In biology, an organism(s) is any contiguous living system, such as an animal, plant, fungus, or bacterium. All known types are capable of some degree of response to stimuli, reproduction, growth and development and homeostasis. When it has one cell it is known as a unicellular organism when it has more than one it is known as a multicellular organism. Most unicellular are of microscopic size and are thus classified as microorganisms. Humans are multicellular organisms composed of many trillions of cells grouped into specialized tissues and organs.
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