Carbohydrates: Structure, Classification, and Functions

Carbohydrates

Overview and Learning Outcomes

Carbohydrates are essential biological macromolecules composed of sugars and their polymers. They play critical roles in energy storage, structural support, and cellular recognition. Understanding their structure and classification is fundamental in 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 with 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, fructose)

• Disaccharides: Two monosaccharides joined together (e.g., sucrose, lactose)

• Oligosaccharides: Few (typically 3-10) monosaccharide units

• Polysaccharides: Hundreds to thousands of monosaccharide units (e.g., starch, cellulose)

Example: Glucose is a monosaccharide, while sucrose is a disaccharide composed of glucose and fructose.

Monosaccharides

Structure and Properties

Monosaccharides are the simplest carbohydrates, often referred to as simple sugars. They are water-soluble and typically sweet-tasting. Their chemical formula is usually a multiple of CH2O.

• Glucose is the most common monosaccharide.

• Monosaccharides can exist as isomers, such as α-glucose and β-glucose, which differ in the orientation of the hydroxyl group at carbon 1.

• 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: Glucose (C6H12O6) is a hexose aldose; fructose is a hexose ketose.

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 (loss of water).

• Sucrose: Glucose + Fructose

• Lactose: Glucose + Galactose

• Maltose: Glucose + Glucose

Example: Lactose intolerance results from the lack of the enzyme lactase, which breaks down lactose into its monosaccharide components.

Polysaccharides

Structure and Functions

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

• Storage: Polysaccharides such as starch (in plants) and glycogen (in animals) store energy for later use.

• Structure: Polysaccharides such as cellulose (in plant cell walls), chitin (in fungal cell walls and exoskeletons of arthropods), and peptidoglycan (in bacterial cell walls) provide physical support and protection.

Example: Starch is a polymer of α-glucose used for energy storage in plants, while cellulose is a polymer of β-glucose providing structural support.

Structural Comparison of Polysaccharides

Starch vs. Cellulose

Both starch and cellulose are polymers of glucose, but they differ in the type of glycosidic linkage and their biological function.

Polysaccharide	Monomer	Linkage	Function
Starch	α-glucose	α(1→4) glycosidic bonds	Energy storage in plants
Cellulose	β-glucose	β(1→4) glycosidic bonds	Structural support in plant cell walls

Additional info: Humans cannot digest cellulose due to the lack of enzymes that break β(1→4) linkages.

Functions of Carbohydrates in Cells

Major Roles

Carbohydrates perform several essential functions in living cells:

• Energy Storage: Glucose and its polymers are used to store and provide energy. Photosynthesis produces glucose, which is then used to make ATP.

• Structural Support: Cellulose, chitin, and peptidoglycan provide rigidity and protection to cells and organisms.

• Cell Recognition: Carbohydrates on cell surfaces are involved in cell-cell recognition, helping cells identify themselves and foreign invaders (important in immune response and organ transplantation).

Example: Glycoproteins on the surface of red blood cells determine blood type.