The Structure and Function of Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Molecules of Life

All living organisms are composed of four major classes of large biological molecules, each with distinct structures and functions essential for life.

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Macromolecules are large, complex molecules. Their unique properties arise from the specific arrangement of their atoms.

Overview of Biological Macromolecules

Three of the four classes—carbohydrates, proteins, and nucleic acids—are macromolecules that are typically polymers, meaning they are made up of many similar or identical building blocks called monomers. Lipids, while large and important, are not true polymers.

• Polymer: A long molecule consisting of many similar building blocks (monomers).

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

Examples:

• Carbohydrates: Starch (polymer of glucose)

• Proteins: Polymers of amino acids (e.g., alcohol dehydrogenase)

• Nucleic acids: DNA (polymer of nucleotides)

• Lipids: Phospholipids (not polymers)

Carbohydrates

Carbohydrates are sugars and polymers of sugars, serving as fuel and building material for cells.

• Monosaccharides: Simple sugars (e.g., glucose)

• Disaccharides: Double sugars (two monosaccharides joined)

• Polysaccharides: Polymers of many sugar building blocks (e.g., starch, cellulose)

Monosaccharides generally have molecular formulas that are multiples of CH2O. For example, glucose is C6H12O6.

Lipids

Lipids are a diverse group of hydrophobic molecules, including fats, phospholipids, and steroids. Unlike other macromolecules, lipids are not polymers.

• Fats: Constructed from glycerol and fatty acids

• Phospholipids: Major component of cell membranes

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

Lipids are hydrophobic due to their nonpolar hydrocarbon chains.

Proteins

Proteins are polymers made from amino acid monomers. They perform a wide range of functions in cells, including catalysis, defense, transport, storage, and structural support.

• Amino acids: Organic molecules with amino and carboxyl groups

• Polypeptide: Polymer of amino acids

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

Nucleic Acids

Nucleic acids store, transmit, and help express hereditary information. DNA and RNA are polymers of nucleotides.

• Nucleotide: Composed of a nitrogenous base, a pentose sugar, and one or more phosphate groups

• DNA: Contains deoxyribose sugar

• RNA: Contains ribose sugar

In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose.

Macromolecule	Monomer	Polymer	Example
Carbohydrate	Monosaccharide	Polysaccharide	Starch, Cellulose
Protein	Amino acid	Polypeptide	Alcohol dehydrogenase
Nucleic acid	Nucleotide	Polynucleotide	DNA, RNA
Lipid	None (not a true polymer)	None	Phospholipid

Key Terms and Definitions

• Macromolecule: A very large molecule, such as a protein, nucleic acid, or polysaccharide.

• 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.

• Deoxyribose: The pentose sugar found in DNA.

• Ribose: The pentose sugar found in RNA.

Example: Nucleoside Components

• Deoxyribose (in DNA): C5H10O4

• Ribose (in RNA): C5H10O5

Formulas

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

• Glucose: $C_6H_{12}O_6$

Summary Table: Biological Macromolecules

Class	Monomer	Polymer	Function
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structural support
Proteins	Amino acid	Polypeptide	Catalysis, transport, structure, defense
Nucleic acids	Nucleotide	Polynucleotide	Genetic information storage and transfer
Lipids	None	None	Energy storage, membrane structure, signaling

Additional info: These notes are based on introductory slides for Chapter 5 of Campbell Biology, focusing on the structure and function of large biological molecules. The content is foundational for understanding biochemistry in general biology courses.