HKDSE Biology — Energy Flow in Ecosystems Lesson-Prep Guide (for tutors)

HKDSE Biology — Energy Flow in Ecosystems Lesson-Prep Guide (for tutors) | pyramids, 10% rule, NPP

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can draw an energy pyramid but answer why each level shrinks with it was eaten. This lesson makes them write an energy budget for every level.

Objectives — Students can: 1) compute net primary production as GPP minus respiration with the correct unit; 2) apply the 10% rule to move up or down a food chain; 3) name the three routes by which energy is lost between levels.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Percentages	What percentage of 88,000 is 61,600?	70%
Units	What does kJ m-2 yr-1 describe?	Energy per square metre per year
Subtraction	80,000 minus 22,000	58,000

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Hook question	Ask what percentage of a tree energy reaches a hawk; students write an estimate first	Write an estimate
5-15 min	Prerequisite check	Three quick items; students read units aloud	Answer and circle units
15-35 min	Three-act script	Deliver the energy budget, the 10% rule and the loss routes	Restate one line per act
35-45 min	Worked examples	Model one GPP to NPP item and one pyramid item	Copy the working and unit
45-55 min	Independent practice	Circulate; ask only which level they started from	Three items, self-check
55-60 min	Exit check	Collect answers and tally errors	Written work

03 Teaching script (read it out)

Scene 1: Act 1: the energy budget

“Plants bank energy first: everything fixed by photosynthesis is gross primary production, GPP. But the plant respires and spends part of it, so what is left for growth is net primary production, NPP. So NPP equals GPP minus respiration. With GPP 88,000 kJ m-2 yr-1 and respiration 26,400 kJ, the net figure is 61,600 kJ, about 70 percent. Only that portion is available to the next level.”

How is NPP written, and why subtract respiration?

Scene 2: Act 2: the 10% rule

“Roughly one tenth of the energy survives each step. From 120,000 kJ of producers, primary consumers get 12,000, secondary 1,200, tertiary only 120. Why so steep? Three leaks: part is never eaten; part of what is eaten is indigestible and leaves as faeces; the rest is respired and mostly lost as heat. That is why an energy pyramid is always upright.”

What are the three routes of energy loss?

Scene 3: Act 3: why only some pyramids can invert

“An energy pyramid can never invert, because energy only decreases. A pyramid of numbers can: one tree carries thousands of aphids, so the lower bar is narrower in individuals. A biomass pyramid can invert briefly too, when phytoplankton reproduce fast and are grazed fast, so standing crop is small while production is high. First ask which pyramid the question draws, then decide whether inversion is allowed.”

Which pyramid can never be inverted, and why?

04 Question types and worked steps

Type	Example	Steps	Answer
GPP to NPP	GPP 88,000 kJ m-2 yr-1 and respiration 26,400 kJ m-2 yr-1. Find NPP and its percentage of GPP.	1) Subtract: 88,000 minus 26,400; 2) unit kJ m-2 yr-1; 3) divide by GPP and multiply by 100.	61,600 kJ m-2 yr-1, 70%
Pyramid, moving up	Producers fix 120,000 kJ. Energy available to tertiary consumers under the 10% rule?	1) Multiply by 0.1 per level; 2) count three levels; 3) state the unit.	120 kJ
Pyramid, moving down	Tertiary consumers need 80 kJ. How much must producers fix at least?	1) Divide by 0.1 per level; 2) three levels means divide by 0.001; 3) 80 / 0.001.	80,000 kJ
Explain the loss	Why is transfer efficiency only about 10%?	1) Not eaten; 2) undigested and lost as faeces; 3) respired as heat.	All three, each naming which portion of energy

Sanity check (teach this): Sanity check: NPP must be below GPP, and the percentage between 0 and 100. 120,000 divided by 1000 gives 120 because three levels means dividing by ten three times. If an upper level holds more energy than the level below, the multiplication went the wrong way.

05 Independent practice (3 questions)

Question	Answer	What to watch
GPP 50,000 kJ, respiration 18,000 kJ. Find NPP and its percentage.	32,000 kJ, 64%	Subtract before dividing, and do not drop the percent sign
Producers fix 250,000 kJ. Maximum energy for secondary consumers?	2,500 kJ	Did they count two levels rather than three?
Quaternary consumers need 80 kJ. How much must producers fix?	80,000 kJ	Check the number of zeros when dividing by 0.001

06 Common mistakes and what to say

Mistake	Student writes	Say this
Using GPP as NPP	Multiplying GPP by 0.1 directly	The next level only receives net production, so subtract respiration first.
Miscounting levels	Treating secondary consumers as the third level multiplied three times	Producers are level one and primary consumers level two, so count before multiplying.
Dropping m-2 yr-1	Writing only kJ	Energy density carries area and time; kJ alone is not the quantity asked.
Energy cycles	Saying energy is recycled in a food chain	Matter cycles, energy does not: it enters once and ends as heat.

07 Homework (paper, section and question numbers)

Secondary Biology chapter index (cells, genetics, ecology, human physiology) — Work the ecology chapter first
Lesson-prep overview — All subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
GPP 64,000 kJ with respiration 16,000 kJ: NPP and percentage?	48,000 kJ, 75%	Wrong: reteach the Act 1 formula
Producers fix 500,000 kJ. Maximum for secondary consumers?	5,000 kJ	Wrong: recount the levels
Secondary consumers hold 300 kJ. Minimum for primary consumers?	3,000 kJ	Two or more wrong: return to the 10% rule

09 Tutor FAQ

Which year groups is this for?

S6 DSE Biology ecology, and usable in S5 once genetics is finished.

Must students memorise 10%?

No: remember about one tenth and be able to multiply by 0.1; always use the numbers given.

Is there practice attached?

The homework row links to the biology index so ecology and genetics can be scheduled together.

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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.