Chapter 5: Carbohydrates – Structure, Classification, and Biological Roles

Chapter 5: Carbohydrates

Overview of Biological Macromolecules

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules play essential roles in cell structure, function, and energy storage.

• Carbohydrates: Serve as energy sources and structural components.

• Lipids: Important for membrane structure and energy storage.

• Proteins: Function as enzymes, structural elements, and signaling molecules.

• Nucleic acids: Store and transmit genetic information.

Macromolecules are polymers built from monomers by condensation (dehydration) reactions and broken down by hydrolysis.

Carbohydrates

Definition and General Properties

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the empirical formula $(CH_2O)_n$ where n is the number of carbon atoms.

• Characterized by the presence of carbonyl (C=O) and multiple hydroxyl (–OH) groups.

• Serve as important sources of energy, structural materials, and play roles in cell recognition.

Example: Glucose, with the formula $C_6H_{12}O_6$, is the most common monosaccharide.

Classification of Carbohydrates

• Monosaccharides: Simple sugars (e.g., glucose, fructose) that cannot be hydrolyzed into smaller carbohydrates.

• Oligosaccharides: Short chains of monosaccharide units (e.g., sucrose).

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

Monosaccharides: Structure and Classification

Structural Variability

Monosaccharides vary in several key ways, which determine their identity and function:

1. Number of Carbon Atoms: Monosaccharides are classified by the length of their carbon skeleton (e.g., triose (3C), pentose (5C), hexose (6C)).

2. Location of the Carbonyl Group:

   • Aldose: Carbonyl group at the end of the carbon chain (e.g., glucose).

   • Ketose: Carbonyl group within the carbon chain (e.g., fructose).

3. Spatial Arrangement of Atoms: Isomers differ in the arrangement of hydroxyl groups around asymmetric carbons (e.g., glucose vs. galactose).

4. Linear vs. Ring Forms: In aqueous solutions, monosaccharides often form ring structures, which can have different functional properties.

Each monosaccharide has a unique structure and function due to these variations.

Table: Classification of Monosaccharides

Criterion	Example	Description
Number of Carbons	Triose, Pentose, Hexose	3, 5, or 6 carbon atoms in backbone
Carbonyl Position	Aldose (Glucose), Ketose (Fructose)	End or middle of chain
Spatial Arrangement	Glucose vs. Galactose	Different arrangement of –OH groups
Ring vs. Linear	α-Glucose, β-Glucose	Ring closure creates isomers

The Synthesis and Breakdown of Carbohydrates

Polymerization: Formation of Glycosidic Linkages

Monosaccharides are joined together by dehydration (condensation) reactions, which remove a water molecule to form a covalent bond known as a glycosidic linkage.

• Glycosidic linkage: Covalent bond formed between two monosaccharides by a dehydration reaction.

• Example: Formation of maltose from two glucose molecules.

$\text{Monosaccharide}_1 + \text{Monosaccharide}_2 \xrightarrow{\text{dehydration}} \text{Disaccharide} + H_2O$

Hydrolysis: Breakdown of Polymers

Polysaccharides and oligosaccharides can be broken down into their monomer units by hydrolysis, a reaction that adds water to break glycosidic linkages.

• Hydrolysis is essential for digestion and cellular metabolism.

Biological Functions of Carbohydrates

Energy Storage

• Carbohydrates such as starch (in plants) and glycogen (in animals) serve as major energy storage molecules.

• They are easily hydrolyzed to release glucose for cellular respiration.

Structural Roles

• Cellulose: Major component of plant cell walls, providing structural support.

• Chitin: Structural polysaccharide in fungal cell walls and arthropod exoskeletons.

Cell Identity and Recognition

• Carbohydrates on cell surfaces (often as part of glycoproteins and glycolipids) play key roles in cell-cell recognition and signaling.

Summary Table: Major Polysaccharides and Their Functions

Polysaccharide	Organism	Function	Monomer
Starch	Plants	Energy storage	Glucose
Glycogen	Animals	Energy storage	Glucose
Cellulose	Plants	Structural support (cell wall)	Glucose
Chitin	Fungi, Arthropods	Structural support (cell wall, exoskeleton)	N-acetylglucosamine

Key Terms and Concepts

• Monosaccharide: The simplest carbohydrate, acting as a monomer for disaccharides and polysaccharides.

• Glycosidic linkage: Covalent bond joining two carbohydrate molecules.

• Dehydration reaction: Chemical reaction that joins two molecules by removing water.

• Hydrolysis: Chemical reaction that breaks bonds by adding water.

• Isomer: Molecules with the same molecular formula but different structures.

Additional info: The notes also reference the importance of comparing the structure and function of carbohydrates with other macromolecules (proteins, nucleic acids, lipids), and the general principle that the structure of a biological molecule determines its function.