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.
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A typical prokaryote
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Human blood cells
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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.
Eukaryotic cells reproduce by means of mitosis or meiosis; prokaryotic cells, by binary fission.
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Spermatozoa swarming an ovum
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