Inheritance and ecology · GCSE Biology
Food chains and biomass
Build food chains for GCSE Biology: producer, primary and secondary consumers, pyramids of biomass drawn to scale, and why only about 10% of biomass is passed on at each trophic level.
Arrow means “eaten by” and shows energy flow. Producers photosynthesise. Pyramids of biomass are drawn to scale and almost always get smaller up the chain because of the ~10% transfer.
The important bits
What you need to know
- 1
A food chain shows feeding relationships: producer → primary consumer → secondary consumer → tertiary consumer. The arrows show the direction of energy and biomass, not “what eats what” backwards.
- 2
Producers are photosynthetic organisms (usually plants or algae) that transfer light energy into chemical energy in glucose (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂).
- 3
Consumers eat other organisms. Herbivores are primary consumers; carnivores feeding on them are secondary consumers. A food web is many linked chains.
- 4
On Higher tier, pyramids of biomass are drawn to scale. Bars represent biomass at each trophic level (often g/m²). They are almost always pyramid-shaped. Pyramids of number can invert (thousands of insects on one oak).
- 5
Only a small percentage of biomass (often taught as about 10%) is transferred to the next level. The rest is lost in uneaten parts, egestion, and energy transferred to the surroundings by respiration (heat from the exothermic reaction) and movement.
- 6
That 10% rule explains why food chains are short and why feeding people on grain uses the producer level more efficiently than feeding the same grain to cattle then eating beef.
- 7
If a producer dies or a pesticide kills insects, every consumer above can fall. Bioaccumulation: toxins such as DDT or mercury become more concentrated up the chain because they are not broken down.
- 8
Apex predators are few because little biomass remains at the top. Removing them can make mid-level consumers boom and over-graze producers — a cascade.
Quotations worth analysing
Short evidence. Real method.
“The arrows in a food chain show the direction of energy transfer.”
Producer on the left, or at the base of a pyramid. Reversing arrows is an instant zero.
“About 10% of biomass is transferred to the next trophic level.”
Explain the losses: not eaten, egested, respired as heat. Do not write that energy is destroyed.
“Pyramids of biomass are drawn to scale.”
Width represents biomass. A sketch that is not to scale cannot show a 10 000 kg producer bar versus a 1 000 kg consumer bar fairly.
Go deeper
How do I construct a pyramid of biomass that would actually score?
Put the producer at the bottom as the widest bar. Primary consumers next, then secondary, then tertiary. The width of each bar is proportional to biomass, not to a doodle of the animal. If producers are 500 g/m² and herbivores 80 g/m², the bottom bar is 500/80 = 6.25 times wider. Label trophic levels. A pyramid of numbers for an oak tree can have a tiny producer bar (one tree) and a huge insect bar — that is why biomass is preferred. If the question gives dry mass, use those figures; water content would inflate wet mass unfairly. Units belong on the axis. Do not draw a triangle and call it a pyramid of biomass without bars.
Go deeper
Where does the “lost” 90% actually go?
Not every leaf is eaten. Of what is eaten, some is egested as faeces (indigestible cellulose for many animals). What is absorbed is partly used in respiration: glucose + oxygen → carbon dioxide + water, transferring energy that ends up heating the surroundings — that energy is not available as new biomass in muscle or fat. Movement, keeping warm (mammals and birds), and excretion of urea all take a cut. The small remainder is new growth (biomass) that a predator could eat. Plants also respire, so even producers do not lock 100% of photosynthetic glucose into biomass. In the exam, “energy is used up” is sloppy; write transferred to the environment as heat in respiration, or not consumed. Conservation of energy still holds.
Go deeper
How do I use a food chain to explain a pesticide or a population crash?
Name the trophic level that is hit, then follow the arrows. Insecticide kills primary consumers → secondary consumers starve (or also die if they eat poisoned prey). A toxin that does not break down becomes more concentrated at each step (bioaccumulation), so the top predator gets the highest dose — classic for otters, birds of prey, or humans eating large fish. If producers fail (drought, disease, deforestation), the whole chain shrinks from the bottom. If you are given a table of numbers over years, describe the pattern first, then name the feeding link. “Everything is connected” without a named producer or consumer is not ecology; it is a poster.
See the idea in action
Grass (10 000 kg) → rabbit (1 000 kg) → fox (120 kg). Arrows show energy from grass to rabbit to fox. The pyramid of biomass has a wide grass bar, a narrower rabbit bar, a narrow fox bar, drawn to scale. Transfer from grass to rabbit is 10%; from rabbit to fox is 12% — both near the taught 10% because of uneaten grass, faeces and heat from respiration. A student who draws the fox at the bottom or writes arrows towards the grass has reversed the chain. Feeding humans on grain uses the 10 000 kg level more efficiently than cycling it through rabbits first.
Exam technique
Turn knowledge into marks
Start every chain with a producer and draw arrows towards the consumer. For biomass pyramids, scale the bars and put producers at the base. Explain 10% with not eaten, egested, and heat from respiration — never “energy disappears”.
Common mistakes
Do not give these marks away
- 01
Drawing food-chain arrows the wrong way, or putting the producer at the top of a biomass pyramid.
- 02
Saying energy is used up or destroyed, rather than transferred as heat in respiration.
- 03
Treating a pyramid of numbers as if it were biomass when an oak tree inverts the numbers.
Why is a pyramid of biomass almost always narrower at each higher trophic level?
APredators are always smaller than their prey
BOnly a small percentage of biomass is transferred; much is not eaten, egested, or lost as heat from respiration
CProducers do not photosynthesise
DEnergy is created at the top of the chain
Show the answer
Only a small percentage of biomass is transferred; much is not eaten, egested, or lost as heat from respiration. About 10% of biomass typically passes up each step. Predators can be larger than individual prey (whale and krill), so size of one animal is not the reason. Energy is not created at the top.
Quick questions
If this is the bit you searched
Which way do the arrows go in a food chain?
From the organism that is eaten to the organism that eats it. They show energy and biomass transfer. Start with a producer.
What is a pyramid of biomass?
A scale diagram of the biomass at each trophic level, producers at the bottom. It is usually pyramid-shaped because little biomass is transferred up the chain.
Why is only about 10% of biomass transferred to the next level?
Not all of an organism is eaten, some is egested, and much of the energy in glucose is transferred to the surroundings as heat during respiration.
What is a producer?
An organism that makes its own food by photosynthesis, usually a plant or alga. It is the start of the food chain.