Basic unit of life

Online Biology Dictionary

Prokaryote cell
Human blood cells
Human blood cells

Spermatozoa swarming an ovum

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The cell is to biology what the atom is to chemistry. All living things other than viruses are composed of cells. They are membrane-bounded compartments, usually microscopic, filled with a complex liquid called cytoplasm. Within the cytoplasm are various structures with specific functions.

Broadly speaking, all cellular organisms can be divided into two categories, prokaryotes and eukaryotes (viruses, which are not cellular, constitute an additional major category).

Many eukaryotes are multicellular (animals, plants, and most fungi). That is, they are organisms composed of multiple cells. There are also many single-celled eukaryotes. Eukaryotic chromosomes are linear and enclosed in a nuclear envelope. A prokaryotic chromosome is single, circular, and not so enclosed (see figure left).occurrence in disease.

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