Fish and plants in a stream Living things are called organisms. We know if something is an organism, rather than non-living, from several features. First, an organism grows and develops at some stage, usually changing its shape and getting bigger. Second, life processes happen inside the organism that change chemical substances from one form to another and which use up energy. Third, an organism must take in raw materials for its growth and also take in energy to power its life processes. Fourth, an organism reproduces—it produces more of its own kind.
Organisms come in many shapes and sizes. Some are too small to see except with a microscope, like an amoeba in the mud. Some are quite small and seen better with a magnifying lens, like a ladybird. Some are gigantic, like the sequoia tree. Some, like the dinosaurs, lived in prehistoric times but no longer survive. We humans are also living things.
In any particular environment such as, for example, rainforest or grassland, there is a wide variety of organisms. A measure of this variety is called biodiversity.
The oldest living animal is a marine bivalve, Arctica islandica, whose common name is the ocean quahog. These animals are reported to be more than 500 years old.
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