FUNDAMENTALS OF MICROBIOLOGY - V. M. Samygin - 2015

CHAPTER 5. CHEMICAL COMPOSITION OF MICROBIAL CELLS

Studying The chemical composition of microorganisms is essential for understanding the mechanisms of their vital activity and metabolic processes. Biochemical processes occurring within a Cell—and consequently, the chemical composition—vary among different Bacteria. Even within the same species, The Nature of METABOLISM and cellular composition will fluctuate depending on its physiological state and environmental conditions.

5.1. Inorganic Substances

Living Cells contain about 90 different chemical elements; roughly 25 of these are found in virtually all cells, while the total mineral content in bacteria grown on standard nutrient media ranges from 2% to 14% of the dry cell weight.

Based on their Abundance in living systems, all chemical elements are divided into three major groups: Macronutrients, micronutrients, and ultramicronutrients. Macronutrients account for a total of about 99%, micronutrients for less than 1%, and ultramicronutrients for less than 0.01% of the cellular mass.

Macronutrients are typically divided into two broad groups. The first group includes oxygen (65–75%), carbon (15–18%), hydrogen (8–10%), and nitrogen (1.5–3%), which together make up about 98% of The Cell mass. They are often referred to as organogenic elements. These four elements are constituents of Organic compounds; moreover, hydrogen and oxygen form Water, which accounts for at least two-thirds of the contents of most cells. The second group comprises phosphorus (0.2–1%), potassium (0.15–0.4%), sulfur (0.15–0.2%), chlorine (0.05–0.1%), calcium (0.04–2%), magnesium (0.02–0.03%), sodium (0.02–0.03%), and iron (0.01–0.015%), collectively making up about 1.9%.

Micronutrients include manganese, copper, zinc, cobalt, nickel, iodine, and fluorine.

Examples of ultramicronutrients include silver, gold, bromine, molybdenum, boron, arsenic, and selenium, among others. It should be kept in mind, however, that in high concentrations many micronutrients and ultramicronutrients are toxic to organisms.

A special place among the Inorganic Compounds of living cells is occupied by water. The water content in the Cytoplasm of most bacterial species ranges from 75% (E. coli) to 85% (Vibrio cholerae). The highest concentration of water is characteristic of capsule-forming bacteria, whereas spores contain the lowest amount of water (40–50%). Quantitatively, water is the principal constituent of the cell; it exists either in a free state or bound to other cellular components.

Bound water is a structural element of the cytoplasm and cannot act as a solvent. Free water serves as a dispersion medium for colloids and a solvent for crystalline substances, a source of hydrogen and hydroxyl ions, and a participant in Chemical Reactions.

An important property of water is the ability of its molecules to dissociate into hydrogen ions (Н⁺) and hydroxyl ions (OH^-), which participate in numerous biochemical processes, primarily oxidation-reduction reactions. Within cells, the hydrogen ion concentration (medium pH) is maintained at a specific level close to neutrality, which is necessary for normal enzyme function. A water molecule can also participate, for example, in Hydrolysis reactions (the Cleavage of a covalent bond with The addition of hydrogen and hydroxyl to the resulting products) or hydrogenation (the addition of a water molecule across a double bond).

The high heat capacity of water (absorbing heat without significant changes in its own Temperature) protects living organisms from drastic temperature fluctuations, while its high heat of vaporization shields them from overheating at elevated temperatures. The high thermal conductivity of water ensures rapid temperature equalization in different parts of Multicellular Organisms.

5.2. Organic Substances

Organic substances are complex carbon-containing compounds whose abundance in living cells vastly exceeds that of inorganic compounds. Whereas at the beginning of the 19th century about 80 naturally occurring organic substances were known, by the early 20th century their number had reached 100,000, and today the number of organic compounds isolated from natural sources or produced synthetically exceeds several million and continues to grow.

Most organic substances are formed by a small number of elements. Virtually all of these substances contain hydrogen In addition to carbon, and many contain oxygen and nitrogen. These four elements readily form covalent bonds through the pairing of electrons in the outer orbitals of the atoms. To completely fill its outer orbital, carbon lacks four electrons and is therefore capable of forming four electron pairs (four covalent bonds); nitrogen lacks three electrons, oxygen two, and hydrogen one. Furthermore, carbon, nitrogen, and oxygen are capable of forming double bonds, which significantly increases The Diversity of organic substances and imparts new properties to them. Sulfur, phosphorus, and halogens are also constituents of A large number of organic compounds.

Living cells contain low-molecular-weight organic substances (Amino Acids, sugars, organic acids, NUCLEOTIDES, Lipids, etc.) and high-molecular-weight substances, or Biopolymers. Polymers are molecules composed of a large number of repeating units—monomers—connected to one another by covalent bonds to form a long unbranched or branched chain. A polymer is termed a homopolymer if its constituent monomers are identical (e.g., Polysaccharides such as starch and Glycogen, composed of glucose molecules), or a heteropolymer if it is built from several different monomers (Proteins built from 20 amino acids, Nucleic Acids consisting of 4 types of nucleotides).

Lipids. A distinct group of cellular organic compounds consists of lipids (fats and fat-like substances), which vary in Structure and perform diverse Functions. Lipids are found in all bacteria, with their content in different species ranging from 0.2% to 41%. Lipids are Components of the cell envelope, act as reserve nutrient stores, and determine the toxic and antigenic properties of bacteria. The breakdown of fats into carbon dioxide and water yields a large amount of energy (38.9 kJ/g), which accounts for the energy-storage function of fats.

All lipids are hydrophobic compounds, meaning they are insoluble in water but soluble in non-polar organic Solvents (chloroform, benzene, ether). The simplest in structure are neutral lipids, or fats, which are esters of the trihydric alcohol glycerol and Fatty acids. If all hydroxyl groups of glycerol are bound to fatty acids, triglycerides are formed; if two or one are bound, diglycerides and monoglycerides are formed, respectively. Neutral fats can be deposited in the cytoplasm in the form of droplets.

The basis of Introduction/36.html">Biological Membranes is formed by Phospholipids. They are also esters of glycerol and fatty acids, but in phospholipid molecules only two alcohol groups are linked to fatty acids, while the third forms an ester bond with a phosphoric acid residue. Phosphoric acid can form an additional ester bond with low-molecular-weight alcohols, giving rise to various classes of phospholipids. Thus, the molecules of all phospholipids contain hydrophobic (fatty acid residues) and hydrophilic (phosphoric acid and the attached alcohol) components, enabling the molecules to interact with both polar and non-polar solvents (such substances are termed amphiphilic).

In water and aqueous solutions, phospholipids spontaneously form micelles (i.e., particles of the dispersed phase of a sol surrounded by a layer of molecules or ions of the dispersion medium), Liposomes—bilayer phospholipid vesicles whose internal volume is filled with water—or extended flat bilayer structures, which form the basis of all biological membranes. Because phospholipids contain a large amount of unsaturated and polyunsaturated fatty acids, biological membranes at the ambient temperature of living organisms exist in a "fluid" (liquid-crystalline) state, ensuring their flexibility, elasticity, and high mobility of the proteins embedded within them.

CARBOHYDRATES. Carbohydrates (sugars, saccharides) are organic compounds with the general formula (CH₂O)_n. By chemical nature, they are most commonly aldehyde-alcohols or ketone-alcohols. Depending on the number of carbon atoms (3, 4, 5, 6, etc.) incorporated into the sugar molecule, they are classified as trioses, tetroses, pentoses, hexoses, etc.

Among Monosaccharides, The most significant are pentoses: ribose and deoxyribose, which are components of RNA and DNA nucleotides; ribulose, which plays an important role in carbon dioxide fixation during Photosynthesis; and hexoses: glucose, the primary energy source, along with FRUCTOSE AND GALACTOSE. Trioses and tetroses (glyceraldehyde, etc.) are formed as intermediates in various metabolic processes.

Living organisms contain both monosaccharides as well as di- and polysaccharides. Among Disaccharides, the most widespread are sucrose (fruit sugar), consisting of glucose and fructose molecules, and lactose (milk sugar), consisting of glucose and galactose molecules. Polysaccharides are generally homopolymers composed of repeating monomers—glucose molecules—linked by covalent bonds between C1 and C4 (and in branched polysaccharides, also between C1 and C6) carbon atoms.

The total amount of carbohydrates in a microbial cell ranges from 12% to 28%. The bulk of carbohydrates consists of complex compounds—polysaccharides—which break down into simple sugars upon hydrolysis. Based on structural complexity, bacterial polysaccharides can be divided into two groups: nitrogen-free and more complex ones containing 1% to 5% nitrogen. Most frequently, nitrogen is present in the form of amino derivatives of glucose and galactose.

Proteins. Proteins are high-molecular-weight organic compounds, unbranched heteropolymers composed of α-L-amino acid residues linked by peptide bonds (-CO-NH-). The amino acids making up proteins are called proteinogenic; about 100 Amino acids are found in living organisms. The enormous Diversity of proteins is determined by The sequence of their constituent amino acid residues and the length of the polypeptide chain, which dictate differences in the Spatial Structure, as well as the chemical and physical Properties of Proteins. Many of them serve as components of various biologically active compounds and participate in metabolic processes.

There are 4 Levels of Protein structural Organization: Primary Structure (the sequence of amino acid residues in the polypeptide chain; stabilized by covalent peptide bonds), Secondary structure (α-helical regions stabilized by Hydrogen Bonds between the CO and NH groups of peptide bonds), tertiary structure (the three-dimensional spatial conformation of the polypeptide chain; stabilized by disulfide bridges between Cysteine residues, as well as hydrophobic, electrostatic, and hydrogen bonds between amino acid residues), and quaternary structure (the association of multiple polypeptide chains into a single functional unit) (Fig. 7).

Class="center">Fig. 7. Levels of protein structural organization

Not all proteins possess a quaternary structure. Sometimes different proteins form complex supramolecular complexes that function as a single entity (for example, Ribosomes or components of the mitochondrial Respiratory Chain). Based on their composition, proteins are divided into simple proteins, or proteins proper, consisting solely of amino acids, and complex proteins, or proteinids, which, in addition to amino acids, contain carbohydrates (Glycoproteins), lipids (Lipoproteins), nucleotides or nucleic acids (Nucleoproteins), as well as Metalloproteins, Phosphoproteins, etc.

The total protein content in bacteria accounts for 50–75% of dry matter. The most critical biological properties of microbes are determined by the quality of the proteins synthesized by the cell. Both simple and complex proteins are present in the bacterial cell. The proteins of bacteria also include amino acids not found in the cells of animal and plant organisms.

Proteins perform a wide variety of functions: structural, transport, regulatory, and nutritive. Motility is mediated by flagellar proteins. The Breakdown of Proteins and utilization of amino acids during Energy Metabolism are accompanied by the release of energy and its storage in the form of ATP—the energy function. Every Organism synthesizes its own unique proteins, and their composition and quantity may change throughout its lifetime. Proteins within the organism are constantly being renewed; this property underlies metabolism. However, the most crucial is widely recognized to be the catalytic function of protein Enzymes, which take part in all chemical reactions occurring in living organisms.