Carbohydrates: Structure, Classification, and Functions

Carbohydrates

Overview and Learning Outcomes

Carbohydrates are essential biological molecules composed of sugars and their polymers. They play critical roles in energy storage, structural support, and cellular recognition. Understanding their structure and function is fundamental in general biology.

• Identify if a molecule is a carbohydrate

• Compare and contrast linkages between monosaccharides and other macromolecules

• Compare and contrast the structure and function of starch, glycogen, cellulose, chitin, and peptidoglycan

• Describe the three major functions of carbohydrates in cells using examples

Classification of Carbohydrates

Types of Carbohydrates

Carbohydrates are classified based on the number of sugar units (monomers) they contain:

• Monosaccharides: Single sugar units (e.g., glucose)

• Disaccharides: Two sugar units joined together (e.g., sucrose)

• Oligosaccharides: Few (more than two) sugar units

• Polysaccharides: Hundreds to thousands of sugar units

Example: Glucose is a monosaccharide, while starch is a polysaccharide.

Monosaccharides

Structure and Properties

Monosaccharides are the simplest carbohydrates and serve as building blocks for more complex sugars. They are water-soluble and typically sweet-tasting. Their chemical formula is usually a multiple of CH2O.

• Glucose (C6H12O6) is the most common monosaccharide.

• Monosaccharides can exist as isomers, such as alpha (α) and beta (β) forms of glucose.

• They can be classified by:

  • Location of the carbonyl group (aldose vs. ketose)

  • Length of the carbon skeleton (triose, pentose, hexose)

  • Arrangement around asymmetric carbons (stereoisomers)

Example: Ribose (pentose), fructose (hexose, ketose), glucose (hexose, aldose).

Disaccharides

Formation and Examples

Disaccharides are formed when two monosaccharides are joined by a covalent bond known as a glycosidic linkage. This process involves a dehydration reaction.

• Sucrose: Glucose + Fructose

• Lactose: Glucose + Galactose

• Maltose: Glucose + Glucose

• Lactose intolerance results from the lack of functional lactase enzyme, which breaks down lactose.

Equation:

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

Polysaccharides

Structure and Functions

Polysaccharides are large molecules composed of hundreds to thousands of monosaccharides joined by glycosidic linkages. They serve two main purposes in cells:

• Storage: Extra energy (e.g., starch in plants, glycogen in animals)

• Structure: Physical support (e.g., cellulose in plant cell walls, chitin in fungal cell walls and exoskeletons, peptidoglycan in bacterial cell walls)

Example: Starch and glycogen are storage polysaccharides; cellulose, chitin, and peptidoglycan are structural polysaccharides.

Summary Table: Types of Carbohydrates

Type	Number of Sugar Units	Example	Main Function
Monosaccharide	1	Glucose	Energy source
Disaccharide	2	Sucrose	Transport/storage
Oligosaccharide	Few (3-10)	Glycoproteins	Cell recognition
Polysaccharide	Hundreds-thousands	Starch, cellulose	Storage/structure

Key Concepts

• Structure determines function: The arrangement of monosaccharides and the type of glycosidic linkage affect the properties and roles of carbohydrates.

• Glycosidic linkage: The covalent bond joining two monosaccharides; can be α or β, influencing digestibility and function.

• Cellular functions: Carbohydrates provide energy, structural support, and facilitate cell-cell recognition.

Additional info: The notes infer the importance of carbohydrates in health (e.g., lactose intolerance) and their role in cell recognition, which is crucial for immune response and organ transplantation.