Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Introduction to Microbiology
Cell Structure
Prokaryotic Cells

Electron Cell/15.html">Microscopy has revealed that There are two fundamentally Different types of Cells. Although they share some common features, cells of one type differ so significantly from those of the other in Structure and function that it is appropriate to consider them separately. All currently known cells belong to either the first or the second type.

Prokaryotic cells lack a membrane-bound Nucleus. Prokaryotes are characterized by relatively small sizes and structural simplicity. They usually exist in isolation, independently of other cells. The linear dimensions of these cells, which can be spherical, rod-shaped, or spiral, typically range from 0.5 to 3 µm*. To visualize these dimensions, it is helpful to compare them with the sizes of other objects in the universe. As shown in Fig. 1.1, a Introduction/4.html">Prokaryotic Cell is as much smaller than a HUMAN AS A human is smaller than the Earth, or as a hydrogen atom is smaller than a cell. As we will see later, these relationships become highly significant in the detailed study of cell Functions. The volume of a single prokaryotic cell is about 10^-12 mL, of which 50–80% is Water. To a first approximation, the mass of a single prokaryote can be assumed to be 10^-12 g.

* 1 m (meter) = 10³ mm (millimeters) = 10⁶ µm (micrometers; the term micron was also previously used) = 10⁹ nm (nanometers) = 10¹⁰ Å (angstroms).

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Fig. 1.1. Characteristic sizes of objects in the universe. The size range of living organisms is extremely wide. (From: Loewy, A., Siekevitz, P., Cell Structure and Function. — Moscow: Mir, 1984, p. 45.)

Microorganisms of this type grow very rapidly and are widely distributed in nature. Some prokaryotes can double in size, mass, and number in 20 min. Prokaryotes are generally biochemically versatile in the sense that they can utilize A wide variety of nutrients and, moreover, are capable of selecting the best nutrients from a mixture available in the medium. This feature of prokaryotes, along with some of their other properties that we will discuss later, enables prokaryotic cells to adapt to a wide variety of conditions. Since prokaryotes usually exist as isolated single-celled organisms, they have virtually no means of controlling their environment. For this reason, their flexibility in nutrient Selection is essential for their survival. The rapid growth and biochemical versatility of prokaryotes make them indispensable in biological research and biochemical technology.

FIG. 1.2. Electron micrograph of the prokaryote Bacillus subtilis. This soil bacterium, shown here near the end of the division process, is used industrially to produce certain biological catalysts and Antibiotics. The Cell is about 2 µm long and 1 µm wide. B. subtilis is also an important host cell for recombinant DNA. (Photograph courtesy of A. Ryter.)

Figure 1.2 illustrates the main Structural Features of a prokaryotic cell. The cell is surrounded by a rigid wall about 200 Å thick. The wall maintains the integrity of the cell, which is essential for its survival in changing environmental conditions. Directly beneath the wall lies The cell membrane, which is typically about 70 Å thick and does not differ in structure from the membranes of other cells. It is sometimes called The Plasma Membrane. The most important function of the membrane is the Transport of substances into and out of the cell, with the membrane determining which substances are transported and at what rate. Inside the cell, There is a relatively large, poorly defined region called the nucleoid, which plays a central role in controlling vital cell functions. The dark, irregularly shaped spots inside the cell represent Ribosomes—the centers of Protein Synthesis, a crucial biochemical process. The Cytoplasm is the fluid that occupies the rest of the cell volume. Prokaryotic Cells also contain light, vesicle-like areas called reserve granules; these are not visible in this figure but can be distinguished in some other micrographs. We will examine the structure and function of these prokaryotic cell components in greater detail later, after the necessary basic principles have been presented and the relevant terms explained.

While sharing many Structural and functional elements, different prokaryotes can also differ significantly from one another. Blue-green Algae, for example, possess membranes capable of capturing light energy and utilizing it for Photosynthesis. In this complex process of solar energy utilization, the cells are provided with the organic matter necessary for their life processes and release oxygen into the atmosphere.