Unicellular organism
Unicellular organisms, or single cell organisms, consist of only one cell. This one cell has it's own metabolism, which is usually based on oxygen usage, but there are enough organisms to which oxygen is actually toxic.
All unicellular organisms are able to reproduce themselves without help of other organisms, as opposed to viruses. Bacteria, for example, are unicellular organisms which even can move forward. There are also unicellular organisms (even a few bacteria) which use photosynthesis to support themselves.
Unicellular organisms werde probably the first form of life that appeared on earth. Today they have an important place in all ecosystems and in most higher multicellular organisms. For mankind they are important for the creation of certain types of food, medical substances and biological weapons.
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