S5 Biology: Energy Flow in Ecosystems — Tutor Lesson Plan (60 min, NPP to carbon cycle, four errors)

2026-11-02

Tutor lesson-prep guides · Secondary biology (DSE)

Students can recite the ten per cent rule and still start the calculation from the wrong number. This lesson makes net primary production the only starting line, then uses one set of figures to explain why a food chain is short and why meat costs land.

Objectives — the student can distinguish gross from net primary production and compute the photosynthetic efficiency, apply the ten per cent figure level by level to explain the shape and length of a food chain, and explain the decomposer’s double role, separating the cycling of matter from the one-way flow of energy.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Photosynthesis	What does photosynthesis need?	Light, carbon dioxide and water
Respiration	What does respiration release?	Energy, carbon dioxide and water
Food chains	How does a producer differ from a consumer?	A producer makes its own food; a consumer eats

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Hook	One square metre of forest and a year of sunlight	Guess how much becomes wood
8-22 min	Scene 1	GPP, respiration and NPP, 0.4 per cent efficiency	Get 20000 and 23.1 per cent
22-34 min	Scene 2	Ten per cent rule and pyramid shape	Get 2000, 200, 20
34-46 min	Scene 3	Decomposers, matter cycles and energy does not	Separate the two statements
46-54 min	Scene 4	Diet and land, the thousandfold comparison	Get the factor
54-60 min	Exit check	Three items in section 08	Marked at once

03 Teaching script (read it out)

Scene 1: Sunlight in, a fraction of a per cent out

Incident solar energy on a square metre of forest over a year is about 5.0 times 10 to the sixth kilojoules. Net primary production is about 20000 kilojoules, so the photosynthetic efficiency is 20000 divided by 5.0 million, which is 0.4 per cent. Gross production is 26000 and respiration takes 6000, leaving 20000, so respiration has already spent 23.1 per cent of what was fixed. The sentence to kill is that plants turn sunlight into biomass: more than 99 per cent of the light never enters the food chain at all, and only net production is available to the next level, which is why every pyramid calculation starts from NPP.

Ask: “Why must the pyramid use NPP rather than GPP?” Because the respired share has already left as heat and cannot be eaten by anything.

Scene 2: Ten per cent, level by level

From 20000 kilojoules the primary consumers take about 2000, the secondary 200 and the tertiary 20, so the fourth level holds 2.00 kilojoules, which cannot support a breeding population, and that is the honest reason food chains are short. Where does the other ninety per cent go? Respiration, excretion, the parts that are never eaten and become detritus for decomposers, and the cost of movement. Energy pyramids are always upright because each level holds less than the one below. Biomass pyramids can invert in the sea, where phytoplankton turn over fast, and pyramids of numbers invert easily, since one tree feeds many insects, so the type of pyramid must be named before it is read.

Warn: treat ten per cent as an average and use the figure the question supplies. A student who writes that exactly ten per cent always transfers has turned a rule of thumb into a false law.

Scene 3: Matter cycles, energy does not

Decomposers, bacteria and fungi, oxidise the organic carbon in detritus back to carbon dioxide and return nitrogen as ammonium, so the nutrients reach the producers again: matter cycles. Energy cannot cycle, because every transfer loses heat, so an ecosystem needs a continuous input of sunlight and stops without it. The two sentences are separate claims and must be said separately. The human consequence follows from the same arithmetic: eating plants directly leaves 2000 kilojoules available from that square metre, while eating a tertiary consumer leaves 20, so the same diet needs roughly a thousand times the land. That is a resource calculation, not a moral one, and presenting it that way keeps the lesson honest.

Ask: “Do decomposers give energy back to plants?” They give matter back; the energy has already left as heat.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	NPP is 20000 kilojoules. Find the energy available to primary consumers.	Multiply by 0.1, and start from NPP not GPP	2000 kilojoules
Graph reading	A pyramid narrows at the top. Name the type and the reason.	Energy pyramid, each level loses heat	Only energy pyramids must be upright
Explanation	Explain why a food chain rarely has six levels.	Ten per cent transfer and ninety per cent loss leaves too little at the sixth level	The arithmetic, not the adjectives
Application	Compare the land needed for a plant diet and a meat diet.	Compare the available energy at each trophic level	About a thousandfold

Sanity check (teach this): Every calculation is checked by the starting value: NPP, never GPP, and never the incident sunlight. Every percentage must be below a hundred, and every pyramid answer must name which pyramid is meant. An answer that moves energy around a cycle has contradicted the second law, and the correction is the sentence about heat, said once and then practised with a diagram.

05 Independent practice (3 questions)

Question	Answer	What to watch
NPP is 20000 kilojoules. Find the energy available to primary consumers.	2000 kilojoules.	Multiply by 0.1 once
GPP is 30000 and respiration 7500. Find NPP.	22500 kilojoules.	Subtract, do not multiply
Starting from 20000, how much reaches the tertiary consumers?	20 kilojoules.	Three multiplications
Why can an energy pyramid never invert?	Each level holds less energy than the one below, because heat is lost.	Say lost, not recycled

06 Common mistakes and what to say

Mistake	Student writes	Say this
Starting from GPP instead of NPP	Multiplying 26000 by 0.1	Only net production is available to the next level; the respired share never leaves the plant
Believing energy cycles	Saying decomposers return energy to plants	Decomposers return matter; the energy has already gone as heat
Treating ten per cent as a law	Writing that exactly ten per cent always transfers	It is an average, so use the figure the question gives
Ignoring unit conversion	Adding kilocalories to kilojoules	One kilocalorie is 4.18 kilojoules, and skipping the conversion spoils the whole answer

07 Homework (paper, section and question numbers)

Secondary biology chapter index — Read this chapter with the photosynthesis and the ecology chapters
HKDSE Biology Paper 1 practice paper — Ecosystem questions; redo section 05 questions 1 to 4 of this guide

08 Exit check (last 5 minutes)

Question	Answer	Decision
NPP is 20000 kilojoules. How much reaches secondary consumers?	200 kilojoules.	All correct: move to succession
Can an energy pyramid invert?	No, because energy falls at every level.	Wrong: redo the ten per cent rule
Name the decomposer’s two roles.	Break down organic matter and release inorganic nutrients.	Two or more wrong: back to scene one

09 Tutor FAQ

Do students need to remember 5.0 times 10 to the sixth?

Only as an order of magnitude, so that 0.4 per cent looks plausible rather than mysterious.

Is the land comparison examinable?

The arithmetic is, and it is the strongest application of the ten per cent rule, which is why it is worth the last eight minutes.

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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.