Energy Flow & Food Chains IGCSE Biology

Energy Flow and Food Chains: From Producers to Top Consumers

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Energy enters ecosystems through photosynthesis by producers. It flows along food chains from producers to primary consumers, secondary consumers and tertiary consumers. Energy is lost at each trophic level through respiration, excretion and as heat.

Energy flow questions require students to explain why food chains are short and why energy transfer between levels is inefficient. Supplement students must quantify energy losses and calculate efficiency.

Food chains and trophic levels

A food chain shows the flow of energy from one organism to the next. Each step is a trophic level. Producers (green plants and algae) form the first trophic level because they convert light energy into chemical energy in organic molecules through photosynthesis.

Primary consumers (herbivores) eat producers. Secondary consumers (carnivores) eat primary consumers. Tertiary consumers eat secondary consumers. Decomposers (bacteria and fungi) break down dead organisms and waste at every level, recycling nutrients back to the soil.

Why energy is lost at each trophic level

Not all energy from one trophic level is transferred to the next. Energy is lost in several ways:

Respiration: All organisms respire, releasing energy as heat to the surroundings. This energy cannot be used by the next trophic level. Excretion and egestion: Urine, faeces and other waste products contain energy that is not passed to the consumer. Uneaten parts: Not all of an organism is consumed; bones, roots and other parts are left behind.

Typically only about 10% of the energy at one trophic level is transferred to the next. This is why food chains rarely have more than four or five trophic levels: by the top there is not enough energy to support another level.

Calculating energy transfer efficiency (Supplement)

At Supplement level, students may be asked to calculate the efficiency of energy transfer between trophic levels:

Efficiency = (energy transferred to next level / energy available at current level) x 100

For example, if producers contain 10 000 kJ of energy and primary consumers contain 1 200 kJ, the efficiency is (1 200 / 10 000) x 100 = 12%. Students should be able to perform this calculation and explain why the remaining 88% is lost to respiration, waste and uneaten material.

The role of decomposers

Decomposers are microorganisms (mainly bacteria and fungi) that break down dead organic matter and waste products. They secrete enzymes onto the dead material, digest it externally, and absorb the soluble products. This process returns nutrients such as nitrates and phosphates to the soil, where they can be absorbed by plant roots.

Without decomposers, nutrients would remain locked in dead organisms and waste, and would not be recycled back into the ecosystem. Decomposition is fastest in warm, moist conditions with plenty of oxygen.

Try a focused question (3 marks)

Explain why a food chain rarely has more than four trophic levels.

Show the answer and marking guidance

Energy is lost at each trophic level through respiration (as heat), excretion and in uneaten parts.
Only about 10% of the energy at one level is transferred to the next.
By the fourth or fifth level, there is not enough energy remaining to support another population of organisms.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying energy is "used up" rather than lost as heat through respiration.
Forgetting that producers are the first trophic level, not sunlight.
Claiming energy is recycled in ecosystems (energy flows through and is lost; nutrients are recycled).
Confusing decomposers with detritivores (decomposers break down dead matter externally using enzymes).