Energy FlowCambridge IGCSE Biology: Revision notes

The Sun is the principal source of energy input to biological systems. Energy flows through living organisms as follows: light energy from the Sun is captured by producers (via photosynthesis) and converted into chemical energy stored in organic molecules. This chemical energy is then passed from organism to organism along a food chain, before eventually being transferred to the environment — mostly as heat lost through respiration.

A pyramid of energy shows the amount of energy present at each trophic level in a food chain, usually over a fixed time period (e.g. per year). Because energy is always lost at each stage of a food chain, a pyramid of energy is always the expected pyramid shape — smaller at each level going up — unlike pyramids of numbers or biomass, which can sometimes be distorted or unusual in shape. This makes pyramids of energy the most accurate and reliable way to represent energy transfer through an ecosystem.

The transfer of energy from one trophic level to the next is often not efficient. Much of the energy an organism takes in is lost:

• As heat, through respiration
• In waste materials, such as faeces and urine
• In movement and other life processes, rather than being converted into new biomass

Because so much energy is lost at each stage, food chains usually have fewer than five trophic levels — beyond this, too little energy remains to support a further level.

This inefficiency also explains why it is more energy efficient for humans to eat crop plants directly than to eat livestock that have themselves been fed on crop plants: eating the plant directly avoids the energy losses that occur when that energy is first transferred into an animal before reaching humans.