The shape of the lesson
By the end of the lesson, learners can
- Follow the energy: Press Release sunlight and follow the glowing packets of energy leaving the Sun.
- Why energy flows but does not cycle: Carbon and nitrogen atoms go round and round between living things and their surroundings.
- Source: the Sun is the principal source of energy input to biological systems.
- Forms: light energy → chemical energy in organisms → heat in the environment.
- Between organisms: energy passes on by feeding, as chemical energy in food.
- Words: energy is transferred or lost to the environment, never made or used up.
One picture every idea returns to
The Sun is the principal source of energy input to biological systems. Producers capture light energy in photosynthesis and store it as chemical energy in organic molecules such as glucose, starch and proteins. Chemical energy passes from organism to organism by feeding, but at every stage some is transferred to the environment, mainly as heat from respiration. Energy flows through an ecosystem in one direction; it is not recycled.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, From sunlight to heat.. Almost every living thing on Earth runs on energy that left the Sun a few minutes ago. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Follow the energy | Press Release sunlight and follow the glowing packets of energy leaving the Sun. Project tab 1 of the lab (Energy flow from the Sun through a food chain) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why energy flows but does not cycle | Carbon and nitrogen atoms go round and round between living things and their surroundings. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why energy flows but does not cycle, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Energy is never "used up" or "made". | Say it is transferred: from light to chemical energy, then to the environment as heat. And energy is not recycled: nutrients cycle, energy flows through. |
| Heat lost to the environment cannot be used by plants for photosynthesis. | Only light energy can be captured, which is why the flow is one-way. |
Energy flow: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
State the principal source of energy for biological systems and name the process that brings this energy into a food chain.
Describe the flow of energy through the food chain grass → rabbit → fox, including the forms of energy involved.
Explain why an ecosystem needs a continuous input of energy from the Sun, whereas it does not need a continuous input of carbon.
A student writes: "When the fox respires, the energy from the rabbit is used up." Identify the error and rewrite the statement correctly.
A field receives 1 000 000 kJ of light energy per m2 per year. The grass stores 10 000 kJ per m2 per year as chemical energy. (a) Calculate the percentage of light energy stored by the grass. (b) Suggest two reasons why this percentage is so low.
Dead leaves fall from a tree. State what happens to the energy they contain.
Answer key and marking notes
- The Sun (sunlight / light energy).
- Photosynthesis by producers (green plants).
- Light energy from the Sun is absorbed by chlorophyll in the grass and used in photosynthesis.
- It is stored as chemical energy in organic compounds such as glucose and starch.
- Chemical energy passes to the rabbit when it eats the grass, and to the fox when it eats the rabbit.
- At each level some energy is released by respiration and is eventually transferred to the environment as heat.
- Energy leaves the ecosystem as heat, which producers cannot capture again.
- So energy flows through the ecosystem in one direction and must be replaced by sunlight.
- Carbon atoms are recycled between the air, organisms and decomposers, so the same atoms are used again.
- Energy cannot be used up (or destroyed).
- The chemical energy from the rabbit is transferred during respiration.
- Correct version: respiration releases the chemical energy, which the fox uses for movement and other processes, and it is eventually transferred to the environment as heat.
- (a) 10 000 ÷ 1 000 000 × 100 = 1%.
- (b) Much light misses the leaves or is reflected, or passes through them.
- Some wavelengths are not absorbed by chlorophyll, and some captured energy is released by the plant's own respiration as heat (any two).
- Decomposers (bacteria and fungi) feed on the dead leaves and take in the chemical energy.
- The decomposers respire, and the energy is transferred to the environment as heat.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is the principal source of energy input to most food chains?
- ALight energy from the Sun
- BChemical energy in the soil
- CHeat energy from the environment
- DEnergy released by decomposers
In which form is energy stored in the tissues of a plant?
- AChemical energy in organic compounds
- BLight energy trapped in chlorophyll
- CHeat energy in the cell sap
- DKinetic energy of the leaves
A rabbit eats grass. Which statement describes the energy transfer?
- AChemical energy in the grass passes to the rabbit when it feeds
- BLight energy in the grass passes to the rabbit when it feeds
- CThe rabbit makes new energy from the grass by respiration
- DAll the energy in the grass becomes chemical energy in the rabbit
What eventually happens to all the energy that enters a food chain?
- AIt is transferred to the environment as heat
- BIt is stored permanently in the top consumer
- CIt is recycled back to the producers by decomposers
- DIt is destroyed by the respiration of every organism
Use the diagram of energy flow in a grassland. What percentage of the light energy is stored by the grass?
- A1%
- B10%
- C0.1%
- D8%
Use the diagram. Of the 15 000 kJ in the grass, how much is passed to the sheep, and is this consistent with the losses shown?
- A1200 kJ; 15 000 − 7500 − 6300 = 1200 kJ
- B1200 kJ; 15 000 − 7500 = 7500 kJ, so it does not fit
- C7500 kJ; the respiration arrow goes to the sheep
- D6300 kJ; decomposers pass energy to the sheep
Why must an ecosystem receive a continuous supply of light, but not a continuous supply of new carbon atoms?
- AEnergy leaves as heat that cannot be reused; carbon atoms are recycled
- BCarbon atoms leave as heat that cannot be reused; energy is recycled
- CLight is used up in photosynthesis; carbon is made in respiration
- DDecomposers destroy energy but release carbon dioxide unchanged
A cow eats 100 kJ of grass but only about 10 kJ ends up in new cow tissue. Where does most of the remaining energy go?
- AReleased by respiration and lost as heat, or lost in faeces
- BStored in the cow's muscles as light energy
- CReturned to the grass through the cow's roots
- DDestroyed during digestion in the stomach
Quiz key
Producers capture light energy from the Sun in photosynthesis, so the Sun is the principal source for almost all food chains. Soil minerals are not an energy source for plants. Heat in the environment cannot be captured by producers. Decomposers release energy from dead material, but that energy originally came from sunlight.
Photosynthesis uses light energy to make glucose, which is built into starch, proteins and fats; the energy is stored as chemical energy in these organic compounds. Chlorophyll absorbs light but does not store it as light. Heat is energy lost to the environment, not stored. Kinetic energy is energy of movement, not a store in plant tissue.
Feeding passes on chemical energy stored in the grass. The grass contains chemical energy, not light energy. Respiration releases energy; it never makes new energy. Not all the energy passes on: some grass is not eaten, some is lost in faeces, and much is released as heat in respiration.
Every organism releases energy in respiration and it ends up as heat in the surroundings, so eventually all the energy is transferred to the environment. Top consumers die and are decomposed, so energy is not stored permanently. Nutrients are recycled, but energy is not: producers cannot use heat. Energy cannot be destroyed, only transferred.
15 000 ÷ 1 500 000 × 100 = 1%. 10% would mean 150 000 kJ was stored. 0.1% comes from slipping a decimal place when dividing. 8% is the sheep's share of the grass energy (1200 ÷ 15 000), not the grass's share of the light.
The grass has 15 000 kJ; 7500 kJ is released in respiration and 6300 kJ goes to decomposers, leaving 15 000 − 7500 − 6300 = 1200 kJ, exactly the value for the sheep. Subtracting only the respiration loss ignores the decomposer pathway. Respiration energy leaves as heat to the environment, not to the sheep, and decomposers do not feed the sheep.
Energy enters as light and leaves as heat, which producers cannot capture again, so it must be constantly replaced. Carbon atoms move round the carbon cycle and are used again. Carbon atoms are matter, not heat, and energy is not recycled. Light is converted to chemical energy, not used up, and respiration does not make carbon. Decomposers do not destroy energy; they release it as heat.
Most of the energy is released in respiration for movement and keeping warm and leaves as heat; some leaves in faeces and urine. Muscles store chemical, not light, energy. Cows have no roots and energy does not flow back to producers. Digestion breaks food into smaller molecules but cannot destroy energy.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.