

Large-scale deforestation in tropical areas has reduced biodiversity. Describe the effects:

• Habitats and food sources are lost, so species die or move away. Many tropical species live in only one small area, so they can be lost completely.
• Fewer trees means less photosynthesis, so less carbon dioxide is removed from the air.
• Burning the trees releases carbon dioxide by combustion, and the decay of the remaining material releases more by microorganisms’ respiration.

Worked example 4. Six 20 m × 20 m plots in uncleared forest and six in cleared land are surveyed for tree and shrub species.

Plot	1	2	3	4	5	6
Forest	31	28	35	30	26	33
Cleared	9	12	7	10	11	8

Forest mean = 183 ÷ 6 = 30.5. Cleared mean = 57 ÷ 6 = 9.5. Reduction = (30.5 − 9.5) ÷ 30.5 × 100 = 68.9%. The ranges (26 to 35 and 7 to 12) do not overlap, which supports the conclusion that clearing reduces species richness. The data cover one place and one survey, so they do not show that all deforestation has the same effect.

3.3.4 d: Carbon dioxide, methane and global warming

Carbon dioxide and methane in the atmosphere are increasing and contribute to global warming. An increase in the Earth’s temperature of only a few degrees Celsius may:

• cause big changes in the Earth’s climate
• cause a rise in sea level
• cause changes in migration patterns, for example in birds
• result in changes in the distribution of species.

Worked example 5. The table gives yearly average carbon dioxide at a monitoring station, in parts per million (ppm).

Year	1960	1980	2000	2020
CO₂ (ppm)	317	339	370	414

Rise per 20 years: 22, 31 and 44 ppm. In 1960 to 2020 the rise is 414 − 317 = 97 ppm, which is 97 ÷ 317 × 100 = 30.6%. The rise is faster in each period (1.1, then 1.6, then 2.2 ppm per year), so the concentration is accelerating.

Analysing data and evaluating methods

Throughout this section you may be given data and asked to interpret it, or a method and asked to judge it.

• Valid means the method measures what it claims to. A single water sample from one place does not show the state of a whole lake.
• Reliable means repeating gives similar results. Several samples, repeated measures and a mean improve this.
• Check sample size, whether sites are representative, whether other variables were controlled, and whether the instrument is precise enough.
• Correlation is not proof of cause. CO₂ and temperature rising together fits the explanation, but the data alone do not prove it.

Common errors

• Writing that decay needs “heat” rather than warmth, or ignoring that very high temperatures stop it.
• Saying microorganisms “eat” the material without mentioning digestion by enzymes.
• Showing arrows in the carbon cycle from animals to plants without photosynthesis in between.
• Forgetting that microorganisms respire too.
• Saying algae “use up” the oxygen in eutrophication.
• Writing “pollution” without naming the pollutant and what it does.

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.