The Molecules of Life: Structure and Function of Biologically Relevant Molecules

The Molecules of Life

Introduction

This unit introduces the fundamental chemical principles underlying the structure and function of biologically relevant molecules. Understanding these molecules is essential for studying microbiology, as they form the basis of cellular structure, metabolism, and genetic information.

Learning Objectives

• Understand what an atom is.

• Describe covalent (non-polar and polar) bonds, ionic bonds, and hydrogen bonds.

• Understand the idea of pH (acid and alkalinity).

• Explain why water is the universal solvent of life.

• Discuss the composition of the major macromolecules (proteins, polysaccharides, nucleic acids, and lipids).

Section 2.1: Elements and Atoms

Atoms and Elements

Atoms are the smallest units of matter that retain the properties of an element. Elements are substances that cannot be broken down into simpler substances by chemical means.

• Element: A pure substance consisting of only one type of atom (e.g., carbon, hydrogen, oxygen).

• Atom: Consists of a nucleus (protons and neutrons) and electrons orbiting the nucleus.

• Atomic Number: Number of protons in the nucleus, unique to each element.

• Atomic Mass: Sum of protons and neutrons in the nucleus.

Example: Carbon has 6 protons, so its atomic number is 6.

Section 2.2: Chemical Bonds and Reactions

Types of Chemical Bonds

Chemical bonds are forces that hold atoms together in molecules. The main types are:

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

• Ionic Bonds: Electrons are transferred from one atom to another, creating charged ions that attract each other.

• Hydrogen Bonds: Weak attractions between a hydrogen atom (already covalently bonded to an electronegative atom) and another electronegative atom. Important in stabilizing the structures of proteins and nucleic acids.

Example: Water molecules are held together by hydrogen bonds, which give water its unique properties.

Water: The Universal Solvent

Water's polarity allows it to dissolve many substances, making it the universal solvent. This property is essential for biochemical reactions in cells.

• Polarity: Water has a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen Bonding: Responsible for water's high cohesion, surface tension, and ability to dissolve ionic and polar compounds.

pH: Acidity and Alkalinity

pH measures the concentration of hydrogen ions ($[H^+]$) in a solution.

• Acid: Substance that increases $[H^+]$ in solution (pH < 7).

• Base: Substance that decreases $[H^+]$ in solution (pH > 7).

• Neutral: pH = 7 (pure water).

Equation:

$\text{pH} = -\log_{10}[H^+]$

Section 2.4: Organic Molecules

Macromolecules

Organic molecules in cells are mainly macromolecules, which are large molecules composed of smaller subunits (monomers). The four major classes are:

• Proteins

• Carbohydrates

• Lipids

• Nucleic acids

Carbohydrates

• Serve as energy sources and structural components.

• Monosaccharides (simple sugars) are the building blocks (e.g., glucose, fructose).

• Polysaccharides (e.g., starch, glycogen, cellulose) are polymers of monosaccharides.

Lipids

• Hydrophobic molecules including fats, oils, phospholipids, and steroids.

• Fats are composed of glycerol and fatty acids; can be saturated or unsaturated.

• Phospholipids form the main structure of cell membranes.

• Steroids (e.g., cholesterol) have a characteristic four-ring structure.

Proteins

• Polymers of amino acids linked by peptide bonds.

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

• Protein structure has four levels: primary, secondary, tertiary, and quaternary.

Nucleic Acids

• Polymers of nucleotides; store and transmit genetic information.

• Two main types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Nucleotide structure: phosphate group, five-carbon sugar (deoxyribose or ribose), and nitrogenous base (adenine, guanine, cytosine, thymine [DNA], uracil [RNA]).

• Base pairing: A-T (DNA), A-U (RNA), G-C (both).

Table: Macromolecules, Subunits, and Functions

Name	Subunit	Function
Protein	Amino acid	Enzymes, structural support, transport, regulation
Carbohydrate	Monosaccharide	Energy storage, structural components
Lipid	Glycerol and fatty acids	Energy storage, membrane structure, signaling
Nucleic acid	Nucleotide	Genetic information storage and transfer

Concluding Comments

All living organisms are composed of the macromolecules proteins, carbohydrates, nucleic acids, and lipids. Understanding their structure and function is essential for further study in microbiology, as these molecules are involved in the physiology and metabolism of microorganisms.

Self-Test and Review

• Review the key concepts and complete the self-test questions to reinforce your understanding.

• Practice summarizing the subunits and functions of each macromolecule.

Additional info: The notes reference figures and textbook sections for further reading, which are not included here. Students are encouraged to consult their textbooks for detailed diagrams and additional examples.