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. Each class serves unique functions, such as energy storage, structural support, catalysis, and genetic information transfer.

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

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

• Proteins: Catalysts, structural elements, and transporters (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. Carbohydrates are the primary energy source for most organisms and also serve structural and recognition roles.

• 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 joined by a glycosidic bond (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 serve as building blocks for more complex carbohydrates.

• General formula:

• Smallest monosaccharides: Dihydroxyacetone and D-/L-glyceraldehyde (n=3).

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

• Structural isomerism: Glucose, galactose, and fructose all have the formula but differ in structure due to the arrangement of functional groups around chiral carbons.

Structure and Isomerism

Monosaccharides can exist as linear chains or ring-shaped molecules. In aqueous solutions, the ring form predominates. The ring closure creates a new asymmetric carbon (anomeric carbon), leading to alpha (α) and beta (β) anomers.

• Alpha (α) anomer: OH group below the plane at the anomeric carbon.

• Beta (β) anomer: OH group above the plane at the anomeric carbon.

Classification by Carbon Number and Functional Group

Monosaccharides are further classified by the number of carbon atoms and the presence of an aldehyde or ketone group.

Functions of Monosaccharides

• Glucose: Primary energy source in humans and plants.

• Galactose: Component of lactose (milk sugar).

• Fructose: Found in fruits and honey, contributes to sweetness.

• Ribose: Structural component of nucleic acids (RNA) and some coenzymes.

• Mannose: Important in glycoproteins and mucoproteins.

Disaccharides

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

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

• Glycosidic bond: Covalent bond linking two monosaccharides, with the elimination of water.

Reducing and Non-Reducing Disaccharides

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

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

Functions of Disaccharides

• Sucrose: Major transport form of sugar in plants, product of photosynthesis.

• Lactose: Main sugar in milk, energy source for mammals.

• 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 linked by glycosidic bonds. They serve as energy storage (e.g., starch, glycogen) or structural components (e.g., cellulose, pectin).

• General formula:

• Examples: Starch (plants), glycogen (animals), cellulose (plants), pectin (plants, used in jam making).

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 denoted by D- or L- prefixes, based on the orientation of the hydroxyl group on the highest-numbered chiral center in the Fischer projection.

• D-sugars: –OH on the right side.

• L-sugars: –OH on the left side.

Cyclic Structures and Anomers

• Furanose: 5-membered ring structure.

• Pyranose: 6-membered ring structure.

• Anomers: Differ in the configuration at the anomeric carbon (α or β).

Summary Table: Classification of Carbohydrates

Key Equations and Concepts

• General formula for carbohydrates:

• Glycosidic bond formation:

• Empirical formula for simple carbohydrates:

Additional info: This guide expands on the provided notes with academic context, definitions, and examples to ensure completeness and clarity for biochemistry students.