The Structure and Function of Large Biological Molecules

Chapter Five: The Structure and Function of Large Biological Molecules

Introduction

This chapter explores the four major classes of large biological molecules essential to life: carbohydrates, lipids, proteins, and nucleic acids. These molecules, often called macromolecules, play critical roles in the structure and function of cells.

Macromolecules

Definition and Importance

• Macromolecules are large molecules composed of thousands of covalently connected atoms.

• Within cells, small organic molecules are joined together to form these larger molecules.

• The structure of a macromolecule is closely related to its function.

Polymers and Monomers

Basic Concepts

• A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• These building blocks are called monomers.

• Examples: A train (polymer) is made up of individual cars (monomers).

Diversity of Polymers

• Each cell contains thousands of different kinds of macromolecules.

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

• An immense variety of polymers can be built from a small set of monomers.

Synthesis and Breakdown of Polymers

Synthesis of Polymers

• Polymers are synthesized by dehydration reactions (also called condensation reactions), where two monomers bond together through the loss of a water molecule.

• Enzymes are macromolecules that speed up dehydration reactions.

Equation: $\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Enzyme}} \text{Polymer} + H_2O$

Breakdown of Polymers

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

• Example: Digestion in the human body involves hydrolysis of polymers.

Equation: $\text{Polymer} + H_2O \xrightarrow{\text{Enzyme}} \text{Monomer}_1 + \text{Monomer}_2$

Classes of Large Biological Molecules

• Carbohydrates (polymers)

• Proteins (polymers)

• Nucleic acids (polymers)

• Lipids (not polymers, but large and diverse biological molecules)

Carbohydrates

Overview

• Include sugars and polymers of sugars.

• Most names for sugars end in -ose.

• Carbohydrate polymers are called polysaccharides, made of monosaccharide monomers.

Monosaccharides

• The simplest carbohydrates; single sugars.

• General formula: $(CH_2O)_n$ (e.g., glucose: $C_6H_{12}O_6$)

• Contain a carbonyl group ($C=O$) and multiple hydroxyl groups ($-OH$).

Classification of Monosaccharides

• By number of carbons: trioses (3C), pentoses (5C), hexoses (6C).

• By location of carbonyl group:

  • Aldose: carbonyl at end (e.g., glucose)

  • Ketose: carbonyl within skeleton (e.g., fructose)

Linear and Ring Forms

• Monosaccharides often form rings in aqueous solutions, which are more stable under physiological conditions.

• Example: Glucose forms a six-membered ring; fructose forms a five-membered ring.

Functions of Monosaccharides

• Major fuel for cells (energy source).

• Raw material for building other molecules.

• Serve as monomers for disaccharides and polysaccharides.

Disaccharides

• Formed by joining two monosaccharides via a dehydration reaction.

• The covalent bond formed is called a glycosidic linkage.

• Examples:

  • Maltose: glucose + glucose

  • Sucrose: glucose + fructose

  • Lactose: galactose + glucose

Facts about Disaccharides

• Must be broken down into monosaccharides to be used for energy.

• Lactose intolerance results from the inability to digest lactose due to lack of the enzyme lactase.

Polysaccharides

• Polymers of sugars with various structures and functions.

• Structure and function determined by sugar monomers and glycosidic linkages.

Functions of Polysaccharides

• Storage polysaccharides: hydrolyzed as needed to provide monosaccharides for cells (e.g., starch in plants, glycogen in animals).

• Structural polysaccharides: building material for structures that protect cells or organisms (e.g., cellulose in plants, chitin in fungi and arthropods).

Storage Polysaccharides

• Starch: storage in plants, polymer of glucose, stored in plastids.

• Glycogen: storage in animals, polymer of glucose, stored in liver and muscle cells.

• Starch types:

  • Amylose: unbranched

  • Amylopectin: branched (1-6 linkages at branch points)

• Glycogen: extensively branched (1-6 linkages at branch points)

Structural Polysaccharides

• Cellulose: major component of plant cell walls, polymer of glucose with β(1-4) linkages, straight and unbranched, forms microfibrils.

• Chitin: found in exoskeletons of arthropods and cell walls of fungi, similar to cellulose but with nitrogen-containing side groups.

Comparison Table: Storage vs. Structural Polysaccharides

Type	Example	Function	Monomer	Linkage
Storage	Starch (plants), Glycogen (animals)	Energy storage	Glucose	α(1-4), α(1-6) (branched)
Structural	Cellulose (plants), Chitin (fungi/arthropods)	Structural support	Glucose (cellulose), N-acetylglucosamine (chitin)	β(1-4) (cellulose), β(1-4) with N-acetyl group (chitin)

Additional info: The notes above are based on the provided slides and expanded with standard academic context for clarity and completeness.