BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.3. Prokaryotes

The prokaryotic kingdom comprises organisms commonly known as Bacteria. This is the most ancient group, which emerged approximately 3.5 billion years ago, as well as the smallest organisms with a cellular Structure. The properties of prokaryotes are summarized in Table 2.2. As a rule, prokaryotes exist as single Cells, although blue-green Algae (cyanobacteria, Cyanobacteria) can form chains of cells called filaments.

Class="center">Table 2.2. Main DIFFERENCES BETWEEN PROKARYOTES and eukaryotes

Some bacteria adhere to one another, forming characteristic clusters resembling bunches of grapes (Fig. 2.10); however, the aggregated cells remain entirely independent of each other. Individual bacterial cells can only be seen with a microscope, which is why they are called microorganisms. The science that studies bacteria, bacteriology, constitutes an important branch of microbiology.

Bacteria vary in size, ranging from 0.1 to 10 µm in length, with an average diameter of about 1 µm. Consequently, a bacterial cell has ample room for 200 medium-sized globular protein molecules (5 nm in diameter) to span its width. Since such molecules are capable of diffusing approximately 60 µm per second, these organisms require no special transport mechanisms.

Bacteria can be found everywhere: in soil and dust, in Water and air, and both inside and On the surface of plants and animals. Some bacteria thrive in hot springs at temperatures of 78 °C or higher. Others can survive extreme cold and even endure certain periods of freezing in ice. Bacteria are also found in deep-sea trenches under extremely high pressure and temperatures of 360 °C, forming the base of unique food webs in these oceanic regions.

The number of bacteria is unimaginably vast; it has been established that a single gram of fertile soil contains 2.5 billion bacteria, while 1 cm³ of fresh milk may harbor over 3 billion. Along with fungi, bacteria are vital to all other organisms because, by breaking down organic matter through their metabolic activity, they drive the cycling of nutrients in nature. Furthermore, they are gaining increasing importance in human life—not only because some species are pathogenic agents of various diseases, but also because, owing to The Diversity of their biochemical reactions, they can be utilized in numerous biotechnological processes. This topic is discussed in more detail in Chapter 12.

2.3.1. Structure of Bacteria

Figure 2.5 illustrates The structure of a generalized bacterium—a typical Introduction/4.html">Prokaryotic Cell. Figures 2.6 A–D depict the well-known rod-shaped bacterium Escherichia coli. It is typically completely harmless, and its presence in water can serve as a very reliable indicator of fecal water contamination. Of all bacteria, E. coli has been studied the most thoroughly and is also one of the bacteria whose complete genetic map has been sequenced. Note that E. coli has far fewer visible intracellular structures than a Eukaryotic Cell (Figs. 5.10 and 5.11). Figure 2.7 shows another rod-shaped bacterium which, unlike E. coli, possesses a flagellum.

Fig. 2.5. Structure of a generalized rod-shaped bacterium (typical prokaryotic cell). The number of subcellular structures in such bacteria is significantly smaller than in a eukaryotic cell.

Fig. 2.6. A. Structure of Escherichia coli. E. coli is a rod-shaped bacterium inhabiting the intestines of vertebrates. B. Stained cells viewed under a high-power Light Microscope (×1000). C. Scanning electron micrograph of E. coli. D. Transmission electron micrograph of an E. coli cell section during division (×50,000). Light areas contain DNA. The DNA-containing region is often referred to as the nucleoid.

Fig. 2.7. Transmission electron micrograph of a rod-shaped bacterium, clearly showing its shape, cell wall, pili, and long, wavy flagella (×28,000). The specimen was shadowed with a heavy metal impermeable to electrons. The shielded areas remained uncoated, forming electron-transparent regions. The photograph is shown as a negative so that these regions appear black. This technique is known as shadow casting and is used to reveal the surface structure of small objects.

Cell Wall

The Bacterial cell wall is a fairly rigid structure that enables The Cell to maintain its shape; this is due to the presence of murein (peptidoglycan), a molecule constructed of parallel polysaccharide chains cross-linked at regular intervals by short amino acid chains. Thus, each cell is enclosed in a mesh-like sac that is effectively a single giant molecule. The cell wall protects the cell from bursting when water enters (e.g., via osmosis). Water ions and small molecules pass into the cell through tiny pores within the cell wall.

In 1884, the Danish microbiologist Hans Christian Gram developed a staining technique that established that bacteria fall into two natural groups, which is now known to be due to differences in their cell wall structure. Bacteria that take up the Gram stain are called Gram-positive, while those that do not are called Gram-negative. Practical exercises involving Gram staining are described in Section 12.9.2.

In Gram-positive bacteria, such as Staphylococcus, Bacillus, and Lactobacillus, other components—mainly polysaccharides and proteins—are incorporated into the murein mesh, making the cell wall relatively thick. In Gram-negative bacteria, such as Salmonella, E. coli, and Azotobacter, the cell wall is thinner and has a more complex structure (Fig. 2.8). In these bacteria, the murein layer is covered on the outside by a smooth, thin, membrane-like layer of Lipids and polysaccharides, which protects the cells from Lysozyme—an antibacterial enzyme found in tears, saliva, other biological fluids, and egg white. Lysozyme cleaves the polysaccharide backbone of murein, leading to perforation of the cell wall and cell lysis, i.e., osmotic Swelling and bursting. The lipid-polysaccharide layer also confers resistance to penicillin in Gram-negative bacteria. This antibiotic blocks cross-linking in the murein of growing Gram-positive bacteria, rendering their cells more vulnerable to osmotic Shock.

Fig. 2.8. STRUCTURE OF THE cell wall in Gram-positive (left) and Gram-negative (right) bacteria. During the decolorization step of Gram staining, the stain is easily washed out of the thin murein layer in Gram-negative bacteria.

Plasma Membrane, Mesosomes, and Photosynthetic Membranes

Like all other living organisms, the living matter of a bacterial cell is surrounded by a semipermeable membrane. In terms of Structure and function, the plas

ma membrane of bacterial cells does not differ from the Plasma Membranes of Eukaryotic cells (Sec. 5.9). It also serves as the localization site for respiratory Enzymes, while in some bacteria it forms mesosomes and/or photosynthetic membranes.

Mesosomes are folded structures formed by invaginations of the cell plasma membrane (Fig. 2.5). During cell division, mesosomes apparently associate with DNA, which ensures the Separation of two daughter DNA molecules after Replication and promotes The formation of a septum between the daughter cells.

In photosynthetic bacteria, sac-like, tubular, or lamellar invaginations of the plasma membrane contain Photosynthetic Pigments (including, invariably, bacteriochlorophyll). Similar membrane structures are also involved in nitrogen fixation.

Genetic material (bacterial "chromosome")

Bacterial DNA is a single circular molecule about 1 mm in length (i.e., it is significantly longer than the cell itself) consisting of approximately 5 million Base Pairs. The total DNA content (genome), and consequently the Amount of Information encoded within it, is significantly smaller in a bacterial cell than in a eukaryotic one: typically, bacterial DNA contains several thousand genes, which is 500 times less than in a human cell (see also Table 2.2 and Fig. 2.5).

Ribosomes

Ribosomes serve as the sites of protein synthesis (see Table 2.2 and Fig. 5.5).

Capsules

In some bacteria, mucous or sticky secretions form capsules; capsules are clearly visible after negative staining (where the Background is stained rather than the preparation itself). Sometimes these secretions serve to form colonies from single bacteria. Through these secretions, bacteria acquire The ability to adhere to various surfaces, such as Teeth, silt particles, or rocks. In addition, capsules provide extra protection for the bacterial cell. For instance, encapsulated strains of pneumococci multiply freely in The Human Body, causing Pneumonia, whereas unencapsulated strains are easily attacked and destroyed by phagocytes and are therefore completely harmless.

Spores

Some bacteria, mainly belonging to the genera Clostridium and Bacillus, form endospores (i.e., spores located inside the cells). Spores are thick-walled, long-lived structures characterized by very high resistance, particularly to heat, short-wave radiation, and desiccation. The Intracellular Localization of spores varies and serves as an important characteristic for the identification and Classification of Bacteria (see Fig. 2.10).

Flagella

Many bacteria are motile due to the presence of one or more flagella. A flagellum is a simple hollow cylinder formed by identical protein molecules. Despite their wavy shape, they are quite rigid (Fig. 2.7). Bacterial motility is achieved by the Rotation of the flagellar base; the flagellum essentially screws itself into the medium without making chaotic beating movements, thereby propelling the bacterium forward. Examples of flagellated bacteria include Rhizobium (a single flagellum) and Azotobacter (multiple flagella); both bacteria participate in the natural nitrogen cycle.

Motile bacteria can move in response to specific stimuli, meaning they are capable of taxis. Aerobic bacteria, for example, move toward higher oxygen concentrations in the environment (exhibiting positive aerotaxis), while motile photosynthetic bacteria swim toward light (exhibiting positive phototaxis).

Flagella are best viewed using an Electron microscope with shadowing techniques (Fig. 2.7).

Pili

Numerous thin, rod-like outgrowths called pili or fimbriae are visible on the cell wall of some Gram-negative bacteria (Fig. 2.7). Pili are shorter and thinner than flagella and serve for attachment to specific cells or surfaces. Various types of pili are known, but of greatest interest are F-pili, which are involved in sexual reproduction (Sec. 2.3.3).

Plasmids

In addition to the single DNA molecule present in all bacteria, some of them harbor one or more plasmids (Fig. 2.9). A plasmid is a small, circular molecule of additional DNA capable of self-replication. A plasmid carries only a few genes that confer enhanced cell survival. Some plasmids make cells resistant to Antibiotics. For example, cells of certain staphylococci contain a plasmid carrying the Gene for penicillinase, an enzyme that breaks down penicillin. As a result, the cell becomes resistant to penicillin. The spread of such genes via conjugation (Sec. 2.3.3) is of great medical importance. Other known plasmid genes include, in particular, genes

1) conferring resistance to disinfectants;

2) causing various diseases;

3) responsible for milk Fermentation by lactic acid bacteria in cheesemaking;

4) conferring the ability to use complex chemical substances such as Hydrocarbons as food, making them potentially useful in combating oil spills and producing protein from petroleum products.

Fig. 2.9. Transmission electron micrograph of a bacterial plasmid.