Chapter 5: The Structure and Function of Large Biological Molecules

Overview of Biological Macromolecules

Large biological molecules, also known as macromolecules, are essential for life and perform a wide variety of functions in living organisms. There are four major classes of biological molecules: carbohydrates, proteins, nucleic acids, and lipids. Three of these classes (carbohydrates, proteins, and nucleic acids) are polymers, which are long chains built from repeating units called monomers. Lipids, while crucial, are not true polymers.

• Carbohydrates: Serve as fuel and building material. Monomer: monosaccharide (e.g., glucose).

• Proteins: Perform a vast array of functions including catalysis, structure, and transport. Monomer: amino acid.

• Nucleic acids: Store and transmit genetic information. Monomer: nucleotide.

• Lipids: Diverse group, mainly hydrophobic, important for energy storage and membrane structure. Not polymers.

Polymers and Monomers

Polymerization and Dehydration Synthesis

Most macromolecules are polymers, constructed from monomers through a process called polymerization. The formation and breakdown of polymers involve specific chemical reactions:

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

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

Dehydration Reaction (Synthesis of Polymers)

• Monomers are joined together by dehydration reactions, in which a water molecule is removed to form a new covalent bond.

• This process results in the formation of a longer polymer.

Equation:

Hydrolysis (Breakdown of Polymers)

• Polymers are disassembled to monomers by hydrolysis, a reaction that adds a water molecule, breaking a bond.

• This process is essential for digestion and recycling of biological molecules.

Equation:

The Four Classes of Biological Molecules

Carbohydrates

Carbohydrates are sugars and polymers of sugars. They serve as energy sources and structural materials in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose, galactose) with the general formula .

• Disaccharides: Formed by joining two monosaccharides via a glycosidic linkage (e.g., maltose, sucrose).

• Polysaccharides: Long chains of monosaccharides; can be storage (e.g., starch in plants, glycogen in animals) or structural (e.g., cellulose in plants, chitin in arthropods and fungi).

Example: Starch is a storage polysaccharide in plants, composed of glucose monomers.

Proteins

Proteins are polymers of amino acids and are the most functionally diverse macromolecules in cells. They perform structural, enzymatic, transport, and regulatory roles.

• Amino acids: The monomers of proteins, each with a central carbon, amino group, carboxyl group, hydrogen atom, and variable R group.

• Polypeptide: A polymer of amino acids linked by peptide bonds.

• Protein: One or more polypeptides folded into a specific three-dimensional structure.

Example: Alcohol dehydrogenase is an enzyme (protein) that catalyzes the breakdown of alcohol in the liver.

Nucleic Acids

Nucleic acids store, transmit, and help express hereditary information. The two types are DNA and RNA.

• Nucleotide: The monomer of nucleic acids, consisting of a nitrogenous base, a pentose sugar, and a phosphate group.

• DNA (deoxyribonucleic acid): Stores genetic information; double-stranded helix.

• RNA (ribonucleic acid): Various roles in gene expression; usually single-stranded.

Example: DNA encodes the instructions for building proteins.

Lipids

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

• Fats (triacylglycerols): Composed of glycerol and three fatty acids; function as energy storage molecules.

• Phospholipids: Composed of glycerol, two fatty acids, and a phosphate group; major component of cell membranes, forming bilayers.

• Steroids: Characterized by a carbon skeleton with four fused rings (e.g., cholesterol, hormones).

Example: Phospholipids form the structural basis of biological membranes.

Summary Table: Biological Molecules

Additional info: The notes above are based on standard content from introductory biology textbooks, such as Campbell Biology, and are expanded for clarity and completeness.