S5 Biology: Energy Flow in Ecosystems prep guide | Photosynthetic efficiency, energy pyramids and the carbon cycle

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can recite that energy pyramids narrow upwards, then answer “because energy is lost” when asked why food chains stop after four or five links, without saying how much is lost or where it goes. Every stage in this lesson carries a number.

Objectives — Students can: (1) use gross and net primary productivity with respiration to account for energy at the producer level; (2) apply the ten percent rule to compute energy at each trophic level and explain the shape of the pyramid; (3) describe the role of decomposers in energy flow and the carbon cycle, and compare the land needed for plant and meat diets.

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, water
Respiration	What does respiration release	Energy, carbon dioxide, water
Food chains	Producer against consumer	Makes its own food versus feeds on others

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-12 min	Sunlight to producers	Incident energy 5.0 million kJ per square metre per year	NPP 20000, efficiency 0.4 percent
12-26 min	Gross, net and respiration	GPP 26000, NPP 20000	Respiration takes 23.1 percent
26-42 min	The ten percent rule	20000, then 2000, 200, 20 kJ	One tenth survives each step
42-50 min	Decomposers and the carbon cycle	Organic to inorganic	Matter cycles, energy does not
50-56 min	The land cost of a meat diet	1000 times the area for a third-level consumer	Shorter chains need less land
56-60 min	Exit check	Three questions collected	Done independently

03 Teaching script (read it out)

Scene 1: Sunlight to producers: efficiency under one percent

Incident solar energy is about 5.0 million kJ per square metre per year and a forest fixes roughly 20000 kJ in net primary productivity, so the photosynthetic efficiency is 0.4 percent.
Gross productivity of 26000 minus respiration of 6000 leaves that 20000, so respiration already consumes 23.1 percent of what is fixed. The claim that plants turn all sunlight into biomass is therefore false; more than 99 percent never enters the food chain at all.
Only net productivity is available to the next trophic level, so any pyramid calculation must start from NPP.

Task: with gross productivity 30,000 and respiration 7,500 kJ per square metre per year, find net productivity and the respiration share: 22,500 and 25.0 percent.

Scene 2: The ten percent rule: why food chains are short

Starting from net productivity of 20000 kJ per square metre per year, roughly one tenth passes to each level: 2000 to primary consumers, 200 to secondary, 20 to tertiary.
Where does the other 90 percent go? Respiration as heat, excretion and uneaten parts that become detritus for decomposers, plus the energy spent on movement.
By the fifth level only 2.00 kJ remains, too little to support a breeding population, which is exactly why food chains have a ceiling.
Energy pyramids are always upright. Biomass pyramids can invert in the ocean because phytoplankton turnover is fast, and pyramids of numbers invert easily, as one tree supports many insects.

Task: explain the narrowing pyramid in two sentences, one tenth passed on and nine tenths lost, and give an example of an inverted biomass pyramid.

Scene 3: Decomposers and the carbon cycle: matter cycles, energy does not

Decomposers, mainly bacteria and fungi, oxidise organic carbon in detritus back to carbon dioxide and return nitrogen as ammonium, so nutrients reach producers again. Matter cycles.
Energy cannot cycle: every transfer dissipates heat, so an ecosystem needs a continuous solar input or it stops.
The dietary consequence: eating plants directly, 20000 kJ supports 2000 kJ of human tissue; eating a tertiary consumer leaves 20 kJ, so the same land feeds the same number of people only if it is 1000 times larger.
Shorter chains therefore use less land. That is the scientific basis for the claim about meat and land pressure, not a moral judgement.

Task: if a population needs 2000 kJ, work out how many identical plots are needed when the food comes from a tertiary consumer: 1000 plots.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Energy transfer	Multiply by 0.1 each level, start from NPP	2000, 200, 20
Graph	Reading a pyramid	Decide first: energy, biomass or numbers	Only energy must be upright
Explanation	Chain length	One tenth passes, nine tenths lost	The fifth level cannot sustain a population
Application	Diet and land	Compare energy available along the chain	Factor of 1000

Sanity check (teach this): Keep units consistent: everything in kJ per square metre per year, and convert first if the question gives joules or kilocalories, since 1 kcal is about 4.18 kJ. Check the denominator: photosynthetic efficiency divides by incident solar energy, while transfer efficiency divides by the previous level, and the two must not be mixed.
A figure of 0.4 percent looks low but it is an annual average with respiration already deducted. An answer near 40 percent should trigger a check, since that is impossible. Ten percent is an average and real values run from 5 to 20, so write “about” rather than treating it as a law.

05 Independent practice (3 questions)

Question	Answer	What to watch
Energy available to primary consumers when NPP is 20,000	2000 kJ	Multiply by 0.1
Net productivity with gross 30,000 and respiration 7,500	22,500 kJ	Subtract
Energy left at the tertiary level from 20,000	20 kJ	Multiply by 0.1 three times
Why an energy pyramid cannot invert	Each level holds less energy	Heat loss is irreversible

06 Common mistakes and what to say

Mistake	Student writes	Say this
Starting from gross instead of net productivity	Multiplying 26,000 by 0.1	“Only net productivity is passed on. What respiration burned never reaches the next level.”
Treating energy as recyclable	Saying decomposers return energy to plants	“Decomposers return matter, not energy. The energy has already left as heat, and the two must be kept apart.”
Turning ten percent into a law	Writing that exactly 10 percent always transfers	“Ten percent is an average. Use whatever the question gives and never present it as fixed.”
Skipping the unit conversion	Adding kilocalories to kilojoules	“1 kcal is 4.18 kJ. Without converting, the whole answer is wrong.”

07 Homework (paper, section and question numbers)

Secondary biology chapter index — All chapters
Biology practice paper — Ecology and energy flow, questions 9 to 14

08 Exit check (last 5 minutes)

Question	Answer	Decision
Energy at the secondary level when NPP is 20,000	200 kJ	All correct: move to succession
Can an energy pyramid invert	No, energy always decreases	Wrong: reteach the ten percent rule
Two roles of decomposers	Break down organics, release inorganic nutrients	Two wrong: back to act one

09 Tutor FAQ

Why can a marine biomass pyramid invert?

Phytoplankton reproduce and are eaten so quickly that the standing stock is small at any moment, yet annual production supports a larger standing stock of zooplankton.

Where does the lost ninety percent go?

Mostly respiration as heat, then excretion and the parts never eaten, all of which eventually pass to decomposers.

Why do shorter food chains use less land?

Every level loses about ninety percent of the energy, so removing a level removes one such loss and the same land supports more people.

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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.