Chapter 2: The Chemistry of Microbiology – Study Notes

Atoms and Elements

Basic Concepts of Matter

Understanding the chemical basis of life is essential in microbiology. Atoms and elements form the foundation of all matter, including living organisms.

• Matter: Anything that occupies space and has mass.

• Atoms: The smallest chemical units of matter.

• Element: A pure substance consisting of only one type of atom.

Atomic Structure

• Protons (p+): Positively charged particles in the nucleus.

• Neutrons (n0): Uncharged particles in the nucleus.

• Electrons (e-): Negatively charged particles orbiting the nucleus in electron shells.

• Atomic Number: Number of protons in an atom.

• Atomic Mass: Sum of protons and neutrons ($\text{Atomic Mass} = p + n$).

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon has three naturally occurring isotopes: Carbon-12 (6 protons, 6 neutrons), Carbon-13 (6 protons, 7 neutrons), and Carbon-14 (6 protons, 8 neutrons).

• Radioactive Isotopes: Unstable isotopes that release energy through radioactive decay.

Common Elements of Life

Major Biological Elements

Living organisms are primarily composed of a few key elements, each with specific biological roles.

Element	Symbol	Atomic Number	Biological Significance
Hydrogen	H	1	Component of organic molecules and water; H+ release affects pH
Carbon	C	6	Backbone of organic molecules
Nitrogen	N	7	Component of amino acids, proteins, and nucleic acids
Oxygen	O	8	Component of many organic molecules and water; required for aerobic metabolism
Phosphorus	P	15	Component of nucleic acids and ATP
Sulfur	S	16	Component of proteins
Sodium	Na	11	Principal cation outside cells
Potassium	K	19	Principal cation inside cells; essential for nerve impulses
Calcium	Ca	20	Used in many intracellular signaling processes; essential for cell wall stability in some organisms
Iron	Fe	26	Component of energy-transferring proteins; transport of oxygen
Magnesium	Mg	12	Component of many enzymes; used in photosynthesis
Copper	Cu	29	Component of vitamin B12
Zinc	Zn	30	Component of some enzymes
Molybdenum	Mo	42	Component of some enzymes
Iodine	I	53	Component of many brown and red algae

Chemical Bonds

Valence and Reactivity

The chemical reactivity of an atom is determined by the number of electrons in its outermost (valence) shell. Atoms with incomplete valence shells tend to gain, lose, or share electrons to achieve stability.

• Valence Shell: The outermost electron shell of an atom.

• Atoms with unfilled valence shells are more likely to form chemical bonds.

Types of Chemical Bonds

• Covalent Bonds: Atoms share pairs of electrons. Can be nonpolar (equal sharing) or polar (unequal sharing).

• Ionic Bonds: One atom donates electrons to another, resulting in oppositely charged ions that attract each other.

• Hydrogen Bonds: Weak attractions between a partially positive hydrogen atom and a partially negative atom (often oxygen or nitrogen). Important in stabilizing large molecules like DNA and proteins.

Example: Water molecules are held together by hydrogen bonds, contributing to water's unique properties.

Chemical Reactions

Types of Chemical Reactions

Chemical reactions involve the making or breaking of chemical bonds, transforming reactants into products. In living organisms, these reactions are collectively called metabolism.

• Synthesis (Anabolic) Reactions: Combine smaller molecules to form larger, more complex molecules. Often require energy input (endothermic). Example: Dehydration synthesis forms polymers from monomers, releasing water.

• Decomposition (Catabolic) Reactions: Break down larger molecules into smaller components, releasing energy (exothermic). Example: Hydrolysis reactions use water to break covalent bonds.

• Exchange (Replacement) Reactions: Atoms or groups of atoms are exchanged between molecules. Example: Phosphorylation of glucose in cellular metabolism.

General Equations:

• Synthesis: $A + B \rightarrow AB$

• Decomposition: $AB \rightarrow A + B$

• Exchange: $AB + CD \rightarrow AD + CB$

Water, Acids, Bases, and Salts

Water

Water is the most abundant inorganic compound in living organisms and is essential for life.

• High cohesion due to hydrogen bonding, leading to surface tension.

• Excellent solvent for many substances.

• Participates in chemical reactions (hydrolysis and dehydration synthesis).

• Stabilizes temperature due to high specific heat.

Acids, Bases, and pH

• Acids: Substances that dissociate in water to release hydrogen ions (H+).

• Bases: Substances that dissociate to release hydroxide ions (OH-), or bind H+.

• pH: A measure of hydrogen ion concentration; $\text{pH} = -\log_{10}[H^+]$

• Most organisms maintain a narrow internal pH range using buffers, which absorb excess H+ or OH-.

Salts and Electrolytes

• Salts: Compounds that dissociate in water into cations and anions, neither of which is H+ or OH-.

• Salts are important as electrolytes, conducting electricity and maintaining cellular functions.

Organic Macromolecules

Functional Groups

Organic molecules often contain specific groupings of atoms called functional groups, which confer characteristic chemical properties.

• Examples: Hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), phosphate (-PO4).

Lipids

Lipids are hydrophobic molecules composed mainly of carbon and hydrogen. They serve as energy storage, structural components, and signaling molecules.

• Fats (Triglycerides): Glycerol + 3 fatty acids. Saturated fats have no double bonds (solid at room temperature); unsaturated fats have one or more double bonds (liquid at room temperature).

• Phospholipids: Glycerol + 2 fatty acids + phosphate group. Major component of cell membranes, forming bilayers with hydrophilic heads and hydrophobic tails.

• Steroids: Four fused carbon rings. Cholesterol is a key steroid in membranes and hormone synthesis.

Carbohydrates

Carbohydrates are composed of carbon, hydrogen, and oxygen (1:2:1 ratio). They provide energy and serve as structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined by dehydration synthesis (e.g., sucrose, lactose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Proteins

Proteins are polymers of amino acids linked by peptide bonds. They perform structural, enzymatic, and regulatory functions.

• Amino Acids: Contain a central carbon, amino group, carboxyl group, hydrogen, and variable R group.

• Peptide Bonds: Covalent bonds formed by dehydration synthesis between amino acids.

• Protein Structure: Four levels—primary (sequence), secondary (alpha-helix, beta-sheet), tertiary (3D folding), and quaternary (multiple polypeptides).

• Denaturation: Loss of structure and function due to environmental changes.

Nucleotides and Nucleic Acids

Nucleic acids (DNA and RNA) store and transmit genetic information. Nucleotides are their building blocks.

• Nucleotide Structure: Phosphate group, pentose sugar (ribose or deoxyribose), and nitrogenous base (A, T, G, C, U).

• DNA: Double helix of antiparallel strands held by hydrogen bonds between complementary bases (A-T, G-C).

• RNA: Single-stranded; involved in protein synthesis.

• ATP (Adenosine Triphosphate): The primary short-term energy carrier in cells.

Example: ATP hydrolysis releases energy: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$