What is Prokaryotic and eukaryotic cells? — Biology

Definition 5.4 University Biology — Year 1 · Chapter 5 — The Cell: Unit of Life

A prokaryotic cell (bacteria, archaea) has no nucleus: its DNA, usually one circular chromosome, lies in a region of the cytoplasm called the nucleoid, and it has no membrane-bounded compartments. It is small ( $0.5\text{ to }5\,\text{µ}\mathrm{m}$ ) and bounded by a plasma membrane and, outside it, a cell wall. A eukaryotic cell (protists, fungi, plants, animals) keeps its DNA, as several linear chromosomes, inside a nucleus bounded by a double membrane, and divides its cytoplasm into membrane-bounded organelles (Chapter 6). It is larger ( $10\text{ to }100\,\text{µ}\mathrm{m}$ ) and has an internal skeleton of protein filaments.

A bacterium (Escherichia coli) cut open. No nucleus, no organelles: a circular chromosome loosely folded in the cytoplasm among thousands of ribosomes, a wall outside the plasma membrane, flagella for swimming and pili for attachment. Length about 2\, µ m. — A bacterium (*Escherichia coli*) cut open. No nucleus, no organelles: a circular chromosome loosely folded in the cytoplasm among thousands of ribosomes, a wall outside the plasma membrane, flagella for swimming and pili for attachment. Length about $2\,\text{µ}\mathrm{m}$ .

Escherichia coli under the scanning electron microscope (the bar at the foot of the image is 2\, µ m). Rods 2\, µ m long, a thousand of which would fit across a millimetre; a single cell divides every twenty minutes in a rich medium. Image: NIAID, public domain. — *Escherichia coli* under the scanning electron microscope (the bar at the foot of the image is $2\,\text{µ}\mathrm{m}$ ). Rods $2\,\text{µ}\mathrm{m}$ long, a thousand of which would fit across a millimetre; a single cell divides every twenty minutes in a rich medium. Image: NIAID, public domain.

Examples

Example 5.7 (Why the eukaryotic cell is compartmented)

A cell of $20\,\text{µ}\mathrm{m}$ has a thousand times the volume of a bacterium but only a hundred times its membrane surface (Proposition 1.6). Membranes are where a cell carries its energy-transducing chains and its transport systems; the eukaryote recovers surface by folding membranes inside itself — the inner membrane of a mitochondrion, the sheets of the endoplasmic reticulum — and gains, besides, separate chambers in which incompatible chemistries (digestion in the lysosome, synthesis in the cytosol) can run side by side.

Example 5.11 (A bacterium and a mitochondrion)

Under the light microscope at $1000\times$ , an E. coli cell is a rod $2\,\mathrm{mm}$ long on the retina’s scale, with no visible interior; a mitochondrion is a dot at the limit of resolution. In a TEM section the same bacterium shows its two membranes, its wall, its nucleoid as a pale fibrous region and its ribosomes as dark grains; the mitochondrion shows its double membrane and the folds of the inner one. Neither is alive.

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Examples

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