Chapter 5: The Structure and Function of Macromolecules

Learning Outcomes

This chapter introduces the four major classes of biological macromolecules, their structures, and their functions in living organisms. Students will learn to:

• Identify the four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

• Describe the basic structures of these macromolecules and their monomeric units.

• Explain how polymers are synthesized by dehydration synthesis and broken down by hydrolysis.

• Relate the structure of each macromolecule to its primary biological function.

• Discuss how changes in macromolecular structure (e.g., protein denaturation) can affect cellular processes.

Introduction to Biology and Macromolecules

Overview of Biological Organization

Biology studies living organisms and their interactions with the environment. Key themes include order, energy processing, growth, reproduction, evolution, and response to the environment.

• Order: Living things are highly organized, from molecules to cells to organisms.

• Energy: Organisms obtain and use energy for growth and maintenance.

• Growth and Reproduction: Living things grow and reproduce, passing on genetic information.

• Evolution: Populations evolve over generations.

• Response to Environment: Organisms detect and respond to stimuli.

Macromolecules: Definition and Classes

What are Macromolecules?

Macromolecules are very large molecules composed of thousands of atoms. They are typically polymers, which are long chains of repeating subunits called monomers.

• Polymer: A large molecule made up of repeating monomer units.

• Monomer: The basic building block of a polymer (e.g., monosaccharides, amino acids, nucleotides).

The four major classes of macromolecules in biology are:

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Polymer Synthesis and Breakdown

Dehydration Synthesis and Hydrolysis

Polymers are assembled and disassembled by two key chemical reactions:

• Dehydration Synthesis (Condensation Reaction): Monomers are joined together by covalent bonds, releasing a molecule of water for each bond formed.

• Hydrolysis: Polymers are broken down into monomers by the addition of water, breaking the covalent bonds.

Equation for Dehydration Synthesis:

Equation for Hydrolysis:

Carbohydrates

Structure and Classification

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, usually in a ratio of 1:2:1 (CH2O). They serve as energy sources and structural materials.

• Monosaccharides: Simple sugars (e.g., glucose, fructose, galactose) that are the monomers of carbohydrates.

• Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose, lactose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Carbohydrates are classified by the number of sugar units:

• Monosaccharide: One sugar unit

• Disaccharide: Two sugar units

• Polysaccharide: Many sugar units

All monosaccharides have a carbonyl group (C=O) and multiple hydroxyl groups (–OH). They can be classified as aldoses (aldehyde sugars) or ketoses (ketone sugars), and by the number of carbons (triose, pentose, hexose, etc.).

Examples and Functions of Carbohydrates

• Starch: A storage polysaccharide in plants, composed of glucose monomers.

• Glycogen: A storage polysaccharide in animals, also composed of glucose.

• Cellulose: A structural polysaccharide in plant cell walls, composed of glucose but with different glycosidic linkages than starch or glycogen.

Table: Comparison of Major Polysaccharides

Example: Humans can digest starch and glycogen but not cellulose, due to the different glycosidic linkages.

Carbohydrate Digestion and Utilization

• Carbohydrates are broken down by hydrolysis into monosaccharides, which are used for energy.

• Some animals (e.g., cows) have symbiotic microbes that help digest cellulose.

Lipids

Structure and Types

Saturated vs. Unsaturated Fats

• Saturated Fats: No double bonds between carbon atoms; saturated with hydrogen. Solid at room temperature (e.g., butter).

• Unsaturated Fats: One or more double bonds; causes kinks in the fatty acid chain. Liquid at room temperature (e.g., oils).

Example: The presence of double bonds in unsaturated fats prevents tight packing, making them liquid at room temperature.

Phospholipids and Membranes

• Phospholipids have a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails.

• In water, phospholipids spontaneously form bilayers, which are the basis of all cell membranes.

Table: Structure of a Phospholipid

Example: The amphipathic nature of phospholipids (having both hydrophilic and hydrophobic regions) is essential for the formation of biological membranes.

Steroids

• Steroids are lipids with a structure of four fused carbon rings.

• Cholesterol is an important steroid in animal cell membranes and a precursor for other steroids, including hormones.

Summary

• Macromolecules are essential for life and include carbohydrates, lipids, proteins, and nucleic acids.

• Carbohydrates and lipids serve as energy sources and structural components.

• Polymers are synthesized by dehydration synthesis and broken down by hydrolysis.

• The structure of each macromolecule determines its function in the cell.