OxfordAQA International GCSE Biology: Energy transfer, biomass pyramids, adaptations and competition (9201)

This guide is aligned to the OxfordAQA International GCSE Biology specification, Version 5.2 (for exams May/June 2018 onwards). It covers sections 3.3.1 (energy transferred in ecosystems) and 3.3.2 (adaptations, interdependence and competition). The qualification is untiered, so every candidate meets all of this content. Revise with the revision notes and test yourself with the practice questions. Course hub: OxfordAQA IGCSE Biology. Tick off outcomes on the printable checklist. To find your weak spots first, take a short diagnostic.

What this unit covers

Section	What you must be able to do
3.3.1 a	Say that the Sun is the energy source for most communities, and that plants and algae transfer about 1% of the incident light energy
3.3.1 b	Explain why only about 10% of biomass passes to the next trophic level
3.3.1 c	Interpret pyramids of biomass and construct them to scale from data
3.3.1 d	Explain how shortening a food chain improves the efficiency of food production
3.3.2 a	State what organisms need from their surroundings and from other living things
3.3.2 b, c	Describe what plants and animals compete for
3.3.2 d, e, f	Define adaptations and extremophiles; recognise structural, behavioural and functional adaptations
3.3.2 (parasites)	Explain how fleas, tapeworms and malaria parasites are adapted to their hosts
3.3.2 (application)	Suggest how an organism is adapted when you are given information about it

Pyramids of numbers are not required. Constructing food webs and food chains is not required either, although you will still meet chains in questions.

3.3.1 a: the Sun as the energy source

Almost all the energy in most communities began as radiation from the Sun. Plants and algae absorb light and use it in photosynthesis (see the bioenergetics guide). They transfer only about 1% of the incident light energy. That energy is stored in the substances that make up their cells: glucose, starch, cellulose, proteins and lipids.

Why so little? Typical reasons are that some light is reflected, some passes straight through leaves, some lands on parts without chloroplasts, and some wavelengths are not absorbed. These reasons are common explanations rather than a required list, so the number matters more than the reasons.

Worked example 1. A field receives 3.2 × 10⁶ kJ of light energy per m² in a year. Plants store about 1% of it.

Energy stored = 3.2 × 10⁶ × 0.01 = 3.2 × 10⁴ kJ per m² (32 000 kJ).

3.3.1 b: why only about 10% moves up

Only approximately 10% of the biomass from each trophic level is transferred to the level above. The rest is lost for two reasons:

• some materials and energy are always lost in waste materials (faeces, urine and, for plants, shed leaves that are not eaten);
• respiration supplies all the energy needed for living processes, including movement. Much of that energy is eventually transferred to the surroundings as heat.

Another point often used in answers is that some parts of an organism are not eaten, such as bones, shells or roots. Remember that the respired carbon leaves as carbon dioxide, so it is no longer biomass.

Worked example 2. Biomass in a pond was measured per hectare: algae 4 800 kg, water fleas 520 kg, minnows 61 kg.

Algae to water fleas: 520 ÷ 4 800 × 100 = 10.83, so 10.8%. Water fleas to minnows: 61 ÷ 520 × 100 = 11.73, so 11.7%.

Both values are close to 10%, which fits the rule. The rule is an approximation, and real values vary between ecosystems.

Worked example 3. Using the 1% and 10% figures from example 1, the energy stored by plants is 3.2 × 10⁴ kJ per m². The next levels hold about:

Level	Energy stored (kJ per m²)
Producers	32 000
Primary consumers (10%)	3 200
Secondary consumers (10%)	320
Tertiary consumers (10%)	32

Because so little is left after three transfers, food chains rarely have more than four or five trophic levels.

3.3.1 c: pyramids of biomass

A pyramid of biomass has one horizontal bar for each trophic level, with the producers at the base. Each bar is drawn to scale, so its length is proportional to the biomass at that level. The bars are centred on a vertical line.

Constructing a pyramid

1. Choose a scale that lets the largest value fit on the page.
2. Divide every value by the scale to get each bar’s length.
3. Draw the bars, producers at the bottom, all the same height, centred.
4. Label each bar with the organism or trophic level, and give the units and the scale.

Worked example 4. Grassland biomass (dry mass, kg per hectare): grass 3 000, grasshoppers 360, frogs 45.

Use a scale of 1 mm = 50 kg.

• Grass: 3 000 ÷ 50 = 60 mm
• Grasshoppers: 360 ÷ 50 = 7.2 mm
• Frogs: 45 ÷ 50 = 0.9 mm

The frog bar is under 1 mm. That is correct, not a mistake: the pyramid is narrow because each transfer is about 10%. Here the transfers are 360 ÷ 3 000 = 12.0% and 45 ÷ 360 = 12.5%.

Interpreting a pyramid

• A wide base and narrowing bars show that biomass falls at each level.
• The ratio of one bar to the one below is the percentage transferred.
• The biomass is usually dry mass, because water content varies and dry mass is a better measure of the material that has been built.
• Say what the pyramid represents. A bar shows the biomass at the time of sampling.

3.3.1 d: efficiency of food production (B)

Each extra stage in a food chain loses most of the biomass and energy. Reducing the number of stages therefore increases the proportion of the producers’ energy that reaches humans. Eating plants directly (one step) is more efficient than feeding the plants to livestock and eating the meat (two steps).

Worked example 5. A hectare of crop stores 7 500 MJ of edible energy.

• Eaten directly by people: 7 500 MJ.
• Fed to cattle, which retain about 10%: 7 500 × 0.1 = 750 MJ in the meat.

If a person needs 10 MJ per day, the crop feeds 7 500 ÷ 10 = 750 person-days, but the meat feeds 750 ÷ 10 = 75 person-days. Cutting out the cattle stage gives ten times as much human food from the same land.

A balanced answer also notes that some land (steep hillsides, poor soil) suits grazing animals better than crops, and that meat supplies protein. The question usually asks only for the efficiency argument.

3.3.2 a to c: needs and competition

To survive and reproduce, organisms need a supply of materials from their surroundings and from other living organisms.

	Plants	Animals
From surroundings	Light, carbon dioxide, water, mineral ions, space	Oxygen, water, shelter, space
From other organisms	Pollination and seed dispersal by animals	Food (plants or other animals), mates

Where supplies are limited, organisms compete.

• Plants compete for light and space, and for water and nutrients (mineral ions) from the soil.
• Animals compete for food, mates and territory.

Competition can be between members of the same species or of different species. Organisms also depend on each other: a predator needs prey, and a flowering plant may need an insect to carry its pollen. This is interdependence.

Worked example 6. Mustard seedlings were grown in trays at four densities. The mean dry mass per plant was measured after four weeks.

Seedlings per tray	Mean dry mass per plant (g)	Total dry mass per tray (g)
10	0.92	9.2
20	0.61	12.2
40	0.34	13.6
80	0.19	15.2

Total mass = number × mean mass (for example 80 × 0.19 = 15.2 g).

Percentage decrease in mean mass from 10 to 80 seedlings: (0.92 − 0.19) ÷ 0.92 × 100 = 79.3%.

Explanation: crowded seedlings compete for light, water and mineral ions, so each plant has less to photosynthesise and grow with. The total per tray rises only slowly (9.2 g to 15.2 g) while seedling number rises eightfold, so each extra plant adds less and less.

3.3.2 d to f: adaptations

An adaptation is a feature that enables an organism, including a microorganism, to survive in the conditions in which it normally lives. There are three kinds.

Type	Meaning	Examples
Structural	Shape, size or colour	A flattened flea body; a white coat in snow; a thick waxy cuticle
Behavioural	What the organism does	Migration; emperor penguins huddling; sheltering in burrows at midday
Functional	Processes such as reproduction and metabolism	Concentrated urine in a desert rodent; producing very many eggs; flowering soon after rain

The types overlap. Hibernation is a behaviour, but it works by lowering metabolic rate. Give the type the question asks about, with a reason.

Extremophiles

Some organisms live where conditions are extreme, such as very high salt levels, high temperatures or high pressures. These are extremophiles. Examples: archaea in salt lakes that tolerate salt concentrations that would make water leave the cells of most organisms; bacteria such as Thermus aquaticus in hot springs at around 70 °C; and microorganisms living deep in the ocean under great pressure. Their proteins and cell structures work in conditions that would damage those of most organisms.

Using information to suggest adaptations

You may be given facts about an animal or plant and asked how it is adapted. Use three steps: identify the problem the habitat creates, name the feature, then say how it helps survival.

Worked example 7. A sand-dune grass has leaves that roll inwards, and its stomata sit in pits lined with hairs.

• Problem: dunes are windy and dry, so water vapour is easily lost.
• Feature: rolled leaves and hair-lined pits trap a layer of moist air.
• Benefit: this slows the diffusion of water vapour out of the leaf, so less water is lost by transpiration.

That is a structural adaptation.

Parasite adaptations (B)

A parasite lives on or in a host and gains food or shelter at the host’s expense.

Parasite	Where it lives	Adaptations and why they help
Flea	Among the hair of mammals	Body flattened from side to side so it moves between hairs; claws grip hair; piercing mouthparts feed on blood; strong legs for jumping between hosts
Tapeworm	Inside the intestines of mammals	Hooks and suckers on the head hold it to the gut wall; a thick outer covering resists digestive enzymes; a long flat body gives a large surface area to absorb digested food; no digestive system is needed; segments full of eggs give huge reproductive output
Malaria parasite	In humans (single-celled)	Small enough to live inside liver and red blood cells; reproduces rapidly inside them; passed between people by a mosquito, which is its vector

For a full-mark answer, join feature to function: say what the feature is, then how it helps the parasite feed, stay attached or spread.

Common errors

• Writing that energy is “used up” or “lost”. It is transferred to the surroundings as heat.
• Saying 10% of animals are eaten. The 10% refers to biomass (or energy) passing to the next level.
• Drawing a pyramid with unequal bar heights, no scale or no labels.
• Claiming that animals compete for light, or that plants compete for mates. Match the resource to the organism.
• Describing a feature without saying how it helps survival.

Official syllabus

OxfordAQA International GCSE Biology (9201), Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. Sections 3.3.1 and 3.3.2.