Molecules of Life: Biological Macromolecules and Carbohydrates

Molecules of Life

Introduction to Biological Macromolecules

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These molecules are essential for structure, function, and regulation of the body's tissues and organs.

• Macromolecules are large molecules, often polymers, formed by the joining of smaller units called monomers.

• Of the four classes, carbohydrates, proteins, and nucleic acids are true polymers.

• Lipids are not polymers but are grouped with macromolecules due to their large size and biological importance.

Polymers and Monomers

Polymers are long molecules consisting of many similar or identical building blocks linked by covalent bonds. The repeating units that serve as the building blocks of a polymer are called monomers.

• Polymerization is the process of linking monomers to form a polymer.

• Three of the four classes of biological macromolecules are polymers: carbohydrates, proteins, and nucleic acids.

Dehydration and Hydrolysis Reactions

Polymers are synthesized and broken down by two key types of reactions:

• Dehydration Reaction (Condensation Reaction): Monomers are joined together by covalent bonds through the removal of a water molecule. This process forms a longer polymer.

• Hydrolysis: Polymers are disassembled into monomers by the addition of a water molecule, breaking the covalent bond.

Example: The formation of a protein from amino acids involves repeated dehydration reactions, while digestion of proteins involves hydrolysis.

Carbohydrates

Overview and Functions

Carbohydrates serve as fuel and building material for cells. They include sugars and the polymers of sugars.

• The simplest carbohydrates are monosaccharides (single sugars).

• Carbohydrate polymers are called disaccharides (two sugars) and polysaccharides (many sugars).

Monosaccharides: Structure and Classification

Monosaccharides are the simplest carbohydrates and generally have molecular formulas that are multiples of CH2O. Glucose (C6H12O6) is the most common monosaccharide.

• Monosaccharides are classified by:

  • The location of the carbonyl group (as aldose [aldehyde sugar] or ketose [ketone sugar]).

  • The number of carbons in the carbon skeleton (triose: 3C, pentose: 5C, hexose: 6C).

Examples of Monosaccharides

	Aldose (Aldehyde Sugar)	Ketose (Ketone Sugar)
Trioses (3C)	Glyceraldehyde	Dihydroxyacetone
Pentoses (5C)	Ribose	Ribulose
Hexoses (6C)	Glucose, Galactose	Fructose

Linear and Ring Forms of Sugars

Although monosaccharides are often drawn as linear skeletons, in aqueous solutions many sugars form rings. This ring structure is more stable and is the predominant form in biological systems.

• For example, glucose forms a six-membered ring in solution.

Disaccharides

A disaccharide is formed when a dehydration reaction joins two monosaccharides. The covalent bond formed between the two sugar monomers is called a glycosidic bond.

• Example: Sucrose (table sugar) is a disaccharide formed from glucose and fructose.

Summary Table: Classification of Carbohydrates

Type	Definition	Example
Monosaccharide	Single sugar unit	Glucose, Fructose
Disaccharide	Two monosaccharides joined by a glycosidic bond	Sucrose, Maltose
Polysaccharide	Many monosaccharides joined in long chains	Starch, Cellulose, Glycogen

Additional info: Polysaccharides can serve as storage (e.g., starch in plants, glycogen in animals) or structural (e.g., cellulose in plant cell walls) molecules.