The chemical compounds of living things such as microorganisms are known as organic compounds because of their association with organisms. The organic compounds, the subject matter of organic chemistry, are the compounds associated with life processes in microorganisms.
Carbohydrates. Four major categories of organic compounds are found in all microorganisms. The first category is the carbohydrates.
Carbohydrates are used by microorganisms as sources of energy. In addition, carbohydrates serve as structural materials such as in the construction of the microbial cell wall. Carbohydrates are molecules composed of carbon, hydrogen, and oxygen; the ratio of hydrogen atoms to oxygen atoms is 2:1.
The simple carbohydrates are commonly referred to as sugars. Sugars are monosaccharides if they are composed of single molecules and disaccharides if they are composed of two molecules. The most important monosaccharide is glucose, a carbohydrate with the molecular formula C 6H 12O 6. Glucose is the basic form of fuel for many species of microorganisms. It is soluble and is transported by body fluids to all cells, where it is metabolized to release its energy. Glucose is the starting material for cellular respiration, and it is the main product of photosynthesis in microorganisms.
Three important disaccharides are also found in living things. One disaccharide is maltose, a combination of two glucose units covalently linked. Yeast cells break down the maltose from grain starch in the process of alcoholic fermentation. Another disaccharide is sucrose, the table sugar formed by linking glucose to another monosaccharide called fructose. A third disaccharide is lactose, composed of glucose and galactose units. Lactose, the major carbohydrate in milk, is digested to acid by microorganisms when they sour milk and form sour‐milk products such as yogurt and sour cream.
Complex carbohydrates are known as polysaccharides. Polysaccharides are formed by linking eight or more monosaccharide molecules. Among the most important polysaccharides are starches, which are composed of hundreds or thousands of glucose units linked to one another. Starches serve as a storage form for carbohydrates. Microorganisms break down starch to use the glucose it contains for their energy needs.
Another important polysaccharide is glycogen, which is related to starch. Many bacteria have glycogen in thier cytoplasm. Still another is cellulose. Cellulose is also composed of glucose units, but the units cannot be released from one another except by a few species of microorganisms, especially those in the stomach of the cow and other ruminants. The cell walls of algae contain cellulose, and certain fungi have this polysaccharide. Another polysaccharide called chitin is a primary constituent in the fungal cell wall.
Lipids. Lipids are organic molecules composed of carbon, hydrogen, and oxygen atoms. In contrast to carbohydrates, the ratio of hydrogen atoms to oxygen atoms is much higher. Lipids include steroids, waxes, and the most familiar lipids, fats.
Fat molecules are composed of a glycerol molecule and one, two, or three molecules of fatty acids. A fatty acid is a long chain of carbon atoms with associated hydroxyl (–OH) groups. At one end of the fatty acid is an organic acid (–COOH) group. The fatty acids in a fat may be all alike or all different. They are bound to the glycerol molecule during dehydration synthesis, a process that involves the removal of water (Figure ). The number of carbon atoms in a fatty acid may be as few as four or as many as 24.
Certain fatty acids have one or more double bonds in their molecules. Fats that include these molecules are called unsaturated fats. Other fatty acids have no double bonds. Fats that include these fatty acids are called saturated fats.
Some microbial species use fats as energy sources. They produce the enzyme lipase, which breaks down fats to fatty acids and glycerol. An important type of phosphorus‐containing lipid, the phospholipid, is a major constituent of the cell membranes of all microorganisms.
Proteins. Proteins are among the most complex of all organic compounds. They are composed of units called amino acids, which contain carbon, hydrogen, oxygen, and nitrogen atoms. Certain amino acids also have sulfur atoms, phosphorus, or other trace elements such as iron or copper.
Many proteins are immense and complex as compared to carbohydrates or fats. However, all are composed of folded, long chains of the relatively simple amino acids. There are 20 kinds of amino acids, each with an amino (–NH 2) group and an organic acid (–COOH) group. The amino acids differ with respect to the nature of the chemical group that is attached to the base structure. Examples of amino acids are alanine, valine, glutamic acid, tryptophan, tyrosine, and histidine.
Amino acids are linked to form a protein by the removal of water molecules (Figure ). The links forged between the amino acids are called peptide bonds, and small proteins are often called peptides.
All living things, including microorganisms, depend upon proteins for their existence. Proteins are the major molecules from which microorganisms are constructed. Certain proteins are dissolved or suspended in the watery substance of the cells, while others are incorporated into various structures of the cells, such as the cell membrane. Bacterial toxins (metabolic poisons) and microbial flagella and pili are usually composed of proteins.
An essential use for proteins is in the construction of enzymes. Enzymes catalyze the chemical reactions that take place within microorganisms. The enzymes are not used up in the reaction, but remain available to catalyze succeeding reactions. Without enzymes, the metabolic activity of the microorganism could not take place.
Every species manufactures proteins unique to that species. The information for synthesizing these unique proteins is found in the nucleus of the cell. The so‐called genetic code specifies the sequence of amino acids in the protein and thereby regulates the chemical activity taking place within the cell. Proteins also can serve as a reserve source of energy for the microorganism. When the amino group is removed from an amino acid, the resulting compound is energy rich.
Nucleic acids. Like proteins, nucleic acids are very large molecules. The nucleic acids are composed of smaller units called nucleotides. Each nucleotide contains a five‐carbon carbohydrate molecule, a phosphate group, and a nitrogen‐containing molecule that has basic properties and is called a nitrogenous base.
Microorganisms contain two important kinds of nucleic acids. One type is called deoxyribonucleic acid, or DNA. The other is known as ribonucleic acid, or RNA. DNA is found primarily in the nucleus of eukaryotic microorganisms (which have nuclei) and suspended in the cytoplasm of prokaryotic microorganisms (which lack nuclei). DNA is also located in plasmids, the tiny loops of DNA found in bacterial cytoplasm. RNA is found in both the nucleus (if present) and the cytoplasm of the microorganism.
DNA and RNA differ from one another in their components. DNA contains the carbohydrate deoxyribose, while RNA has ribose. In addition, DNA contains the bases adenine, cytosine, guanine, and thymine, while RNA has adenine, cytosine, guanine, and uracil.