Materials move between living and non-living parts
Definition
Nutrient cycle
A nutrient cycle is the repeated transfer and reuse of a nutrient between organisms and the non-living environment.
- Carbon, nitrogen, water and mineral ions are used repeatedly as they move between organisms and their surroundings.
- The biotic components of an ecosystem are its living organisms, including plants, animals and microorganisms.
- The abiotic components are its non-living surroundings, including the atmosphere, water and soil.
- A material can be stored in one component before a biological or physical process transfers it to another.
- For example, nitrogen can be present in dissolved soil minerals, become part of a plant protein and later enter an animal that eats the plant.
- The atoms are rearranged into different substances during these transfers, rather than being created or destroyed.
Organisms take in and return materials
- Plants take in carbon dioxide from the air and absorb water and dissolved mineral ions through their roots.
- They use these raw materials to make the substances needed for growth, including carbohydrates and proteins.
- Animals obtain carbon-containing and nitrogen-containing substances in food, and obtain water by drinking or feeding.
- When one organism eats another, some of the materials in the food become part of the feeding organism.
- Excretion returns metabolic waste to the surroundings, while egestion returns undigested food.
- When an organism dies, its body still contains materials that were taken from the environment during its life.
- Decomposition returns materials from dead organisms and waste to forms that can be taken up again.
- These transfers connect the living organisms with material stores in the air, soil and water.
Example
- A nitrogen atom in a dissolved soil mineral is absorbed by a grass plant and becomes part of a protein.
- A rabbit eats the grass and uses some of this nitrogen to make its own proteins.
- After waste is released or the rabbit dies, decomposition helps return the nitrogen to the surroundings where it can be used again.
Recycling maintains available supplies
- Earth has a finite supply of each element, so continued growth depends on making existing materials available repeatedly.
- Without recycling, more materials would remain locked in dead bodies and waste, leaving fewer available for new growth.
- Recycling maintains a supply of usable raw materials, rather than increasing the total number of atoms.
- Materials can also enter or leave a particular ecosystem, for example when water carries dissolved minerals into or away from a pond.
Exam technique
When describing a material transfer, identify the material, the component it leaves, the component it enters and the process responsible.
Energy flows through ecosystems
- Light energy from the Sun supplies most ecosystems and is transferred to chemical energy stores by photosynthesis.
- Organisms transfer energy to the surroundings as heat, which becomes dispersed and is unavailable for repeated use by the food chain.
- A continuing energy input is therefore needed, even though the materials used by organisms can circulate repeatedly.
- The arrows showing matter returning to plants represent recycled materials, not energy returning from decomposers.
Common Mistake
Do not describe energy as being recycled: materials cycle, while energy is transferred through an ecosystem and dissipated to the surroundings.
Self review
- Which non-living stores supply raw materials to plants?
- How can nitrogen move from soil into an animal?
- Why would a lack of recycling eventually restrict growth?
- Why does an ecosystem need a continuing energy input?
