Teaching sequence

Lesson objective

In this lesson students consider how energy flows into and out of an ecosystem via the pathways of food webs, and how it must be replaced to maintain the sustainability of the system. They consider the impacts of human activity on an ecosystem from a range of different perspectives.


Repeat the food web activity from lesson 1 this time making only food chains. Point out that all chains start with a plant. Discuss how food provides energy for all organisms. Ask students where plants get their energy from and where all living things get their energy.


  1. Watch the video Photosynthesis
  2. Ask students why plants are green. Remind them of the physics concept that green light is reflected not absorbed, thereby giving plants a green appearance.
  3. Introduce the molecule ‘chlorophyll’ and discuss how it uses the energy from the Sun to split the water molecules.
  4. Ask the students if plants would be able to photosynthesise at night. Introduce the idea of plants using the glucose to respire and that this is more obvious at night. Ask students if they have flowers that use up the oxygen in their room at night. Ask if the amount or intensity of light makes a difference to the rate of photosynthesis.
  5. Use a water plant (such as Elodea) to demonstrate oxygen being emitted from the leaves.
  6. Complete the learning object Effects of light intensity on photosynthesis so that a graph is produced indicating the relationship between light intensity and the rate of oxygen production. Ask students what is the relationship between the amount of oxygen produced and the rate of photosynthesis and what other factors would affect the rate of photosynthesis. (Answer: Availability of reactants such as carbon dioxide.)
  7. Remind students how mass is always conserved and ask where the carbon atoms come from for photosynthesis. Ask students what happens to the carbon atoms in the carbon dioxide during photosynthesis. Where do they go? Where does the energy come from to make the glucose molecule? How efficient is the transformation of light energy into chemical energy? Where does that energy go?
  8. Ask students where they get their energy. Is that energy transformation efficient? Where does that energy go? How much energy is stored in their bodies?
  9. Introduce the 10% rule. Show the students how this can be represented diagrammatically with an energy pyramid.
    Teacher note: For details on the 10% rule and the energy pyramid see Background information.
  10. Students complete the worksheet Energy flow in food webs.
  11. Ask students how much phytoplankton must be used to make one tuna sandwich. Show students the Tuna sandwich trophic pyramid.
  12. Divide students into two groups to debate the topic ‘Vegetarians cause less damage to the ecosystem’.


Ask students to keep a food diary for two days. Discuss their food choices and the impact on the ecosystem.

Ask students to answer the question: Are humans a sustainable species?

Lesson Resources


Student activities

Digital resources

Photosynthesis, Teachers’ Domain
(2:24 min) 

L7577 Effects of light intensity on photosynthesis, NDLRN 

Tuna sandwich trophic pyramid,
Science Learning 


S3883 Energy flow in food webs,

Extension activities

How could food be produced to support a colony on Mars? (Word, 323 KB)

Useful links

Basic debating skills,
ACT Debating Union. Debating instructions. Original resource no longer available on original site, this copy made available through the Internet Archive's Wayback Machine.

Illuminating photosynthesis,
NOVA Online. Learning object