B7.1 - Living organisms are independant

Why do we need the Sun?


  • thermal energy that keeps the Earth's atmosphere warm
  • light energy that green plants use to make food chemicals (carbohydrates, fats etc...)

Animals, fungi and bacteria need plants for food, whether it's directly or indirectly. Life on Earth is dependent on energy from the Sun.

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How do plants harness the Sun's energy?

Only the green parts of plants absorb light energy.

They use it to make food in a process called photosynthesis. Plants absorb only a tiny percentage of the Sun's energy. 

The materials made by photosynthesis are made of chemicals like carbohydrates, fats and proteins. These chemicals make new plant cells and any food stores inside those cells. Think of both of these as a store of energy.

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Do all organisms in the ecosystem feed in the same

We call organisms that make their own food autotrophs, these are mostly green plants. Only a few non-green micro-organisms are also autotrophs - they don't use light to make their food though.

Organisms that can't make their own food are called heterotrophs, these are usually animals, bacteria and fungi. They depend on food from autotrophs to keep them alive.

If a heterotroph feeds on plants we call it a herbivore.

If a heterotroph feeds on animals that eat plants we call it a carnivore.

If a heterotroph feeds on waste produced by plants and animals we call it a decomposer.

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How are food and energy transferred between organi

When an animal eats a plant, the energy stored in the plant is then transferred to the animal. The chemicals in waste material and dead plants and animals are also a store of energy. The energy stored in these is transferred to the decomposers who feed on them.

We can show what eats what using a food chain. Food chains also show the direction of energy transfer too.

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Pyramid of numbers

A pyramid of numbers shows the numbers of organisms at each feeding level in an ecosystem.

It gives you no idea of the biomass (mass of each organism) at each level.

If lots of smaller organisms feed on a few larger ones in an ecosystem, a pyramid of numbers is not a pyramid shape, so don't be fooled by the name.

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Pyramid of biomass

A pyramid of biomass shows the mass of organisms at each feeding level in an ecosystem. It shows how much food is available to the next level.

A pyramid of biomass is bigger at the base than at the top because some of the mass does not pass on to each new level.

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How is energy lost from food chains?

There is less biomass at each level in a food chain, so there is less energy as well. The energy that is not passed on is;

  • in uneaten parts
  • used for life processes (movement and keeping warm)
  • lost to the surroundings as heat energy
  • in waste products

As there is less food and energy at each feeding level, there's a limit to the length of food chains.

You need to be able to calculate the percentage efficiency of energy transfer at different stages of a food chain.

If 2 units of energy out of 20 are transferred, the percentage transferred is;

2/20 x 100 = 10%

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What is in soil?

Soil contains;

  • biomass. This includes living organisms and decaying materials.
  • inorganic particles (sand, silt and clay)
  • air
  • water with dissolved and mineral ions

To calculate the percentage of water, find the mass of soil at the start, then the mass of soil when dried to constant mass. Take the mass of soil at start from the mass of soil when dried to find the loss of mass/water, then divide this by the mass of soil at start and times by 100.

So, mass of soil at start - mass of soil when dried = loss of mass(water lost)

Then, loss of mass(water lost) / by mass of soil at start x 100 = percentage of water.

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Thank you

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