Biomolecules: The Chemistry of Life

Overview of Biomolecules

Biomolecules are fundamental to life, serving as the architects, engines, and messengers within cells. They include carbohydrates, lipids, proteins, and nucleic acids, each with distinct roles in biological systems.

• Carbohydrates: Main energy source and structural component.

• Lipids: Long-term energy storage, membrane structure, and signaling.

• Proteins: Catalysts (enzymes), structural elements, and functional molecules.

• Nucleic Acids: Genetic information storage and transmission.

Carbohydrates

Introduction to Carbohydrates

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They are the primary energy source for most organisms and play structural roles in plants and animals.

• General formula:

• Empirical formula:

• Types: Sugars (simple carbohydrates) and starches (complex carbohydrates)

Chemical Nature of Carbohydrates

Chemically, carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield these compounds upon hydrolysis. The functional group determines whether a carbohydrate is an aldose (aldehyde group) or a ketose (ketone group).

• Aldose: Contains an aldehyde group (R-CHO)

• Ketose: Contains a ketone group (RC(=O)R')

c

Classification of Carbohydrates

Types of Carbohydrates

Carbohydrates are classified based on their complexity and the number of sugar units:

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

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

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

• Polysaccharides: More than ten sugar 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)

• Functional groups: Aldoses (aldehyde group), Ketoses (ketone group)

• Examples: Glucose, fructose, ribose, galactose

Structure of Monosaccharides

Monosaccharides exist as linear chains or ring-shaped molecules. In ring forms, the hydroxyl group on the anomeric carbon (carbon-1) can be in the alpha (α) or beta (β) position.

• Alpha (α): OH group below the plane

• Beta (β): OH group above the plane

Functions of Monosaccharides

• Glucose: Main energy source in humans and plants

• Galactose: Component of lactose in milk

• Fructose: Found in fruits and honey

• Ribose: Structural element of nucleic acids

• Mannose: Component of glycoproteins

Classification Based on Carbon Number and Functional Group

Monosaccharides are further classified by the number of carbon atoms and the presence of aldehyde or ketone groups. The following table summarizes this classification:

Disaccharides

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

• Examples: Sucrose, lactose, maltose, trehalose, cellobiose, gentiobiose

• Glycosidic bond: Covalent bond formed between two sugar molecules

Classification of Disaccharides

Disaccharides are classified as reducing or non-reducing sugars based on their ability to undergo oxidation-reduction reactions.

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

• Non-reducing sugars: Bonded through an acetal linkage (e.g., sucrose, trehalose)

Functions of Disaccharides

• Sucrose: Major source of carbon and energy in plants

• Lactose: Major energy source in animals

• Maltose: Intermediate in starch and glycogen digestion

• Trehalose: Essential energy source for insects

• Cellobiose: Important in carbohydrate metabolism

• Gentiobiose: Constituent of plant glycosides

Polysaccharides

Polysaccharides are complex carbohydrates composed of many monosaccharide units. Their general formula is , where n is a large number.

• Examples: Starch, glycogen, cellulose, pectin

• Functions: Energy storage (starch, glycogen), structural support (cellulose), gelling agent (pectin)

Nomenclature of Carbohydrates

Systematic Naming

Carbohydrates are named based on chain length and functional group, with the suffix "-ose". Prefixes such as "tri-", "penta-", or "hexa-" indicate the number of carbon atoms. Stereochemistry is denoted by D or L, depending on the orientation of the hydroxyl group on the highest-numbered chiral center.

• Aldoses: Contain an aldehyde group

• Ketoses: Contain a keto group

• D and L designations: D-sugars have the –OH on the right, L-sugars on the left in Fischer projection

Cyclic Structures and Anomers

Monosaccharides often form ring structures. Furanose refers to a 5-membered ring, and pyranose to a 6-membered ring. The orientation of the OH group at the anomeric carbon gives rise to α-anomer (OH down) and β-anomer (OH up).

• Disaccharides and Polysaccharides: Named according to the monosaccharide units and type of glycosidic linkage (e.g., maltose: α-1,4 linkage; lactose: β-1,4 linkage; sucrose: α-1,2 linkage)

Summary Table: Carbohydrate Classification

Additional info:

• Carbohydrates are essential for energy metabolism, cellular structure, and signaling.

• Understanding carbohydrate structure and function is foundational for biochemistry, molecular biology, and physiology.