Transfers Along a Food Chain

Section 1 of 6Energy entering and moving along a food chain

1Light energy

• Light energy Sunlight is the source of energy for almost all food chains.

2Producers

• Producers Producers use photosynthesis to convert light energy into chemical energy stored in carbon-containing compounds in their biomass.

3Primary consumers

• Primary consumers They eat producers, break down the carbon compounds and use the energy and substances to build new biomass in their own tissues.

4Higher consumers

• Higher consumers Secondary and tertiary consumers repeat the process at each trophic level.

Section 2 of 6Why energy is lost between trophic levels

Only about 10% passes on

Uneaten parts

• Uneaten parts Predators rarely eat everything.
  • Bones, fur, teeth and claws hold energy that never reaches the consumer.

Egestion

• Egestion Not everything eaten is digested and absorbed.
  • Material that is hard to digest, such as plant fibre, is egested as faeces.

Respiration

• Respiration Energy is released by respiration to power movement and keep warm, and ends up as heat in the surroundings.

Excretion

• Excretion Energy is lost in waste products such as urea.

Death and decay

• Death and decay Energy in dead organisms goes to decomposers instead of to the next consumer.

Section 3 of 6Consequences

Short food chains

• Short food chains Only around 10% of the energy available at one trophic level becomes biomass at the next.
  • After four or five levels there is too little left to support another.

Pyramids

• Pyramids Pyramids of biomass get narrower towards the top for the same reason.

Section 4 of 6Calculating the efficiency of transfer

1

Use: percentage efficiency = (biomass in the higher trophic level ÷ biomass in the lower trophic level) × 100.

2

Worked example: clover has 1450 kg of biomass and the snails that eat it have 160 kg.

3

Divide the higher level by the lower level: 160 ÷ 1450 = 0.1103.

4

Multiply by 100: 11.03, which is 11.0% to 3 significant figures.

5

Always check which level is on top of the fraction: the level that gains energy goes on top.

Section 5 of 6Key terms

Term	Meaning
Trophic level	The position of an organism in a food chain.
Producer	An organism that makes its own food by photosynthesis, the first trophic level.
Consumer	An organism that obtains energy by eating other organisms.
Biomass	The mass of living material, which stores chemical energy.
Egestion	The removal of undigested food as faeces.
Efficiency	The percentage of energy or biomass passed from one trophic level to the next.

Section 6 of 6Exam technique

1

Saying energy is 'used up' or 'destroyed'; it is transferred to the surroundings, mostly as heat.

2

Dividing the lower level by the higher level in an efficiency calculation.

3

Confusing egestion (undigested food leaving the gut) with excretion (waste made by cells, such as urea).

4

Forgetting to give the answer to the number of significant figures requested.

5

Writing that food chains are short because animals run out of food, instead of explaining the energy lost at each level.