What is Unicellular, colonial, multicellular? — Biology

Definition 1.1 University Biology — Year 2 · Chapter 1 — Diversity of Unicellular Organisms

An organism is unicellular when a single cell carries out every function of life — nutrition, exchange, movement, reproduction — and lives on its own. It is colonial when cells of one kind stay attached after dividing but each remains able to live alone if separated. It is multicellular when its cells are of several types, depend on one another, and only a few of them (the germ line) leave descendants. Unicellular organisms include all archaea, nearly all bacteria, and most lineages of eukaryotes; the informal name protist covers the unicellular eukaryotes, which are not one group but many.

The sizes of unicellular organisms span five orders of magnitude. Prokaryotes (blue) cluster around a micrometre; eukaryotic cells (orange) are ten to a hundred times larger; the largest single cells — a giant sulfur bacterium, the alga Acetabularia — are exceptions that the text explains. — The sizes of unicellular organisms span five orders of magnitude. Prokaryotes (blue) cluster around a micrometre; eukaryotic cells (orange) are ten to a hundred times larger; the largest single cells — a giant sulfur bacterium, the alga *Acetabularia* — are exceptions that the text explains.

Examples

Example 1.2 (Six inhabitants of a drop)

Escherichia coli, a rod $2\,\text{µ}\mathrm{m}$ long, swims with a tuft of flagella and divides every twenty minutes when fed. Chlamydomonas, a green alga $10\,\text{µ}\mathrm{m}$ across, rows with two flagella toward the light it detects with an eyespot. Paramecium, a ciliate of $200\,\text{µ}\mathrm{m}$ , is covered with thousands of cilia, feeds through a gullet and bails water out with two contractile vacuoles. A diatom lives in a two-part shell of silica, ornamented with pores. Amoeba flows by extending pseudopods and engulfs what it meets. A yeast cell buds a daughter from its side. Six answers, of widely different size and machinery, to one problem.

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Examples

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