Further Biology

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Energy and Organisms

All living things on Earth ultimately get their energy from the Sun's radiation. A small proportion of this energy is absorbed by green plants during photosynthesis.

The sun's energy is stored in chemicals that make up plants' cells. When the plants, and the animals that eat them, are eaten or decompose, the energy is then transferred to other organisms. Decay organisms can feed off dead organisms and the waste products of animals.

Autotrophs and Heterotrophs

Autotrophs are organisms that make their own food. Plants, also known as producers are autotrophs.

Heterotrophs are organisms that are unable to make their own food so instead they obtain their energy by consuming other organisms. Animals and decay organisms, including some bacteria and fungi are heterotrophs.

Animals that eat plants are called herbivores and are primary consumers. Animals that eat other animals are called carnivores and may be secondary or even tertiary consumers.

An ecosystem is a defined area containing a self-sustained community of organisms, autotrophs and heterotrophs, living in the non-living surroundings, eg. a pond or wood. The transfer of food energy in an ecosystem can be represented in a number of ways, including...

  • food chains
  • pyramids of numbers
  • pyramids of biomass

Food Chains

Energy in a food chain always flows in one direction.

  • It starts with light energy from the Sun.
  • The light energy transfers to an autotroph which captures the energy, carries out photosynthesis and stores the energy in cells.
  • A herbivore heterotroph then eats the autotroph. Some of the energy stored in the plant is transferred to the herbivore and stored in it's cells.
  • A carnivore heterotroph then eats the herbivore heterotroph. Some of the energy is then transferred to the carnivore and stored in its cells.

At each stage of the food chain, a large proportion of the energy is...

  • lost to the environment as heat or through respiration
  • excreted as waste products
  • trapped as undigestable material such as bones and hair

This means that as the food chain moves from heterotrophs, there is less energy available at each trophic level. Therefore there is a limit to the number of levels in a food chain, usually arounf four or five levels.

Pyramids of Number

A pyramid of numbers can be used to show the total number of organisms that feed on each other in a food chain. Each horizontal level of the pyramid represents a trophic level. By looking at these levels we can understand the feeding relationships and numbers of autotrophs and heterotrophs.

The chart is called a pyramid because typically the bottom bar, showing the number of autotrophs to be consumed, is the biggest, and the top bar showing the top consumer will be the smallest. This is because only some of the energy and nutrients are passed on from consumer to consumer.

However, pyramids of number can have unusual shapes. For example, one oak tree may have many aphids feeding on it, so the bar representing the oak tree will be smaller than…


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