BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 3. THE CELL AND THE EXTRACELLULAR MATRIX

3.1. Molecular and Supramolecular Organization of the Cell

3.1.1. Prokaryotic Cells

Prokaryotic Cells are typically spherical or elongated in shape, with dimensions of a few micrometers. As a rule, they possess a Cell wall enclosing a single cellular membrane known as The Plasma Membrane. Because there is only one membrane, it bounds a single compartment whose Cytoplasm contains Proteins, Nucleic Acids, other small molecules, and inorganic ions. Prokaryotes reproduce by binary fission. Under optimal conditions, this division occurs approximately every 20 min (the interval between divisions is referred to as the generation time).

Two kingdoms of prokaryotes are distinguished: Bacteria (cyanobacteria, or blue-green Algae) and archaea. The former predominantly inhabit "favorable" ecological niches, such as soil, Water, and other organisms. The latter thrive in "harsh" ecological niches, including hypersaline, thermal, and acidic waters, swamps, and the deep sea.

A Introduction/4.html">Prokaryotic Cell (Fig. 3.1) features a capsule—a mucous or sticky secretion that provides additional cellular protection. Beneath the capsule lies The Cell wall, which is permeable to water, small molecules, and ions, exhibits antigenic properties, and prevents osmotic Swelling. Based on their Structure, bacterial cell walls are divided into two groups: Gram-positive and Gram-negative. Cells of the first group stain positively using Hans Christian Gram's method (they bind methylene blue and other basic Dyes,

and retain the stain after Treatment with iodine, followed by alcohol or acetone). Cells of the second group do not take up this stain. The cell wall contains a "lattice" made of murein (a polysaccharide). Motile bacteria possess one or more flagella composed of flagellin (an Actin-like protein), which forms a hollow cylinder 10-20 nm in diameter. Such bacteria are capable of taxis. The cell wall of certain Gram-positive bacteria bears pili (fimbriae), which serve to attach the cells to surfaces. Beneath the cell wall lies the plasma membrane, whose invaginations form mesosomes (vesicular and tubular membrane structures involved in Cell wall formation, METABOLISM/36.html">DNA Replication, Respiration, and other processes) and photosynthetic membranes containing Photosynthetic Pigments, including bacteriochlorophyll. Bacterial DNA is a single circular molecule consisting of 5 · 10⁶ nucleotide pairs (several thousand genes).

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Fig. 3.1 Simplified diagram of The structure of a generalized bacterial cell:

1 - pili (fimbriae); 2 - mucous layer (loose capsule); 3 - photosynthetic membranes; 4 - capsule; 5 - flagellum; 6 - cell wall; 7 - Ribosomes; 8 - plasma membrane; 9 - nutrient reserves (Lipids, Glycogen, polyphosphates);

10 - cytoplasm; 11 - circular DNA molecule; 12 — mesosome

Bacteria exhibit all four Nutritional types: photoautotrophic (energy source: light; carbon source: CO₂), chemoautotrophic (energy source: oxidation of inorganic substances; carbon source: CO₂), photoheterotrophic (energy source: light; carbon source: Organic compounds from other organisms), and chemoheterotrophic (energy source: oxidation of organic compounds; carbon source: organic compounds from other organisms). The latter group is the largest. It includes Saprophytes (obtaining nutrients from dead organic matter), symbionts (e.g., Escherichia coli, which inhabits the intestine and supplies the host with B-group Vitamins and vitamin K), and parasites (living at the host's expense), including pathogenic parasites (which cause various diseases in the host). In a primitive form, bacteria also undergo a type of sexual reproduction resulting in recombinant DNA (containing genes from both parental cells). Consequently, such offspring—recombinants—display a wider range of traits, which is significant for evolution. Recombinants are formed through transformation (a DNA fragment from a donor cell is taken up by a recipient cell and integrated into its own DNA), conjugation (DNA transfer between cells in direct contact with each other), and Transduction (a small DNA fragment from a donor cell is introduced into a recipient cell by a bacteriophage, a virus that "attacks" bacteria). Although Sexual reproduction in bacteria is relatively rare, due to their sheer numbers it occurs frequently and is of particular importance in acquiring new traits, including environmental resistance.

The vast superkingdom of prokaryotes is of immense importance to humans. In terms of their impact, prokaryotes can be viewed as either "beneficial" or "harmful". The former maintain soil fertility, purify wastewater, form the gastrointestinal microflora as symbionts, and are utilized in industrial Fermentation processes, The production of Antibiotics, Hormones, interferons, and Single-Cell Protein, as well as in agricultural pest control. "Harmful" prokaryotes can cause food and feed spoilage and act as causative agents of diseases such as botulism, diphtheria, tuberculosis, leprosy, pertussis, Gonorrhea, Syphilis, typhus, tetanus, cholera, dysentery, gastroenteritis, salmonellosis, and others. Like viral infections, these diseases are transmitted via droplet infection (coughing, sneezing), contagious contact (Sexually Transmitted Infections), vectors, fecal contamination, improperly cooked food, wounds, and medical instruments.