Photosynthesis, energy flow and ecological pyramids
Explain how producers capture light energy, how energy moves through trophic levels and how ecological pyramids represent that flow.
This original StudyForA sequence supports Cambridge IGCSE Environmental Management 0680 Topics 1–4 for exams in 2027, 2028 and 2029. Named local examples are chosen by the learner and centre; verify current conditions and local context with reliable sources. It is not a Cambridge paper, official grade or complete course.
Before you begin
- Use the 2027–2029 syllabus and check your exam year and centre's entry arrangements. Topics 1–4 are taught on this page; Topics 5–7 are still being built.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the syllabus word equation for aerobic respiration?
What you will learn
- Explain how producers capture light energy, how energy moves through trophic levels and how ecological pyramids represent that flow.
- Use evidence from an unfamiliar case to explain a cause-and-effect link and justify a suitable response.
Describe photosynthesis
Photosynthesis synthesises glucose from carbon dioxide and water. Sunlight supplies energy, and chlorophyll captures light energy. The syllabus word equation is: carbon dioxide + water → glucose + oxygen.
Photosynthesis stores some incoming energy in chemical energy in glucose. It is not the same process as respiration, and the syllabus requires the word equation rather than a balanced chemical equation.
Trace energy through trophic levels
Food chains and food webs show feeding links and energy flow. Trophic levels include producer, primary consumer, secondary consumer and tertiary consumer.
The syllabus uses a 10% transfer between trophic levels. Much energy is lost during movement, respiration, digestion and excretion, so less energy is available at higher levels. Treat 10% as the model used for these syllabus calculations, not a guarantee for every real ecosystem.
Interpret ecological pyramids
A pyramid of numbers represents the number of organisms at each trophic level. It can be inverted when one large producer supports many smaller consumers, so check the scale and organism counts rather than assuming every pyramid is upright.
A pyramid of energy represents energy at each trophic level. Available energy decreases upward because energy is lost during life processes; do not confuse counts of organisms with energy totals.
Explain aerobic respiration
Aerobic respiration breaks down glucose in the presence of oxygen to release energy, producing carbon dioxide and water as waste products. The word equation is: glucose + oxygen → carbon dioxide + water.
Organisms use released energy for life processes. Respiration releases carbon dioxide, linking energy transfer with the carbon cycle.
Sources for this lesson
Pause and explain
Using the 10% model, how much energy passes from 500 units at one level to the next?
Worked example
A producer stores 8000 energy units. Using the syllabus 10% model, estimate the energy passed to a primary consumer and then a secondary consumer. State where some of the remaining energy goes.
Show the worked solution
- Ten per cent of 8000 is 800 energy units passed to the primary consumer.
- Ten per cent of 800 is 80 energy units passed to the secondary consumer.
- Energy is lost during movement, respiration, digestion and excretion; the model does not imply that the remaining energy disappears without processes.
Answer The estimates are 800 units to the primary consumer and 80 to the secondary consumer. Energy is lost through the listed life processes, so less is available at each higher trophic level.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A plant receives sunlight, then a caterpillar eats its leaves. Explain photosynthesis and identify the first two trophic levels.
Give me a hint
Name the inputs and role of chlorophyll, then identify producer and primary consumer.
Compare my reasoning
- The plant uses carbon dioxide and water to synthesise glucose, with sunlight as energy and chlorophyll capturing light.
- The word equation is carbon dioxide + water → glucose + oxygen.
- The plant is the producer and the leaf-eating caterpillar is a primary consumer.
Photosynthesis stores energy in glucose. The plant is the producer and the caterpillar feeding on it is a primary consumer.
Look for these in your work
- Inputs and energy stated
- Equation correct
- Trophic levels correctly identified
A food web has 1200 kJ at the producer level. Estimate energy passed to primary and then secondary consumers using the 10% model. Explain why the transfer is not 10% of the original amount at every step.
Give me a hint
Apply 10% to the amount at the immediately lower trophic level.
Compare my reasoning
- The primary consumer receives about 10% of 1200 kJ, which is 120 kJ.
- The secondary consumer receives about 10% of 120 kJ, which is 12 kJ.
- Each step uses the amount available at the previous level; energy is lost through life processes between transfers.
The estimates are 120 kJ and then 12 kJ. Each 10% calculation uses the previous level's energy, not the original 1200 kJ.
Look for these in your work
- First transfer calculated
- Second transfer calculated from 120
- Losses linked to life processes
A pesticide reduces the number of insects in a woodland food web. Predict one possible change to consumers and explain why the available energy may change at higher trophic levels.
Give me a hint
Follow a feeding link and reason from the lower amount of food energy available.
Compare my reasoning
- Insect-eating consumers may have less prey and could decline, move or switch food sources depending on the food web.
- With fewer insects, less energy may pass from that consumer level to predators above it.
- The effect depends on alternative prey, competition and other food-web connections; use observations to test the prediction.
Reduced insect abundance can affect insect-eating species and reduce energy available to higher trophic levels. Food webs have alternative links, so the outcome needs evidence.
Look for these in your work
- Feeding relationship traced
- Energy pathway explained
- Food-web context acknowledged
Common mistakes
- Reversing the photosynthesis and respiration equations
- Claiming all energy transfers to the next level
- Applying 10% to the original amount at every level instead of the amount at the previous level
- Calling energy lost through life processes destroyed
What can you explain now?
Which substance captures light energy in photosynthesis?
Compare with the explanation
Chlorophyll
The correct answer is “Chlorophyll”. The alternatives “Glucose” and “Oxygen” do not match the syllabus concept. Explain how producers capture light energy, how energy moves through trophic levels and how ecological pyramids represent that flow.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Write the photosynthesis word equation and explain the modelled 10% energy transfer between trophic levels.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.