Structure and Function of Biological Molecules

Chapter 5: Structure and Function of Biological Molecules

Introduction

This chapter explores the four major classes of biological macromolecules—carbohydrates, lipids, proteins, and nucleic acids—focusing on their structure, function, and the chemical principles underlying their formation and diversity. Understanding these molecules is fundamental to the study of biology, as they are essential to the structure and function of all living organisms.

Functional Groups in Biological Molecules

Key Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity. They play a crucial role in the structure and function of biological molecules.

• Hydroxyl group (–OH): Found in alcohols; increases solubility in water due to hydrogen bonding.

• Carbonyl group (C=O): Found in ketones and aldehydes; increases reactivity and polarity.

• Carboxyl group (–COOH): Found in carboxylic acids and amino acids; acts as an acid (can donate H+).

• Amino group (–NH2): Found in amino acids; acts as a base (can accept H+).

• Sulfhydryl group (–SH): Found in some amino acids; forms disulfide bonds that stabilize protein structure.

• Phosphate group (–OPO32–): Found in nucleotides (ATP, DNA, RNA); involved in energy transfer.

• Methyl group (–CH3): Affects gene expression and molecular recognition.

Example: The carboxyl and amino groups in amino acids allow them to link together via peptide bonds to form proteins.

Macromolecules and Monomers

Overview of Biological Macromolecules

Most biological macromolecules are polymers, long chains made by linking together smaller units called monomers. The exception is lipids, which are not true polymers.

Macromolecule	Monomer	Polymer
Carbohydrates	Monosaccharide	Polysaccharide
Proteins	Amino acid	Polypeptide
Nucleic acids	Nucleotide	Polynucleotide (DNA/RNA)
Lipids	Glycerol and fatty acids	Not true polymers

Additional info: Lipids are assembled from smaller molecules but do not form repeating chains like other macromolecules.

Polymerization: Dehydration and Hydrolysis

• Dehydration synthesis: Monomers are joined by covalent bonds through the removal of a water molecule. This process builds polymers.

• Hydrolysis: Polymers are broken down into monomers by the addition of water, breaking covalent bonds.

Equation for Dehydration Synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + H_2O$

Equation for Hydrolysis:

$\text{Polymer} + H_2O \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

Carbohydrates

Structure and Classification

Carbohydrates are sugars and their polymers. They serve as energy sources and structural materials.

• Monosaccharides: Simple sugars (e.g., glucose, fructose, ribose) with the general formula $C_nH_{2n}O_n$.

• Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., maltose, sucrose).

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

Example: Starch is a storage polysaccharide in plants, composed of α-glucose monomers. Cellulose is a structural polysaccharide in plant cell walls, composed of β-glucose monomers.

Functions of Carbohydrates

• Energy storage: Starch (plants), glycogen (animals)

• Structural support: Cellulose (plants), chitin (fungi and arthropods)

Lipids

Structure and Types

Lipids are a diverse group of hydrophobic molecules, including fats, phospholipids, and steroids. They are not true polymers.

• Fats (triglycerides): Composed of glycerol and three fatty acids joined by ester linkages.

• Phospholipids: Glycerol, two fatty acids, and a phosphate group; form cell membranes.

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

Saturated vs. Unsaturated Fats

• Saturated fats: No double bonds in fatty acid chains; solid at room temperature (e.g., animal fats).

• Unsaturated fats: One or more double bonds; liquid at room temperature (e.g., plant oils).

Functions of Lipids

• Energy storage: Long-term energy reserves

• Insulation and protection: Thermal and electrical insulation, cushioning organs

• Structural roles: Major component of cell membranes (phospholipids)

• Hormonal roles: Steroid hormones regulate physiological processes

Proteins

Structure and Levels of Organization

Proteins are polymers of amino acids linked by peptide bonds. They perform a vast array of functions in cells.

• Primary structure: Sequence of amino acids in a polypeptide chain

• Secondary structure: Local folding into α-helices and β-pleated sheets, stabilized by hydrogen bonds

• Tertiary structure: Overall 3D shape of a polypeptide, stabilized by interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges)

• Quaternary structure: Association of multiple polypeptide chains

Functions of Proteins

• Enzymes: Catalyze biochemical reactions

• Structural proteins: Provide support (e.g., collagen, keratin)

• Transport proteins: Move substances across membranes (e.g., hemoglobin)

• Defensive proteins: Protect against disease (e.g., antibodies)

• Hormonal proteins: Coordinate organismal responses (e.g., insulin)

• Receptor proteins: Receive signals from outside the cell

• Motor proteins: Enable movement (e.g., actin, myosin)

• Storage proteins: Store amino acids

Protein Denaturation

• Loss of native structure due to changes in pH, temperature, or salt concentration

• Denatured proteins lose their function

Nucleic Acids

Structure and Types

Nucleic acids store and transmit hereditary information. They are polymers of nucleotides.

• DNA (deoxyribonucleic acid): Double-stranded helix; stores genetic information

• RNA (ribonucleic acid): Single-stranded; involved in gene expression and protein synthesis

Nucleotide Structure

• Components: Phosphate group, five-carbon sugar (deoxyribose or ribose), nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA)

• Phosphodiester bonds: Link nucleotides to form polynucleotide chains

Base Pairing and Chargaff's Rules

• Base pairing: Adenine (A) pairs with Thymine (T) in DNA, or Uracil (U) in RNA; Guanine (G) pairs with Cytosine (C)

• Chargaff's rules: In DNA, the amount of A equals T, and G equals C; the ratio of purines to pyrimidines is constant within a species

Genomics and Proteomics

Modern Biological Inquiry

Genomics is the study of whole sets of genes and their interactions, while proteomics is the study of the full set of proteins expressed by a genome. These fields have transformed biological research, enabling large-scale comparisons and evolutionary studies.

• Applications: Human Genome Project, comparative genomics, evolutionary biology

• Bioinformatics: Use of computational tools to analyze DNA and protein sequences

Summary Table: Biological Macromolecules

Component	Examples	Functions
Carbohydrates	Glucose, starch, cellulose	Energy storage, structural support
Lipids	Triglycerides, phospholipids, steroids	Energy storage, insulation, membrane structure, hormones
Proteins	Enzymes, antibodies, hemoglobin	Catalysis, defense, transport, structure, signaling
Nucleic Acids	DNA, RNA	Store and transmit genetic information