An organism is a living thing. Animals and plants are organisms. Organisms are a biotic part of the environment. Rocks and sunshine are a part of the abiotic, or non-living, environment.
Organisms have five basic needs. They need air, water, nutrients (food), energy and a place to live. Animals and plants are organisms.
Some organisms are made up of millions of cells. They are called "multicellular organisms". Some examples are dogs, trees, and humans. Most "multicellular organisms" can be seen without using a microscope.
Some organisms are so small that they cannot be seen with the naked eye. You need a microscope to see them. They are called microorganisms. Organisms can be made up of just one cell. They are called "unicellular organisms" or "single celled organisms". Examples include bacteria, amoeba and protozoa such as the paramecium.

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