Chemical Principles in Microbiology: Structure, Bonds, and Biomolecules

Chapter 2: Chemical Principles

Atomic Structure and Physical Properties

The structure of an atom determines the physical and chemical properties of elements, which are fundamental to understanding microbial life.

• Atom: The smallest unit of an element, composed of protons, neutrons, and electrons.

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles found in the nucleus.

• Electrons: Negatively charged particles orbiting the nucleus.

• Atomic Number: Number of protons in the nucleus; determines the element.

• Mass Number: Sum of protons and neutrons.

• Physical Properties: Determined by atomic structure, including reactivity, state, and bonding behavior.

• Example: Carbon (atomic number 6) forms four covalent bonds, making it essential for organic molecules.

Chemical Bonds and Definitions

Chemical bonds are forces that hold atoms together, forming molecules essential for life.

• Ionic Bond: Formed when electrons are transferred from one atom to another, resulting in charged ions.

• Covalent Bond: Formed when atoms share electrons.

• Hydrogen Bond: Weak attraction between a hydrogen atom and an electronegative atom (e.g., oxygen or nitrogen).

• Molecular Weight: The sum of the atomic weights of all atoms in a molecule.

• Mole: A unit representing $6.022 \times 10^{23}$ molecules or atoms (Avogadro's number).

• Example: Water molecules are held together by hydrogen bonds, giving water unique properties.

Types of Chemical Reactions

Chemical reactions are processes that change the composition of substances. Three basic types are important in biological systems.

• Synthesis Reaction: Two or more substances combine to form a new compound. $A + B \rightarrow AB$

• Decomposition Reaction: A compound breaks down into two or more simpler substances. $AB \rightarrow A + B$

• Exchange Reaction: Parts of two molecules are exchanged to form new compounds. $AB + CD \rightarrow AD + CB$

• Example: Hydrolysis (decomposition) and dehydration synthesis (synthesis) are common in metabolism.

Properties of Water Important to Living Systems

Water is vital for life due to its unique chemical and physical properties.

• Polarity: Water molecules are polar, allowing them to dissolve many substances.

• Hydrogen Bonding: Leads to high surface tension and cohesion.

• Temperature Regulation: High specific heat helps maintain stable temperatures.

• Solvent Properties: Water is a universal solvent, facilitating biochemical reactions.

• Example: Water's polarity enables transport of nutrients and waste in cells.

Acids, Bases, Salts, and pH

Acids, bases, and salts are important for maintaining cellular homeostasis. pH measures the concentration of hydrogen ions.

• Acid: Substance that releases hydrogen ions ($H^+$) in solution.

• Base: Substance that releases hydroxide ions ($OH^-$) or accepts $H^+$.

• Salt: Compound formed from the reaction of an acid and a base.

• pH: Measure of hydrogen ion concentration; calculated as $pH = -\log_{10}[H^+]$.

• Example: Most bacteria grow best at neutral pH (6.5–7.5).

Organic vs. Inorganic Compounds

Compounds are classified based on the presence of carbon and their structure.

• Organic Compounds: Contain carbon-hydrogen bonds; include carbohydrates, lipids, proteins, and nucleic acids.

• Inorganic Compounds: Do not contain carbon-hydrogen bonds; include water, salts, acids, and bases.

• Example: Glucose (organic) vs. sodium chloride (inorganic).

Functional Groups

Functional groups are specific groups of atoms within molecules that determine their chemical properties and reactions.

• Hydroxyl (-OH): Found in alcohols.

• Carboxyl (-COOH): Found in organic acids.

• Amino (-NH2): Found in amino acids.

• Phosphate (-PO4): Found in nucleic acids and ATP.

• Example: The amino group in amino acids allows them to form proteins.

Carbohydrates: Building Blocks

Carbohydrates are essential energy sources and structural components in cells.

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

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

• Polysaccharides: Many monosaccharides linked (e.g., starch, cellulose).

• Example: Glucose is a primary energy source for microbes.

Lipids: Types and Functions

Lipids are hydrophobic molecules important for energy storage and membrane structure.

• Simple Lipids: Composed of fatty acids and glycerol (e.g., triglycerides).

• Complex Lipids: Contain additional elements like phosphorus (e.g., phospholipids).

• Steroids: Four fused carbon rings (e.g., cholesterol).

• Example: Phospholipids form the basic structure of microbial cell membranes.

Proteins: Structure and Building Blocks

Proteins are polymers of amino acids, performing diverse functions in cells.

• Amino Acids: Building blocks of proteins; contain amino, carboxyl, and side groups.

• Primary Structure: Sequence of amino acids.

• Secondary Structure: Folding into alpha helices or beta sheets.

• Tertiary Structure: Three-dimensional shape.

• Quaternary Structure: Association of multiple polypeptides.

• Example: Enzymes are proteins that catalyze biochemical reactions.

Nucleic Acids: Building Blocks

Nucleic acids store and transmit genetic information in cells.

• Nucleotides: Building blocks; consist of a sugar, phosphate group, and nitrogenous base.

• DNA: Deoxyribonucleic acid; stores genetic information.

• RNA: Ribonucleic acid; involved in protein synthesis.

• Example: Escherichia coli uses DNA to encode enzymes for metabolism.

ATP: Role in Cellular Activities

ATP (adenosine triphosphate) is the primary energy carrier in cells.

• Structure: Adenine, ribose, and three phosphate groups.

• Function: Stores and releases energy for cellular processes.

• Energy Release: Hydrolysis of ATP to ADP releases energy. $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

• Example: Microbial cells use ATP for movement, biosynthesis, and transport.