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

Quick recall notes for OxfordAQA International GCSE Biology, Version 5.2 (for exams May/June 2018 onwards), sections 3.3.1 and 3.3.2. For full explanations and longer worked examples, read the study guide, then try the practice questions. Course hub: OxfordAQA IGCSE Biology. Checklist: printable checklist. To find weak spots first, use the free diagnostics. Photosynthesis and respiration background is in the bioenergetics notes.

Key facts at a glance

Section	Fact
3.3.1 a	Sun’s radiation is the energy source for most communities
3.3.1 a	Plants and algae transfer about 1% of incident light energy
3.3.1 b	About 10% of biomass passes up each trophic level
3.3.1 c	Biomass at each stage is drawn to scale as a pyramid of biomass
3.3.1 d (B)	Fewer stages in a food chain gives more efficient food production
3.3.2 a	Organisms need materials from surroundings and other organisms
3.3.2 b	Plants compete for light, space, water, mineral ions
3.3.2 c	Animals compete for food, mates, territory
3.3.2 d	Adaptations help organisms survive where they normally live
3.3.2 e	Extremophiles live in high salt, heat or pressure
3.3.2 f	Structural, behavioural, functional adaptations

Not required: pyramids of numbers, and constructing food webs or chains.

Energy flow

• Light from the Sun is trapped by plants and algae. About 1% of the incident energy is stored in cells.
• Each trophic level passes on about 10% of its biomass.
• Two reasons for the 90% not passed on:
  • waste materials (faeces, urine) carry materials and energy away;
  • respiration supplies the energy for all life processes, including movement, and much ends up as heat in the surroundings.
• A further useful point: some parts are not eaten (bones, roots, shells).
• Energy is transferred to the surroundings, not destroyed.

Method: percentage transferred

1. Take the biomass of the higher level.
2. Divide by the biomass of the level below.
3. Multiply by 100.

Reminder: producers 8 000 kJ, primary consumers 800 kJ. Transfer = 800 ÷ 8 000 × 100 = 10%. Predicted secondary consumers at 10% = 80 kJ.

Method: 1% then 10%

Light 3.2 × 10⁶ kJ per m². Producers store 1%: 3.2 × 10⁴ kJ. Primary consumers at 10%: 3.2 × 10³ kJ. Secondary consumers: 320 kJ.

Pyramids of biomass

Feature	Rule
Base	Producers
Bar length	Proportional to biomass (use a scale)
Bar height	Equal for every bar
Position	Bars centred on a vertical line
Labels	Each trophic level named; units and scale stated
Measure	Usually dry mass per unit area

Method: constructing a pyramid

1. Pick a scale so the largest bar fits on the page.
2. Divide each value by the scale.
3. Draw and label the bars.

Reminder: 640 kg, 72 kg, 8 kg with 1 mm = 10 kg gives 64 mm, 7.2 mm, 0.8 mm.

Interpreting: the narrowing of the bars shows biomass lost at each transfer. If the second bar is 12% of the first, then 12% was transferred.

Efficiency of food production (B)

• Every extra stage loses about 90% of the biomass.
• Fewer stages means more of the producers’ biomass reaches humans.
• Plant crop eaten directly is more efficient than the same crop fed to livestock.
• Reminder: 7 500 MJ of crop. Directly eaten: 7 500 MJ. Through cattle (10%): 750 MJ. That is a tenfold difference.
• Balanced point: grazing can use land that cannot grow crops, and meat provides protein.

Needs and competition

Group	Needs from surroundings	Needs from other organisms	Competes for
Plants	Light, carbon dioxide, water, mineral ions, space	Pollination, seed dispersal	Light, space, water, mineral ions
Animals	Oxygen, water, shelter, space	Food, mates	Food, mates, territory

• Competition happens when a resource is in short supply.
• Interdependence: organisms rely on each other, for example predators on prey and flowers on pollinators.
• Crowding (high density) reduces mean growth per individual because each gets a smaller share.
• Removing one species can affect others that depend on it.

Reading competition data

Method

1. Find the trend in mean mass (or number) as density or numbers change.
2. Quote two values from the table, and a percentage change if asked: (old − new) ÷ old × 100.
3. Link the trend to a resource in short supply.

Reminder: mean mass per seedling falls from 1.50 g to 0.30 g as density rises. Decrease = (1.50 − 0.30) ÷ 1.50 × 100 = 80%. Crowded seedlings share the same light, water and mineral ions, so each grows less.

Adaptations

Type	Meaning	Examples
Structural	Shape, size, colour	White fur in snow; flattened flea; thick waxy leaf cuticle
Behavioural	What the organism does	Migration; huddling; burrowing
Functional	Reproduction, metabolism and other processes	Concentrated urine; many eggs; hibernation lowers metabolic rate

Method: suggesting an adaptation from information

1. Problem the habitat creates (cold, dry, dark, predators).
2. The feature given in the information.
3. How it helps survival or reproduction.

Reminder: a seabird has dense waterproof feathers and a thick fat layer under the skin. Problem: cold water. Features: feathers trap an insulating layer of air; fat insulates. Benefit: less body heat lost, so less energy from respiration is needed to stay warm.

Extremophiles

• Live where conditions are very extreme: high salt, high temperature, high pressure.
• Examples: archaea in salt lakes, Thermus aquaticus in hot springs, microorganisms in the deep ocean.
• They are adapted: their cell chemistry works in conditions that would harm most organisms.

Parasite adaptations (B)

Parasite	Lives	Key adaptations
Flea	Among mammal hair	Flattened sideways; claws; piercing mouthparts; jumping legs
Tapeworm	Mammal intestine	Hooks and suckers; thick covering resists enzymes; flat and long for absorbing digested food; many eggs
Malaria parasite	Single-celled; inside human liver and red blood cells	Tiny; rapid reproduction; spread by mosquito vector

Always write: feature, then function. For example, hooks on a tapeworm’s head (feature) hold it against the gut wall so it is not carried out with the food (function). A feature without a function earns little credit.

Must-know distinctions

• Energy transferred vs energy lost: lost from the food chain, but still exists as heat.
• Biomass vs number: pyramids here use biomass; pyramids of numbers are not required.
• Competition vs interdependence: competition is for scarce resources; interdependence is reliance on others.
• Adaptation vs habitat: the habitat sets the problem (cold, dry, salty); the adaptation is the feature of the organism that solves it.
• Parasite vs predator: a parasite usually lives on or in a host and feeds on it without killing it quickly.

Quick self-test

1. What percentage of incident light energy do plants and algae transfer?
2. Give two reasons why only about 10% of biomass passes to the next trophic level.
3. Producers hold 2 500 kg and primary consumers 275 kg. Calculate the percentage transferred.
4. Scale 1 mm = 25 kg. What bar length shows 600 kg?
5. Where on a pyramid of biomass are the producers?
6. Why is eating wheat more efficient than eating beef from cattle fed on wheat?
7. Name two things plants compete for and two things animals compete for.
8. What is an extremophile? Give one condition they can tolerate.
9. Classify as structural, behavioural or functional: (a) huddling, (b) camouflage colouring, (c) producing thousands of eggs.
10. State two adaptations of a tapeworm for life in the gut.
11. Why can a flea move easily through fur?

Answers

1. About 1%.
2. Any two: materials and energy lost in waste; energy from respiration used for life processes, including movement; heat transferred to the surroundings.
3. 275 ÷ 2 500 × 100 = 11%.
4. 600 ÷ 25 = 24 mm.
5. At the base (bottom bar).
6. Fewer trophic levels, so less biomass and energy are lost in waste and respiration.
7. Plants: any two from light, space, water, mineral ions. Animals: any two from food, mates, territory.
8. An organism that lives in very extreme conditions, such as high salt, high temperature or high pressure.
9. (a) behavioural; (b) structural; (c) functional.
10. Any two: hooks or suckers; thick covering resisting digestive enzymes; long, flat body with large surface area; many eggs.
11. Its body is flattened from side to side, so it slides between hairs.

Where marks are usually lost

• Writing that energy is “used up” instead of transferred to the surroundings as heat.
• Giving only one reason for the 90% when two are asked for.
• Forgetting units, the scale or labels on a pyramid.
• Making bars different heights, or not centring them.
• Dividing the wrong way round when calculating percentage transfer (always higher level over lower level).
• Mixing up resources: plants and mates, animals and light.
• Naming an adaptation without saying how it helps the organism survive.
• Stating “it is adapted to the cold” with no feature named.
• Describing a parasite feature but not linking it to attachment, feeding or spreading.

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.