All materials in the living world are recycled to provide the building…

The idea

Living organisms are built from elements such as carbon, oxygen, hydrogen, nitrogen and phosphorus. These elements are not created anew when an organism grows; they are taken from the environment, passed along food chains, and returned to the environment by processes such as respiration, decomposition, combustion and excretion. Because the supply of these elements on Earth is finite, the same atoms are reused repeatedly. For example, carbon in a glucose molecule made by a plant may be eaten by a cow, respired into carbon dioxide, absorbed by a different plant and built into new glucose. This recycling means the materials forming one organism become the building blocks for future organisms.

What earns the marks

• Elements such as carbon, oxygen, hydrogen, nitrogen and phosphorus are finite on Earth and are reused rather than newly created.
• Materials pass from the environment into organisms, along food chains, and back to the environment through processes including respiration, decomposition, combustion and excretion.
• The same atoms can be incorporated into many different organisms over time, so one organism's materials become another organism's building blocks.
• Microorganisms such as decomposers break down dead material and waste, releasing simple compounds that plants can absorb and reuse.
• Recycling maintains a roughly constant supply of the key elements needed for growth, so ecosystems can continue to support new organisms.

Common mistakes and how to put them right

• Thinking that new matter is created when organisms grow.

  Put it right: Organisms assemble new tissues from elements already present in the environment, so matter is rearranged, not created.

• Believing that energy is recycled in the same way as materials.

  Put it right: Materials are recycled, but energy flows through ecosystems and is eventually lost as heat.

• Assuming decomposers only remove waste.

  Put it right: Decomposers return mineral ions and carbon compounds to the soil or atmosphere so producers can reuse them.

Exam tips

• Use the word 'recycled' precisely: state which material is recycled, the process that returns it, and the organism that reuses it.
• Link each stage to a named process, such as respiration releasing carbon dioxide or decomposition releasing mineral ions.
• When asked why recycling matters, refer to the finite supply of elements and the need for future growth, not just to 'keeping the environment clean'.