In the 1700s, scientists discovered the chemical and physical basis of living things, and soon they realized that the chemical organization of all living things is remarkably similar. Microorganisms, as forms of living things, conform to this principle and have a chemical basis that underlies their metabolism.
Elements and atoms. All living things on earth, including microorganisms, are composed of fundamental building blocks of matter called elements. Over 100 elements are known to exist, including certain ones synthesized by scientists. An element is a substance that cannot be decomposed by chemical means. Such things as oxygen, iron, calcium, sodium, hydrogen, carbon, and nitrogen are elements.
Each element is composed of one particular kind of atom. An atom is the smallest part of an element that can enter into combinations with atoms of other elements.
Atoms consist of positively charged particles called protons surrounded by negatively charged particles called electrons. A third particle called the neutron has no electrical charge; it has the same weight as a proton. Protons and neutrons adhere tightly to form the dense, positively charged nucleus of the atom. Electrons spin around the nucleus in orbits, or shells.
The arrangement of electrons in an atom plays an essential role in the chemistry of the atom. Atoms are most stable when their outer shell of electrons has a full quota, which may be two electrons or eight electrons. Atoms tend to gain or lose electrons until their outer shells have this stable arrangement. The gaining or losing of electrons contributes to the chemical reactions in which an atom participates.
Molecules. Most of the microbial compounds of interest to biologists are composed of units called molecules. A molecule is a precise arrangement of atoms from different elements; a compound is a mass of molecules. The arrangements of the atoms in a molecule account for the properties of a compound. The molecular weight is equal to the atomic weights of the atoms in the molecule. For example, the molecular weight of water is 18.
The atoms in molecules may be joined to one another by various linkages called bonds. One example of a bond is an ionic bond, which is formed when the electrons of one atom transfer to a second atom, creating electrically charged atoms called ions. The electrical charges attract the ions to one another; the attraction creates the ionic bond. Sodium chloride consists of sodium ions and chloride ions joined by ionic bonds (Figure1 ).
Bond formation in molecules. (a) Formation of an ionic bond in a sodium chloride molecule. (b) Covalent bonding in methane and water molecules.
Figure 1
Syntheses in organic molecules. (a) Bonding of two fatty acids to a glycerol molecule in the formation of a fat. (b) Bonding of two amino acids via a peptide bond in the formation of a protein.
A second type of linkage is called a covalent bond (Figure 1 ), which forms when two atoms share one or more electrons with one another. For example, carbon shares its electrons with four hydrogen atoms, and the resulting molecule is methane (CH4). If one pair of electrons is shared, the bond is a single bond; if two pairs are shared, then it is a double bond. Covalent bonds are present in organic molecules such as proteins, lipids, and carbohydrates.
Acids and bases. Certain chemical compounds release hydrogen ions when the compounds are placed in water. These compounds are called acids. For example, when hydrogen chloride is placed in water, it releases its hydrogen ions, and the solution becomes hydrochloric acid.
Certain chemical compounds attract hydrogen ions when they are placed in water. These substances are called bases. An example of a base is sodium hydroxide (NaOH). When this substance is placed in water, it attracts hydrogen ions, and a basic (or alkaline) solution results.