Energy Flow Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • The Sun is the principal source of energy for biological systems.
  • Light is captured by producers and stored as chemical energy, which passes along the food chain.
  • Energy is eventually transferred to the environment, mostly as heat from respiration.
  • A pyramid of energy is always pyramid-shaped; pyramids of numbers and biomass can be distorted.
  • Energy transfer is inefficient, so food chains rarely have more than five trophic levels.

Energy through a food chain

Light energy from the Sun becomes chemical energy in producers, passes along the food chain and ends up as heat in the environment.

Producers capture light energy in photosynthesis and store it as chemical energy in organic molecules. This energy passes from organism to organism along a food chain. Eventually it is transferred to the environment, mostly as heat lost through respiration.

  1. 1

    Sun

    light energy

  2. 2

    Producer

    photosynthesis stores chemical energy

  3. 3

    Consumers

    chemical energy passed along the food chain

  4. 4

    Environment

    energy lost, mostly as heat from respiration

Energy flow through a food chain

What is the principal source of energy for most food chains?

Pyramids of energy

A pyramid of energy always narrows going up, because energy is lost at every stage.

A pyramid of energy shows the energy at each trophic level, usually over a fixed time such as a year. Energy is lost at each stage, so it is always pyramid-shaped. Pyramids of numbers or biomass can be distorted or unusual, so energy pyramids are the most reliable way to show energy transfer.

Energy

  • Always a pyramid

Numbers

  • Can be distorted

Biomass

  • Can be distorted

Which type of pyramid is always pyramid-shaped?

Why transfer is inefficient

Most energy is lost as heat, in waste and in movement, so little reaches the next level.

Energy taken in is lost as heat through respiration, in waste such as faeces and urine, and in movement and other life processes rather than becoming new biomass. So food chains usually have fewer than five trophic levels.

This is why eating crop plants directly is more energy efficient than eating livestock fed on crops.

  1. 1

    Energy taken in

    from the food eaten

  2. 2

    Lost as heat

    through respiration

  3. 3

    Lost in waste

    faeces and urine

  4. 4

    Passed on

    only the energy in new biomass reaches the next level

Where energy goes at each trophic level

Why is it more energy efficient to eat plants than animals fed on plants?

Try an exam question

Explain why food chains rarely have more than five trophic levels.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.