The Structure and Function of Large Biological Molecules: Carbohydrates

Chapter 5: The Structure and Function of Large Biological Molecules

Overview: The Molecules of Life

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These molecules, known as macromolecules, are typically formed by the joining of smaller units called monomers through covalent bonds. The structure of these molecules is closely linked to their function in biological systems.

Macromolecules: Polymers and Monomers

Macromolecules are often polymers, which are long chains of repeating monomer units. The process of building polymers from monomers is fundamental to the structure and function of biological molecules.

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers) joined by covalent bonds.

• Monomer: The basic unit or building block of a polymer.

Types of Reactions: Building and Breaking Macromolecules

Macromolecules are assembled and disassembled by specific chemical reactions:

• Anabolic reactions: Build organic molecules via condensation (dehydration) reactions, where water is removed to form a bond.

• Catabolic reactions: Break down organic molecules via hydrolysis reactions, where water is added to break a bond.

Carbohydrates

Monosaccharides: The Simple Sugars

Carbohydrates are composed of sugar monomers called monosaccharides. These are classified based on the location of their carbonyl group and the number of carbon atoms:

• Aldose: Carbonyl group at the end of the molecule.

• Ketose: Carbonyl group in the middle of the molecule.

• Common monosaccharides: Glucose, galactose, fructose, ribose, and deoxyribose.

Monosaccharide Structure and Function

Monosaccharides generally have the formula $(CH_2O)_n$. Glucose is the most important monosaccharide for cellular respiration. Galactose is found in milk, and fructose is found in fruit and honey. Ribose and deoxyribose are essential for the backbone of RNA and DNA.

Formation of Disaccharides and Polysaccharides

Monosaccharides are linked together by condensation (dehydration) reactions to form disaccharides and polysaccharides. The bond formed between two monosaccharides is called a glycosidic bond. Each bond formation removes a water molecule.

• Disaccharide: Formed by joining two monosaccharides (e.g., maltose, sucrose, lactose).

• Polysaccharide: Long chains of monosaccharide units (e.g., starch, glycogen, cellulose).

Polysaccharides: Structure and Function

Polysaccharides serve as storage and structural materials in cells. Their properties depend on the types of sugar monomers and the positions of glycosidic linkages.

• Starch: Storage polysaccharide in plants, composed of glucose monomers. Can be branched (amylopectin) or unbranched (amylose).

• Glycogen: Storage polysaccharide in animals, highly branched, stored in liver and muscles.

• Cellulose: Structural polysaccharide in plant cell walls, unbranched chains, high tensile strength, insoluble in water.

Examples and Applications

• Cellulose: Provides structural support in plants; indigestible by humans but digestible by some microbes.

• Glycogen: Provides quick energy storage in animals; does not affect blood glucose levels.

• Starch: Energy storage in plants; easily mobilized for energy.

Summary Table: Features of Macromolecules

The following table summarizes the main features of carbohydrates as macromolecules:

Macromolecule	Monomer	Polymer	Bond Type	Main Function
Carbohydrate	Monosaccharide	Polysaccharide	Glycosidic	Energy storage, structural support
Lipid	Fatty acid, glycerol	Triglyceride, phospholipid, steroid	Ester	Energy storage, membrane structure, signaling
Protein	Amino acid	Polypeptide	Peptide	Enzymatic, structural, transport, signaling
Nucleic Acid	Nucleotide	DNA, RNA	Phosphodiester	Information storage, transmission

Key Equations

Condensation (dehydration) reaction:

$\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

Hydrolysis reaction:

$\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Additional info:

Macromolecules are essential for life, and their structure determines their function. Understanding the chemical bonds and reactions involved in their synthesis and breakdown is fundamental to biology.

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