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: Cn(H2O)n

• Aldose: Carbohydrate with an aldehyde group

• Ketose: Carbohydrate with a ketone group

• Example: Glucose (C6H12O6) is a common monosaccharide

Not all compounds with the formula Cn(H2O)n 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 10 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 CnH2nOn.

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

• Aldoses: Monosaccharides with an aldehyde group (e.g., glucose)

• Ketoses: Monosaccharides with a ketone group (e.g., fructose)

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

Structure of Monosaccharides

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

• α-anomer: OH group below the plane of the ring

• β-anomer: OH group above the plane of the ring

Biological Functions of Monosaccharides

• Glucose: Main energy source in humans and plants

• Galactose: Component of lactose (milk sugar)

• Fructose: Found in fruits and honey

• Ribose: Component of nucleic acids and some coenzymes

• Mannose: Found in mucoproteins and glycoproteins

Classification of Monosaccharides 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.

Disaccharides

Disaccharides are composed of two monosaccharide units joined by a glycosidic bond, formed via a dehydration (condensation) reaction. The general formula is C12H22O11.

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

• Glycosidic bond: Covalent bond linking two monosaccharides

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 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 energy storage (e.g., starch, glycogen) or structural components (e.g., cellulose, pectin).

• General formula: (C6H10O5)n

• Examples: Starch, glycogen, cellulose, pectin

Nomenclature of Carbohydrates

Carbohydrates are named based on the number of carbon atoms and the type of functional group. The suffix "-ose" is common to most sugars. Stereochemistry is indicated by D or L, depending on the orientation of the hydroxyl group on the highest-numbered chiral center.

• Prefixes: tri- (3C), penta- (5C), hexa- (6C)

• Aldoses: Contain an aldehyde group

• Ketoses: Contain a ketone group

• Cyclic structures: Furanose (5-membered ring), Pyranose (6-membered ring)

• Anomers: α-anomer (OH down), β-anomer (OH up) at the anomeric carbon

Summary Table: Representative Monosaccharides

Key Equations and Concepts

• Empirical formula for many carbohydrates: $(CH_2O)_n$

• Glycosidic bond formation (dehydration reaction): $\text{Monosaccharide}_1 + \text{Monosaccharide}_2 \rightarrow \text{Disaccharide} + H_2O$

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