The Structure and Function of Large Biological Molecules (Macromolecules)

Macromolecules: The Building Blocks of Life

Introduction to Macromolecules

Macromolecules are large, complex molecules composed of smaller subunits called monomers. These molecules perform a wide variety of important cellular functions and are fundamental to the structure and function of all living organisms.

• Definition: Macromolecules are polymers built from repeating monomer units, (except for lipids, which are not true polymers).

• Main Categories: The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

• Importance: These molecules are responsible for energy storage, structural support, catalysis of biochemical reactions, and the storage and transmission of genetic information.

Overview of the Four Major Macromolecules

Type	Monomer (Subunit)	Main Functions	Examples
Carbohydrates	Monosaccharides	Energy storage, structural support	Starch, cellulose, glucose
Lipids	Fatty acids, glycerol	Energy storage, membrane structure, signaling	Fats, oils, phospholipids, steroids
Proteins	Amino acids	Catalysis, structure, transport, signaling	Enzymes, keratin, hemoglobin
Nucleic Acids	Nucleotides	Genetic information storage and transfer	DNA, RNA

Polymerization: Synthesis and Breakdown

Most macromolecules (except lipids) are formed by the polymerization of monomers through dehydration reactions and are broken down by hydrolysis.

• Dehydration Reaction: Monomers are joined together by covalent bonds with the removal of a water molecule.

• Hydrolysis: Polymers are broken down into monomers by the addition of water, reversing the dehydration reaction.

• Diversity: The sequence and arrangement of monomers result in a vast diversity of polymers with different properties and functions.

Carbohydrates

Structure and Classification

Carbohydrates are sugars and polymers of sugars, serving as both energy sources and structural materials in cells.

• Monosaccharides: The simplest carbohydrates (e.g., glucose, C6H12O6), usually with a formula of (CH2O)n.

• Disaccharides: Formed by covalent linkage (glycosidic bond) of two monosaccharides (e.g., sucrose, lactose, maltose).

• Polysaccharides: Long chains of monosaccharides; can be storage (starch, glycogen) or structural (cellulose, chitin).

Monosaccharide Classification

• By carbonyl group location: Aldose (aldehyde group at end) or Ketose (ketone group within carbon chain).

• By number of carbons: Trioses (3C), pentoses (5C), hexoses (6C), etc.

• In aqueous solutions, monosaccharides often form ring structures.

Polysaccharide Examples and Functions

• Starch: Storage polysaccharide in plants, composed of α-glucose monomers; stored in plastids.

• Glycogen: Storage polysaccharide in animals, highly branched, stored in liver and muscle cells.

• Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose monomers; forms straight, rigid fibers.

Note: The difference between α- and β-glucose leads to different properties and biological roles for starch and cellulose.

Lipids

Structure and Types

Lipids are a diverse group of hydrophobic molecules, not true polymers, and are characterized by their insolubility in water.

• Fats (Triglycerides): Composed of glycerol and three fatty acids; function in energy storage, insulation, and protection.

• Phospholipids: Glycerol backbone, two fatty acids, and a phosphate group; major component of cell membranes, amphipathic (hydrophilic head, hydrophobic tails).

• Steroids: Four fused carbon rings; include cholesterol (membrane structure) and steroid hormones (signaling molecules).

Saturated vs. Unsaturated Fatty Acids

• Saturated: No double bonds, straight chains, solid at room temperature (e.g., animal fats).

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

• Trans Fats: Unsaturated fats with trans double bonds, produced industrially, associated with health risks.

• Essential Fatty Acids: Not synthesized by the body; must be obtained from diet (e.g., omega-3 fatty acids).

Proteins

Structure and Function

Proteins are polymers of amino acids, folded into specific three-dimensional shapes that determine their function. They are the most diverse and abundant macromolecules in cells.

• Amino Acids: Organic molecules with a central carbon, amino group, carboxyl group, hydrogen atom, and variable R group (side chain).

• Polypeptides: Linear chains of amino acids linked by peptide bonds; one or more polypeptides form a protein.

• Levels of Structure:

  • Primary: Sequence of amino acids.

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

  • Tertiary: Overall 3D shape due to interactions among R groups (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions).

  • Quaternary: Association of multiple polypeptide chains (e.g., hemoglobin).

• Functional Diversity: Enzymes, structural proteins, contractile proteins, regulatory proteins, transport proteins, signaling proteins, receptor proteins, defensive proteins, storage proteins.

Example: Sickle-Cell Disease

• A single amino acid substitution in hemoglobin leads to abnormal protein structure and function, causing sickle-shaped red blood cells and disease symptoms.

Nucleic Acids

Structure and Function

Nucleic acids store and transmit hereditary information. The two main types are DNA and RNA.

• Monomer: Nucleotide, composed of a nitrogenous base, a pentose sugar, and a phosphate group.

• DNA (Deoxyribonucleic Acid): Double-stranded, contains deoxyribose sugar, bases A, T, C, G; stores genetic information.

• RNA (Ribonucleic Acid): Single-stranded, contains ribose sugar, bases A, U, C, G; involved in protein synthesis and gene regulation.

• Phosphodiester Bond: Covalent bond linking nucleotides between the 3' hydroxyl and 5' phosphate groups, forming a sugar-phosphate backbone.

• Base Pairing: In DNA, A pairs with T, and G pairs with C via hydrogen bonds; in RNA, U replaces T.

• Antiparallel Strands: DNA strands run in opposite 5' to 3' directions.

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Strands	Double-stranded	Single-stranded (usually)
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Function	Genetic information storage	Protein synthesis, gene regulation

Key Equations and Concepts

• General formula for monosaccharides: $C_nH_{2n}O_n$

• Dehydration synthesis (example for maltose): $C_6H_{12}O_6 + C_6H_{12}O_6 \rightarrow C_{12}H_{22}O_{11} + H_2O$

• Phosphodiester bond formation: $\text{Nucleotide}_1 + \text{Nucleotide}_2 \rightarrow \text{Dinucleotide} + H_2O$

Additional info: This summary is based on introductory college-level biology content and is suitable for exam preparation in General Biology courses.