S1 Science - Ecosystems and Food Webs: Energy Flow and Nutrient Cycling Lesson Prep Guide

S1 Science – Ecosystems and Food Webs: Energy Flow and Nutrient Cycling Lesson Prep Guide

2026-11-15

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Students can say the big fish eats the small fish, then answer that energy is simply used up, and treat decomposers as consumers of rubbish. This lesson uses the ten per cent rule and three roles to turn a food web into a map of energy with arrows.

Objectives — Students can: (1) describe the roles of producers, consumers and decomposers and draw a complete food chain from a given list of organisms; (2) use the rule that only about one tenth of the energy passes to the next level to compute the energy available at each level; (3) explain how carbon moves through photosynthesis, respiration and burning, and why energy flows one way while matter cycles.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Photosynthesis	Which two raw materials do plants use to make food?	Carbon dioxide and water, releasing oxygen
Respiration	What do animals take in and give out when they respire?	Oxygen in, carbon dioxide out, energy released
Percentages	What is one tenth of 10,000?	1,000

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	A fish tank	Show a tank and ask who clears away a fish that dies	Name the decomposers
5-15 min	Three roles	Write grass, grasshopper, frog and bacteria on the board and sort them	State how each one obtains food
15-29 min	Ten per cent rule	Work down from 10,000 kJ of grass to 1,000, 100 and 10 and ask for the fourth level	Compute one tenth themselves
29-38 min	Food web and cycling	Add a second branch to show a web, then trace carbon in and out	Give the two directions of carbon
38-42 min	Independent practice	Three questions on chains, energy and roles; tour asking what each organism eats	Roles first, numbers second
42-45 min	Exit check	Collect answers and ask whether energy can be recycled	Answer no, only matter cycles

03 Teaching script (read it out)

Scene 1: Three roles: more than big fish eats small fish

Producers make their own food by photosynthesis, turning sunlight, carbon dioxide and water into organic matter, so they are the entry point for energy. Consumers eat other organisms: herbivores form the first level, carnivores the second. Decomposers such as fungi and bacteria break dead bodies and waste into simple inorganic substances, returning matter to the soil and air. Students often treat decomposers as consumers of rubbish, so make them say the difference: decomposers do not swallow food, they break organic matter into inorganic matter that producers can use again.

Students state the difference between a decomposer and a consumer

Scene 2: Ten per cent rule: why chains stay short

As energy passes along a food chain only about one tenth reaches the next level; the rest is used in respiration, movement and lost as heat. So 10,000 kJ of grass supports 1,000 kJ of grasshoppers, 100 kJ of frogs and 10 kJ of snakes. That is why a food chain usually has only four or five levels: above that there is too little energy to support a stable population. Ask students to use the rule to explain why top predators are always few.

Students explain why food chains rarely exceed five levels

Scene 3: Nutrient cycling: energy one way, matter returns

Carbon enters plants from carbon dioxide in the air through photosynthesis, returns to the air through respiration, and also returns when fossil fuels burn. Energy is different: it comes in as sunlight and leaves as heat, and cannot be returned to the sun. So we say energy flows but matter cycles, and the two words are not interchangeable. Ask students where the energy comes from and where it finally goes.

Students distinguish energy flow from matter cycling

04 Question types and worked steps

Type	Example	Steps	Answer
Draw a food chain	A field has grass, grasshoppers, frogs, snakes and fungi. Write one food chain	1) Start with the producer. 2) Link by feeding. 3) Arrow points to the eater	Grass to grasshopper to frog to snake; fungi are decomposers and stay off the chain
Energy loss	Grass holds 10,000 kJ; grasshoppers take one tenth and frogs one tenth of that. Energy in frogs?	1) 10,000 times 0.1 is 1,000. 2) Times 0.1 again	100 kJ
Classify a role	What role do bacteria in soil play?	1) Ask what they do. 2) They break bodies into inorganic matter	Decomposers, not consumers
Biomass ratio	1,000 aphids support 50 ladybirds. Ladybirds as a share of aphids?	1) 50 divided by 1,000. 2) Convert to a percentage	5 per cent, about twenty aphids per ladybird

Sanity check (teach this): Three checks. Direction: arrows run from the eaten to the eater, and a reversed arrow breaks the logic of the whole chain. Levels: each level is 0.1 of the one below it, so a higher level can never hold more energy than a lower one. Roles: decomposers stay off the main chain but must still be named. Numerically 1,000 aphids supporting 50 ladybirds is a ratio of 5 per cent, about twenty to one, which cross-checks any biomass pyramid answer.

05 Independent practice (3 questions)

Question	Answer	What to watch
Grass holds 100,000 kJ. Energy available to the second consumer level?	1,000 kJ	Whether one tenth was applied twice
Draw a food web in which grasshoppers have two predators	A web with more than one chain	Whether only one chain was drawn
Which organisms turn fallen leaves back into inorganic matter?	Decomposers such as fungi and bacteria	Whether decomposers were called consumers

06 Common mistakes and what to say

Mistake	Student writes	Say this
Decomposers treated as consumers	Bacteria eat leaves, so they are consumers	Decomposers break matter down into inorganic substances, they do not swallow it
Arrows reversed	Snake to frog to grasshopper to grass	Arrows run from the eaten to the eater, and energy follows the arrow
Only subtracting one tenth	10,000 minus 10 is 9,990	One tenth is what remains, not what is taken away
Saying energy cycles	Used energy goes back to the grass	Energy is lost as heat; only matter cycles

07 Homework (paper, section and question numbers)

Plants and photosynthesis, question 2 — Both need the roles stated before the process
Classification and dichotomous keys, question 3 — Both decide a category from stated features
States of matter and the particle model, question 1 — Both separate energy from matter

08 Exit check (last 5 minutes)

Question	Answer	Decision
Can energy be recycled along a food chain?	No, it flows one way and is lost as heat	Answering yes means energy and matter are still mixed up
Grass holds 10,000 kJ. Energy at the third level?	10 kJ	Answering 1,000 means one tenth step was missed
What do decomposers do?	Break dead bodies and waste into inorganic matter	Answering that they eat rubbish means re-teach the first script

09 Tutor FAQ

Should the numbers be decimals?

Keep S1 whole. Starting from 10,000 keeps every level an integer so attention stays on the idea rather than arithmetic.

Should the nitrogen cycle be taught?

Mention only that protein and fertiliser are linked. Drawing carbon in three directions is enough; nitrogen belongs to S4 Biology.

How many chains should the web have?

Three or four is enough. The point is that one organism can appear in several chains; a bigger web just gets copied without thought.

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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.