Atoms, Bonds, and Biological Molecules: Foundations for Microbiology

Atoms, Bonds, and Biological Molecules

Introduction

This chapter introduces the fundamental chemical principles underlying microbiology, focusing on the structure of atoms, types of chemical bonds, and the major classes of biological macromolecules essential for life. Understanding these concepts is crucial for success in microbiology.

The Atom

Atomic Structure

• Atoms are the smallest units of matter that retain the properties of an element.

• Each atom is composed of three subatomic particles:

  • Protons: Positively charged particles located in the nucleus.

  • Neutrons: Neutral particles also found in the nucleus.

  • Electrons: Negatively charged particles that orbit the nucleus in electron shells.

• The number of protons defines the atomic number and the element.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

The Role of Electrons

• Electrons are involved in chemical bonding and determine the reactivity of an atom.

• Electrons occupy energy levels or shells around the nucleus.

• Atoms are most stable when their outermost energy shell is full.

Chemical Bonds

Ionic Bonds

• Formed when atoms transfer electrons, resulting in charged ions.

• Cations: Atoms that lose electrons and become positively charged.

• Anions: Atoms that gain electrons and become negatively charged.

• Ionic bonds are the electrostatic attraction between oppositely charged ions.

• Common in salts such as sodium chloride (NaCl).

Covalent Bonds

• Formed when two atoms share one or more pairs of electrons.

• The strength of a covalent bond depends on the number of shared electron pairs.

• The electronegativity of an atom is its ability to attract shared electrons.

• Molecules are formed by covalent bonding between atoms.

Nonpolar Covalent Bonds

• Electrons are shared equally between atoms with similar electronegativities.

• Nonpolar molecules do not have charged regions.

• Example: O2 and H2 molecules.

Polar Covalent Bonds

• Electrons are shared unequally between atoms with different electronegativities.

• Results in partial positive and negative charges within the molecule.

• Example: Water (H2O) is a polar molecule, with oxygen being more electronegative than hydrogen.

Hydrogen Bonds

• Weak attractions between the partial positive charge of a hydrogen atom and the partial negative charge of another atom (often oxygen or nitrogen).

• Important for the structure of water, proteins, and nucleic acids.

Chemical Reactions

Types of Chemical Reactions

• Synthesis reactions: Two or more molecules combine to form a larger molecule. $A + B \rightarrow AB$

• Decomposition reactions: A molecule is broken down into smaller components. $AB \rightarrow A + B$

• Exchange reactions: Atoms are exchanged between molecules. $AB + CD \rightarrow AD + CB$

Reduction and Oxidation Reactions (Redox)

• Involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox reactions are always coupled.

• Cells use electron carriers (e.g., NAD+, FAD) to transfer electrons during metabolism.

Biological Macromolecules

Overview

• Macromolecules are large, carbon-containing molecules essential for life.

• Four major classes: carbohydrates, lipids, proteins, and nucleic acids.

• Each class is composed of smaller subunits called monomers.

Lipids

• Composed mainly of carbon and hydrogen; hydrophobic (water-insoluble).

• Major types include fats, phospholipids, waxes, and steroids.

• Fats: Consist of glycerol and fatty acids; used for energy storage.

• Phospholipids: Contain a phosphate group; major component of cell membranes.

• Waxes: Long-chain fatty acids linked to alcohols; protective coatings.

• Steroids: Four fused carbon rings; include cholesterol and hormones.

Lipid Bilayer Membrane Structure

• Phospholipids arrange in a bilayer, forming the fundamental structure of cell membranes.

• Hydrophilic (water-attracting) heads face outward; hydrophobic (water-repelling) tails face inward.

Carbohydrates

• Organic molecules composed of carbon, hydrogen, and oxygen (CH2O).

• Functions: energy storage, structural support, and cell recognition.

• Types:

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

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

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

Monosaccharides

• Basic building blocks of carbohydrates.

• Common examples: glucose, fructose, galactose.

• Can exist in linear or ring forms.

Disaccharides

• Formed by joining two monosaccharides via a glycosidic bond.

• Examples: sucrose (glucose + fructose), lactose (glucose + galactose).

Polysaccharides

• Polymers of many monosaccharide units.

• Serve as energy storage (e.g., starch, glycogen) or structural support (e.g., cellulose, chitin).

Cell Wall and Membrane Structure

Gram-Positive vs. Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present
Teichoic Acids	Present	Absent
Lipopolysaccharide (LPS)	Absent	Present

Additional info: Gram staining differentiates bacteria based on cell wall structure, which is important for diagnosis and treatment.

Proteins

Structure and Function

• Proteins are polymers of amino acids linked by peptide bonds.

• Functions include catalysis (enzymes), structural support, transport, signaling, and defense.

Amino Acids

• Monomers that make up proteins.

• Each amino acid has a central carbon, amino group, carboxyl group, hydrogen atom, and a variable side chain (R group).

• The properties of the R group determine the characteristics of each amino acid.

Protein Structure

• Primary structure: Sequence of amino acids.

• Secondary structure: Local folding (α-helix, β-sheet) stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape of a polypeptide.

• Quaternary structure: Association of multiple polypeptide chains.

Nucleic Acids

DNA and RNA

• DNA and RNA are polymers of nucleotides and serve as the genetic material of organisms and viruses.

• DNA stores genetic information; RNA is involved in protein synthesis and gene regulation.

Nucleotides

• Monomers of nucleic acids, each composed of:

  • A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

  • A phosphate group

  • A nitrogenous base (adenine, guanine, cytosine, thymine [DNA], or uracil [RNA])

Nucleic Acid Structure

• DNA is typically double-stranded, forming a double helix with complementary base pairing (A-T, C-G).

• RNA is usually single-stranded.

• Strands are antiparallel in DNA.