Is energy transfer always one tenth?

That is only a rough rule; real values run from about 5 to 20 per cent and never exceed 100 per cent.

How does a food web differ from a food chain?

A chain is a single path, while a web joins every feeding link in the community.

S4 Biology: Ecosystems and Energy Flow (Paper 1)

2026-11-01

The first paper starts from the roles in a food chain and uses the ten per cent rule for straightforward energy calculations.

Direct answer　Place every organism in the chain first, then multiply by one tenth level by level, checking whether the question asks what is gained or lost.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Biology: Ecosystems and Energy Flow
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationIn the food chain grass, rabbit, fox, which organism is the producer?
FoundationWhich energy change takes place during photosynthesis?
FoundationGrass holds 10 000 kJ of energy and the rabbit gains only one tenth. How much does the rabbit gain?
FoundationWhat is the main role of decomposers in an ecosystem?
IntermediateOf 5 000 kJ taken in by the rabbit only 500 kJ reaches the fox. What is the efficiency of energy transfer?
IntermediateGrass fixes 1 000 000 kJ of solar energy and energy passes on twice at one tenth each time. How much reaches the second consumer level?
IntermediateA primary consumer takes in 2 000 kJ, respires 1 400 kJ and excretes 300 kJ. How much is left for growth?
ChallengeA four-level chain starts with 200 000 kJ and each transfer keeps one tenth. How much reaches the fourth level?
ChallengeWhy can a pyramid of energy never be inverted?
Error redoA student writes that energy is recycled in an ecosystem. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	The grass	A producer makes its own food from light energy; the rabbit and the fox both eat other organisms, so they are primary and secondary consumers.
2	Light energy is converted into chemical energy	Chlorophyll traps light energy and builds glucose from carbon dioxide and water, storing the energy in the chemical bonds of the organic molecule.
3	1 000 kJ	About one tenth of the energy passes on, so 10 000 x 10% = 1 000 kJ.
4	They break down dead organisms and wastes into inorganic substances returned to the environment	Decomposers break organic matter into inorganic nutrients that producers can take up again.
5	10 per cent	500 divided by 5 000 is 10 per cent; most of the rest is lost as heat from respiration.
6	10 000 kJ	One million multiplied by ten per cent twice gives 10 000 kJ.
7	300 kJ	Subtracting respiration and excretion leaves 300 kJ, and only this portion can pass to the next level.
8	200 kJ	Three transfers give 200 000 x 0.1 cubed, so food chains rarely run beyond four or five levels.
9	Each transfer loses much energy as heat, so a higher level always holds less energy than the level below	Energy flows one way and respiration keeps losing heat, so a higher level can never hold more energy than the level below.
10	Energy flows one way and is lost as heat; it is matter that cycles	Matter cycles, but energy is lost as heat at every level and cannot be reused.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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