Biological Molecules: Structure and Function in Living Systems

Biological Molecules

Introduction to Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life. They are composed of thousands of covalently connected atoms and are responsible for the structure and function of cells. The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

• Macromolecule: A large molecule formed by the joining of smaller molecules, usually by a condensation (dehydration) reaction.

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The subunit that serves as the building block of a polymer.

Main classes of biological macromolecules:

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Polymer Synthesis and Breakdown

Dehydration and Hydrolysis Reactions

Polymers are assembled and disassembled by two main types of reactions: dehydration synthesis and hydrolysis.

• Dehydration reaction (condensation): Two monomers are covalently bonded to each other with the loss of a water molecule. This reaction builds polymers from monomers.

• Hydrolysis reaction: Polymers are broken down into monomers by the addition of a water molecule, which breaks the covalent bond.

Example: The formation of a disaccharide from two monosaccharides via dehydration, and its breakdown via hydrolysis.

Equations:

• Dehydration: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

• Hydrolysis: $\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Macromolecule #1: Carbohydrates

Monosaccharides and Disaccharides

Carbohydrates are sugars and their polymers. The simplest carbohydrates are monosaccharides, or simple sugars. Disaccharides consist of two monosaccharides joined by a glycosidic linkage.

• Monosaccharide: The simplest carbohydrate, active alone or serving as a monomer for disaccharides and polysaccharides. Most have formulas that are multiples of $CH_2O$.

• Hexoses: Six-carbon sugars with the formula $C_6H_{12}O_6$ (e.g., glucose, galactose, fructose).

Examples of hexoses:

Monosaccharide	Structure
Glucose	Linear and ring forms; main energy source for cells
Galactose	Isomer of glucose; differs in arrangement of atoms
Fructose	Structural isomer of glucose; found in fruits

Structural Forms of Monosaccharides

Monosaccharides can exist in linear or ring forms. In aqueous solutions, they often form rings, which are more stable.

• Linear form: Open-chain structure with carbonyl and hydroxyl groups.

• Ring form: Formed when the carbonyl group reacts with a hydroxyl group on the same molecule.

Example: Glucose can cyclize to form a six-membered ring (pyranose form).

Polysaccharides

Polysaccharides are macromolecules formed by the polymerization of monosaccharides. They serve as storage and structural materials.

• Starch: Main storage polysaccharide in plants; composed of glucose monomers joined by α-1,4 glycosidic bonds.

• Glycogen: Main storage polysaccharide in animals; similar to starch but more highly branched.

• Cellulose: Major component of plant cell walls; composed of glucose monomers joined by β-1,4 glycosidic bonds.

Polysaccharide	Function	Monomer Linkage
Starch	Energy storage in plants	α-1,4 glucose
Glycogen	Energy storage in animals	α-1,4 and α-1,6 glucose (branched)
Cellulose	Structural support in plants	β-1,4 glucose

Additional info: Humans can digest starch and glycogen but not cellulose due to the difference in glycosidic linkages.