Chapter 5: The Structure and Function of Large Biological Molecules

Overview of Biological Macromolecules

Large biological molecules, also known as macromolecules, are essential to the structure and function of living organisms. There are four major classes: carbohydrates, proteins, nucleic acids, and lipids. Three of these classes (carbohydrates, proteins, and nucleic acids) are polymers, composed of repeating subunits called monomers. Lipids, while large and important, are not true polymers.

• Carbohydrates: Serve as fuel and building material; monomer is the monosaccharide (e.g., glucose).

• Proteins: Perform a wide range of functions including catalysis, defense, transport, and structure; monomer is the amino acid.

• Nucleic Acids: Store and transmit hereditary information; monomer is the nucleotide.

• Lipids: Diverse group of hydrophobic molecules, including fats, phospholipids, and steroids; not composed of repeating monomer units.

Concept 5.1: Macromolecules are Polymers Built from Monomers

Most macromolecules are polymers, which are long molecules consisting of many similar or identical building blocks linked by covalent bonds. The repeating units are called monomers.

• Polymer: A long molecule made up of many monomers.

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

• Examples: Carbohydrates (monosaccharides), proteins (amino acids), nucleic acids (nucleotides).

Synthesis and Breakdown of Polymers

Polymers are assembled and disassembled by specific chemical reactions, often catalyzed by enzymes.

• Enzymes: Specialized macromolecules that speed up chemical reactions.

• Dehydration Reaction: Two monomers are covalently bonded to each other with the loss of a water molecule, forming a polymer.

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

Diagram Explanation:

• Dehydration: Short polymer + unlinked monomer → longer polymer + H2O (water is released).

• Hydrolysis: Polymer + H2O → shorter polymer + monomer (water is consumed).

Diversity of Polymers

Each cell contains thousands of different macromolecules. The diversity of macromolecules arises from the arrangement of a small set of monomers into unique polymers.

• Variation: Macromolecules vary among cells of an organism, more within a species, and even more between species.

• Combinatorial Diversity: A huge variety of polymers can be built from a small set of monomers by varying the sequence and length.

Summary Table: Classes of Biological Molecules

Key Terms and Definitions

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

• Polymer: A long molecule consisting of many similar or identical monomers linked together.

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

• Dehydration Reaction: A chemical reaction in which two molecules become covalently bonded to each other with the removal of a water molecule.

• Hydrolysis: A chemical reaction that breaks bonds between two molecules by the addition of water; functions in disassembly of polymers to monomers.

• Enzyme: A macromolecule serving as a catalyst, a chemical agent that increases the rate of a reaction without being consumed by the reaction.

Example: Synthesis and Breakdown of Polymers

• Synthesis (Dehydration): Formation of a disaccharide from two monosaccharides by removal of water.

• Breakdown (Hydrolysis): Digestion of starch into glucose monomers by addition of water.