Carbohydrates: Structure, Classification, and Biological Importance

Introduction to Biomolecules

Biomolecules are essential organic compounds that play critical roles in the structure and function of living organisms. The four major classes of biomolecules are carbohydrates, proteins, lipids, and nucleic acids. Carbohydrates serve as the primary energy source and structural components in cells.

• Carbohydrates: Main energy source (e.g., glucose, starch, cellulose)

• Lipids: Long-term energy storage, membrane structure, signaling (e.g., fats, oils, phospholipids)

• Proteins: Catalysts, structure, transport, defense (e.g., enzymes, hemoglobin, keratin)

• Nucleic Acids: Genetic information storage and transfer (e.g., DNA, RNA)

Carbohydrates: Definition and General Properties

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the empirical formula (CH2O)n. They are classified as polyhydroxy aldehydes or ketones, or substances that yield these upon hydrolysis.

• General formula:

• Aldose: Carbohydrate with an aldehyde group (e.g., glucose)

• Ketose: Carbohydrate with a ketone group (e.g., fructose)

• Not all compounds with the formula are carbohydrates (e.g., acetic acid is not a carbohydrate)

Classification of Carbohydrates

Carbohydrates are classified based on the number of sugar units and the type of functional group present.

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

• Disaccharides: Two monosaccharide units (e.g., sucrose, lactose, maltose)

• Oligosaccharides: 2–10 monosaccharide units (e.g., raffinose, stachyose)

• Polysaccharides: More than ten monosaccharide units (e.g., starch, glycogen, cellulose)

Monosaccharides

Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed into smaller units. They are colorless, crystalline solids, soluble in water, and insoluble in nonpolar solvents. The general formula is .

• Examples: Glucose, fructose, galactose, ribose, mannose

• Classification: Based on the number of carbon atoms (triose, tetrose, pentose, hexose) and the presence of an aldehyde (aldose) or ketone (ketose) group

Structure and Isomerism

Monosaccharides can exist as linear chains or ring-shaped molecules. In aqueous solutions, they often form cyclic structures. The orientation of the hydroxyl group at the anomeric carbon determines the alpha (α) or beta (β) form.

• Alpha (α): OH group below the plane at C-1

• Beta (β): OH group above the plane at C-1

Biological Functions of Monosaccharides

• Glucose: Primary energy source in humans and plants

• Galactose: Component of lactose in milk

• Fructose: Found in fruits and honey

• Ribose: Component of nucleic acids and some coenzymes

• Mannose: Found in glycoproteins and mucoproteins

Classification by Carbon Number and Functional Group

Disaccharides

Disaccharides are composed of two monosaccharide units joined by a glycosidic bond, formed via a dehydration (condensation) reaction. The general formula is .

• Examples: Sucrose (glucose + fructose), Lactose (galactose + glucose), Maltose (glucose + glucose)

• Glycosidic bond: Covalent bond linking two monosaccharides

Reducing and Non-Reducing Sugars

• Reducing sugars: Have a free hemiacetal group (e.g., maltose, lactose, cellobiose)

• Non-reducing sugars: No free hemiacetal group due to acetal linkage (e.g., sucrose, trehalose)

Functions of Disaccharides

• Sucrose: Major source of carbon and energy in plants

• Lactose: Major energy source in animals (milk sugar)

• Maltose: Intermediate in starch and glycogen digestion

• Trehalose: Energy source for insects

• Cellobiose: Important in carbohydrate metabolism

• Gentiobiose: Found in plant glycosides and some polysaccharides

Polysaccharides

Polysaccharides are large molecules composed of many monosaccharide units. They serve as storage forms of energy or as structural components in cells. The general formula is .

• Examples: Starch (plants), Glycogen (animals), Cellulose (plants), Pectin (plants)

• Starch: Main storage polysaccharide in plants

• Glycogen: Main storage polysaccharide in animals

• Cellulose: Structural component of plant cell walls

• Pectin: Used in jam making as a gelling agent

Nomenclature and Stereochemistry

Carbohydrate nomenclature is based on the number of carbon atoms and the type of functional group. The suffix "-ose" is used for sugars, with prefixes such as "tri-", "penta-", or "hexa-" indicating chain length. Stereochemistry is indicated by D or L, based on the orientation of the hydroxyl group on the highest-numbered chiral center in the Fischer projection.

• D-sugars: –OH on the right

• L-sugars: –OH on the left

Cyclic Structures and Anomers

• Furanose: 5-membered ring

• Pyranose: 6-membered ring

• Anomers: Differ in the orientation of the OH group at the anomeric carbon (α or β)

Summary Table: Classification of Carbohydrates

Key Equations and Concepts

• General formula for carbohydrates:

• Formation of glycosidic bond (dehydration reaction):

• Empirical formula for polysaccharides: