CHAPTER
2
The Chemistry of Microbiology
Chapter Outline
Atoms (pp. 28–30)
Atomic Structure
Isotopes
Electron Configurations
Chemical Bonds (pp. 31–35)
Nonpolar Covalent Bonds
Chemical Reactions (pp. 35–36)
Synthesis Reactions
Decomposition Reactions
Exchange Reactions
Water, Acids, Bases, and Salts (pp. 36–39)
Water
Acids and Bases
Salts
Organic Macromolecules (pp. 39–52)
Functional Groups
Lipids
Chapter Summary
Atoms (pp. 28–30)
Matter is defined as anything that takes up space and has mass. The smallest chemical units of
matter are atoms.
Instructor’s Manual for Microbiology with Diseases by Body Systems, 5e
Atomic Structure
Atoms contain negatively charged particles called electrons spinning around a nucleus
composed of uncharged particles called neutrons and positively charged particles called
protons. (A hydrogen atom contains only one proton and no neutrons.) The number of electrons
Isotopes
Isotopes are atoms of an element that differ only in the numbers of neutrons they contain. For
example, there are three naturally occurring isotopes of carbon, all of which have 6 protons:
Electron Configurations
Since only the electrons of an atom come close enough to interact with another atom, they
determine an atom’s chemical behavior. Electrons orbit their nucleus in three-dimensional
Chemical Bonds (pp. 31–35)
The number of electrons in the valence shell—the outermost shell—determines the atom’s
valence or combining capacity. Atoms with valence shells not containing the maximum number
Nonpolar Covalent Bonds
A covalent bond is the sharing of a pair of electrons by two atoms. Two hydrogen atoms bind
covalently to form a stable molecule of hydrogen in which both atoms have full valence shells.
Atoms such as oxygen that share two pairs of electrons have a double covalent bond with each
Chapter 2 The Chemistry of Microbiology
these are called nonpolar covalent bonds. Structural formulas are used to represent the atoms
of a molecule and the types of bonds between them. Molecular formulas represent the numbers
Polar Covalent Bonds
When atoms with significantly different electronegativities combine, the electron pair will spend
Ionic Bonds
When two atoms with vastly different electronegativities approach each other, the atom with
the higher electronegativity will attract one or more electrons so strongly that it will gain
electrons from the valence shell of the other. This happens, for example, when chlorine, with
Hydrogen Bonds
Like ionic bonds, hydrogen bonds do not involve the sharing of electrons. Instead, a transiently
charged hydrogen atom is attracted to a full or transient negative charge on either a different
Chemical Reactions (pp. 35–36)
Chemical reactions result from making or breaking chemical bonds in a process in which
Instructor’s Manual for Microbiology with Diseases by Body Systems, 5e
Synthesis Reactions
Synthesis reactions involve the formation of larger, more complex molecules. An important
Decomposition Reactions
Decomposition reactions are the opposite of synthesis reactions in that they break bonds within
larger reactants to form smaller atoms, ions, and molecules. Because these reactions release
Exchange Reactions
Exchange reactions (transfer reactions) involve the breaking and formation of chemical bonds
Water, Acids, Bases, and Salts (pp. 36–39)
Inorganic chemicals usually lack carbon. Many—including water, acids, bases, and salts—are
essential to life.
Water
Water constitutes 50–99% of the mass of living organisms. Water molecules are polar and
cohesive; they stick to one another via hydrogen bonding. This produces most of the special
Acids and Bases
An acid is a substance that dissociates into one or more hydrogen ions (H+) and one or more
anions. A base is a molecule that binds with H+ when dissolved in water. Some bases dissociate
into hydroxyl ions and cations. Acids and bases may be organic or inorganic. The concentration
Chapter 2 The Chemistry of Microbiology
of hydrogen ions in a solution is expressed using a logarithmic pH scale in which acidity
increases as pH (potential hydrogen) values decrease. Organisms can tolerate only a narrow pH
Salts
A salt is a compound that dissociates in water into cations and anions other than H+ and OH−. A
cell uses the cations and anions of salts—electrolytes—to create electrical differences between
its inside and outside environment, to transfer electrons from one location to another, and as
important components of many enzymes. Some organisms use salts to provide structural support
for their cells.
Organic Macromolecules (pp. 39–52)
Organic molecules are large, complex molecules that are the basic structures of organisms or
perform the complex chemical reactions of the metabolism of organisms.
Functional Groups
Carbon atoms can link together to form branched and unbranched chains and ring structures.
Atoms of other elements such as oxygen, nitrogen, phosphorus, and sulfur can be bound to these
Lipids
Lipids are organic macromolecules composed almost entirely of carbon and hydrogen atoms
linked by nonpolar covalent bonds. Being nonpolar, they are hydrophobic; that is, they are
insoluble in water.
The four major groups of lipids are fats, phospholipids, waxes, and steroids:
The molecules of fats, also known as triglycerides, are composed of a glycerol and three
Instructor’s Manual for Microbiology with Diseases by Body Systems, 5e
Phospholipids contain only two fatty acid chains and a phosphate functional group. Various
fatty acids occur in phospholipids. Whereas the fatty acid “tail” of a phospholipid molecule is
Carbohydrates
Carbohydrates are organic molecules composed solely of atoms of carbon, hydrogen, and
oxygen, typically in a ratio of 1:2:1. They are often represented at (CH2O)n where n indicated
the number of CH2O units. They are used for immediate and long-term storage of energy, as
structural components of DNA and RNA and some cell walls, and for conversion into amino
acids. They also serve roles in cellular interactions. There are three basic types:
Monosaccharides are simple sugars, such as glucose and fructose. They usually take cyclic
Proteins
The most complex organic macromolecules are proteins, which are composed mostly of carbon,
hydrogen, oxygen, nitrogen, and sulfur. They function to provide structure as components of
cells; enzymatic catalysis as enzymes; regulation of various activities; in transportation of
substances; and as molecules of defense and offense. Proteins are polymers of amino acids, in
the covalent addition of other groups: glycoproteins (carbohydrates), lipoproteins (lipids), and
nucleoproteins (nucleic acids). Denaturation of a protein disrupts its structure and subsequently
its function.
Nucleotides and Nucleic Acids
The two nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) comprise the
Nucleotides and Nucleosides
The monomer of nucleic acids is a nucleotide. Each nucleotide consists of phosphate, a pentose
Nucleic Acid Structure
Nucleic acids are polymers with bonds formed between the sugars and phosphates of nucleo-
Nucleic Acid Function
The structure of DNA carries the genetic instructions for organisms, allows for genetic diversity,
ATP (Adenosine Triphosphate)
Adenosine triphosphate (ATP) is made up of the nitrogenous base adenine, ribose sugar, and
Instructor’s Manual for Microbiology with Diseases by Body Systems, 5e
New Media Resources
Connecting Concepts: ATP and Metabolic Energy, Hydrogen Bonding and Its Role in DNA
Replication.