The Structure and Function of Large Biological Molecules: Polymers and Carbohydrates

Chapter 5: The Structure and Function of Large Biological Molecules

Introduction

Large biological molecules, also known as macromolecules, are essential for the structure and function of living organisms. This chapter focuses on the four major classes of biological macromolecules: carbohydrates, proteins, nucleic acids, and lipids, with an emphasis on polymers and carbohydrates.

Macromolecules and Polymers

Macromolecules: Definition and Importance

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

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

• The repeating units that serve as building blocks are called monomers.

• Major biological polymers include carbohydrates, proteins, and nucleic acids.

The Synthesis and Breakdown of Polymers

• Enzymes are specialized macromolecules that speed up chemical reactions, including those that make or break down polymers.

• A dehydration reaction occurs when two monomers bond together through the loss of a water molecule, forming a polymer.

• Hydrolysis is the process by which polymers are disassembled to monomers, essentially the reverse of dehydration.

Equation (Dehydration Reaction):

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

Equation (Hydrolysis):

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

The Diversity of Polymers

• A cell contains thousands of different macromolecules.

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

• A huge variety of polymers can be built from a small set of monomers.

Carbohydrates: Fuel and Building Material

Overview of Carbohydrates

• Carbohydrates include sugars and polymers of sugars.

• The simplest carbohydrates are monosaccharides (simple sugars).

• Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks.

Monosaccharides

• Monosaccharides have molecular formulas that are usually multiples of CH2O.

• Glucose ($C_6H_{12}O_6$) is the most common monosaccharide.

• Monosaccharides are classified by:

  • The location of the carbonyl group (as aldose or ketose).

  • The number of carbons in the carbon skeleton.

• In aqueous solutions, many sugars form rings rather than linear structures.

• Monosaccharides serve as major fuel for cells and as raw material for building molecules.

Table: Classification of Monosaccharides

Type	Carbonyl Position	Example
Aldose	Carbonyl group at end	Glucose
Ketose	Carbonyl group within	Fructose

Disaccharides

• A disaccharide is formed when a dehydration reaction joins two monosaccharides.

• The covalent bond between two monosaccharides is called a glycosidic linkage.

Equation (Formation of Disaccharide):

$\text{Glucose} + \text{Fructose} \xrightarrow{\text{Dehydration}} \text{Sucrose} + H_2O$

Polysaccharides

• Polysaccharides are polymers of sugars and have storage and structural roles.

• The architecture and function of a polysaccharide are determined by its sugar monomers and the positions of its glycosidic linkages.

Storage Polysaccharides

• Starch is a storage polysaccharide of plants, consisting of glucose monomers.

• Plants store surplus starch as granules within chloroplasts and other plastids.

• The simplest form of starch is amylose.

• Glycogen is a storage polysaccharide in animals, stored mainly in liver and muscle cells.

• Hydrolysis of glycogen in these cells releases glucose when the demand for sugar increases.

Table: Comparison of Storage Polysaccharides

Polysaccharide	Organism	Structure	Function
Starch	Plants	Unbranched (amylose) and branched (amylopectin)	Energy storage
Glycogen	Animals	Extensively branched	Energy storage

Structural Polysaccharides

• Cellulose is a major component of the tough wall of plant cells.

• Like starch, cellulose is a polymer of glucose, but the glycosidic linkages differ.

• The difference is based on two ring forms for glucose: alpha (α) and beta (β).

• Starch (α configuration) is largely helical, while cellulose molecules (β configuration) are straight and unbranched.

• Some hydroxyl groups on the monomers of cellulose can hydrogen-bond with hydroxyls of parallel cellulose molecules, contributing to its structural strength.

Table: Comparison of Starch and Cellulose

Polysaccharide	Monomer	Glycosidic Linkage	Structure	Function
Starch	Glucose (α)	α 1-4	Helical	Storage
Cellulose	Glucose (β)	β 1-4	Straight, unbranched	Structural (plant cell wall)

Summary

• Macromolecules are essential for life and are built from monomers via dehydration reactions.

• Carbohydrates serve as both fuel and building material, with monosaccharides, disaccharides, and polysaccharides playing key roles.

• Polysaccharides have important storage and structural functions, with starch and glycogen storing energy and cellulose providing structural support in plants.

Additional info: The notes above expand on the brief points in the slides, providing definitions, examples, and comparisons for clarity and completeness.