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DSE Biology | Energy Flow in Ecosystems Lesson-Prep Guide (Tutor Edition) | 10% Transfer, 90% Lost and a Four-Level Pyramid
Students can say the big fish eats the small fish but cannot explain why a food chain stops at four or five links. A run of divisions by ten turns energy flow into arithmetic.
Objectives — Students can: (1) calculate the energy available at each consumer level from the energy fixed by producers and state the transfer efficiency; (2) account for the other ninety per cent (respiration, excretion, uneaten material, dead bodies); (3) use the energy pyramid to explain the length limit of a food chain and note where a biomass pyramid can invert.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Photosynthesis | How do producers fix energy | Light to chemical energy stored in glucose |
| Respiration | Where does respired energy go | Some powers activity, most leaves as heat |
| Percentages | What percentage is 20 of 20,000 | 0.1% |
02 Lesson flow (60 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-8 min | Where energy comes from | Sunlight, photosynthesis, chemical energy | Define fixation |
| 8-20 min | Step down the levels | 20,000 to 2,000 to 200 to 20 | Ten per cent each step |
| 20-32 min | Where ninety per cent goes | Respiration, excretion, uneaten, dead | All four named |
| 32-42 min | Why pyramids stop | The arithmetic behind four or five levels | Only 2 kJ at level five |
| 42-52 min | Inverted pyramids | The tree and insect case | Numbers versus biomass |
| 52-60 min | Exit check | Three questions marked live | All three right |
03 Teaching script (read it out)
Scene 1: Energy flows one way; matter cycles
Energy enters as sunlight and producers fix it by photosynthesis: a meadow fixes 20,000 kJ per square metre per year.
That energy then travels one way only and never returns to the sun. Matter such as carbon and nitrogen cycles, energy does not, and writing that energy cycles loses the marks straight away.
Primary consumers take in only 2,000 kJ, a tenth. The other ninety per cent is never eaten, or lost as heat, or locked in material that cannot be digested.
That energy then travels one way only and never returns to the sun. Matter such as carbon and nitrogen cycles, energy does not, and writing that energy cycles loses the marks straight away.
Primary consumers take in only 2,000 kJ, a tenth. The other ninety per cent is never eaten, or lost as heat, or locked in material that cannot be digested.
Students must say: can energy be recycled like carbon (no, it flows one way).
Scene 2: The ten per cent rule is a division
Start at 20,000 kJ: primary consumers 2,000 (10%), secondary 200 (1%), tertiary 20 (0.1%) and a fourth level only 2 (0.01%).
Transfer efficiency is the next level over the previous one times 100%: 2,000 / 20,000 = 10%, and so on down. It is approximate in reality, between 5% and 20%, but when a question gives numbers the numbers decide.
Four destinations must be named for the missing ninety per cent: respiration lost as heat (the largest), energy in excreted waste, material never eaten, and dead bodies and leaf litter. Writing only that it was used up usually earns half marks.
Transfer efficiency is the next level over the previous one times 100%: 2,000 / 20,000 = 10%, and so on down. It is approximate in reality, between 5% and 20%, but when a question gives numbers the numbers decide.
Four destinations must be named for the missing ninety per cent: respiration lost as heat (the largest), energy in excreted waste, material never eaten, and dead bodies and leaf litter. Writing only that it was used up usually earns half marks.
Students must say: the tertiary level holds only 0.1%, so how much producer energy backs 20 kJ (20,000 kJ).
Scene 3: Why a pyramid cannot keep growing
Supporting a tertiary level of 20 kJ needs 200 kJ of secondary consumers and 20,000 kJ of producers, so the area required multiplies by ten at every step.
By the fifth level only 2 kJ remains, 0.01%, far too little to sustain a breeding population. That arithmetic, not a preference for simplicity, is why food chains run four or five links.
Biomass pyramids usually point the same way as energy pyramids, yet one tree carrying a huge insect population inverts the numbers pyramid, and examiners like asking which pyramid may invert.
By the fifth level only 2 kJ remains, 0.01%, far too little to sustain a breeding population. That arithmetic, not a preference for simplicity, is why food chains run four or five links.
Biomass pyramids usually point the same way as energy pyramids, yet one tree carrying a huge insect population inverts the numbers pyramid, and examiners like asking which pyramid may invert.
Students must say: why a food chain cannot run forever (each level keeps a tenth).
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Calculation | Energy and efficiency per level | Multiply by 10%, then check by division | 2,000, 200, 20 |
| Explanation | Where the ninety per cent goes | Respiration, excretion, uneaten, dead | All four named |
| Comparison | Energy versus biomass pyramids | Usually aligned; numbers can invert | Tree and insects |
| Essay | Limits on food chain length | Too little energy for a breeding population | Close on 0.01% |
Sanity check (teach this): Multiply by 0.1 each step: 20,000, 2,000, 200, 20, 2, and verify all four steps.
Efficiency is division, not subtraction: 2,000 over 20,000 is 10%.
Energy flows one way and matter cycles; reversing that costs the whole answer.
Keep 0.1% and 1% apart: the tertiary level holds 0.1% of producer energy, not 1%.
Efficiency is division, not subtraction: 2,000 over 20,000 is 10%.
Energy flows one way and matter cycles; reversing that costs the whole answer.
Keep 0.1% and 1% apart: the tertiary level holds 0.1% of producer energy, not 1%.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| Transfer efficiency to primary consumers | 10% | 2,000 divided by 20,000 |
| Tertiary consumers as a share of producer energy | 0.1% | 20 divided by 20,000 |
| Producer energy needed for 20 kJ at the tertiary level | 20,000 kJ | 20 divided by 0.001 |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Saying energy cycles | Writing that energy cycles through the chain | Energy flows one way; only matter cycles. |
| Only saying it was used up | One destination for the ninety per cent | Name respiration, excretion, uneaten food and dead bodies. |
| Wrong order of magnitude | Tertiary level answered as 1% | 20 over 20,000 is 0.1%, not 1%. |
| Confusing the pyramids | Claiming an energy pyramid can invert | Numbers or biomass may invert; an energy pyramid always stands upright. |
07 Homework (paper, section and question numbers)
- Secondary biology chapter index — Ecosystem practice
- All lesson-prep guides — Compare other subjects
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Energy at primary consumers | 2,000 kJ | All three right: move to nutrient cycles |
| Tertiary share of producer energy | 0.1% | One wrong: reteach the division |
| Producer energy required | 20,000 kJ | Two wrong: return to act three |
09 Tutor FAQ
Is the ten per cent rule a law?
It is an empirical rule, with real efficiencies between 5% and 20%; use the numbers a question supplies.
Why is respiration the biggest loss?
Keeping warm, moving and running metabolism all shed energy as heat, which is the largest single share.
Do decomposers count as a level?
They are not a consumer level, yet energy from dead bodies and waste still flows to them.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.
