Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Cells
There are two major classes of cells: prokaryotic and eukaryotic

The simplest and smallest Cells of the most ancient origin are called prokaryotes; they are represented by various species of single-celled microorganisms, the Bacteria. Prokaryotes were the first cells to arise during biological evolution: fossil remains of these cells, dating back more than 3 billion years (3 ∙ 10⁹), have been found in ancient shales in Africa (Fig. 2-2) as well as in Australia. Eukaryotic cells, which probably arose about 1 billion years later than prokaryotes, are characterized by much larger sizes, a more complex Organization, extraordinary diversity, and the capacity for differentiation. All Multicellular animals, plants, and Fungi are composed of these cells.

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Fig. 2-2. This bacterium-like microfossil, found in South Africa during excavations of siliceous rock deposits known as 'black chert', is about 3.4 billion years old. One of the oldest fossils currently known, it has been named Eobacterium isolarum, which means 'the solitary bacterium at the dawn (of life)'.

The short straight line under this and all subsequent electron micrographs represents the scale; 1 µm is 1/10000 cm.

The terms 'prokaryote' and 'eukaryote' are derived from the Greek word karyon ('nut' or 'kernel', i.e., Nucleus). Prokaryote means 'before nucleus', and eukaryote means 'with a well-defined nucleus'. In a Introduction/4.html">Prokaryotic Cell, the genetic material is localized in a rather poorly defined, non-membrane-bounded nuclear body, or nucleoid. A Eukaryotic Cell, by contrast, contains a highly organized, very complex nucleus surrounded by a nuclear envelope consisting of two membranes. Let us now consider prokaryotic and eukaryotic cells in somewhat greater detail.