Lecture #31 Carbohydrates: Structure, Glycosidic Bonds, and Polysaccharides

Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates, consisting of single sugar units such as glucose and fructose. They serve as the building blocks for more complex carbohydrates.

• Definition: Monosaccharides are simple sugars with the general formula $(C_nH_{2n}O_n)$, where n is typically 3-7.

• Examples: Glucose, Fructose, Galactose

• Structure: They can exist in linear or cyclic forms. The cyclic form is often a five- or six-membered ring (furanose or pyranose).

• Functional Groups: Monosaccharides contain multiple hydroxyl (-OH) groups and either an aldehyde (aldose) or ketone (ketose) group.

Glycosidic Bonds

Glycosidic bonds are covalent linkages formed between the anomeric carbon of one monosaccharide and a hydroxyl group of another, resulting in disaccharides and polysaccharides.

• Definition: A glycosidic bond is formed via a condensation reaction, releasing water.

• Types: The bond can be α or β depending on the configuration of the anomeric carbon.

• Example: In maltose, two glucose units are joined by an α(1→4) glycosidic bond.

• Equation: $\text{Monosaccharide}_1 + \text{Monosaccharide}_2 \rightarrow \text{Disaccharide} + H_2O$

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units joined by a glycosidic bond.

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

• Properties: Disaccharides can be hydrolyzed to yield their constituent monosaccharides.

• Equation: $\text{Disaccharide} + H_2O \xrightarrow{\text{hydrolysis}} \text{Monosaccharide}_1 + \text{Monosaccharide}_2$

Polysaccharides

Polysaccharides are large carbohydrate molecules formed by the polymerization of many monosaccharide units via glycosidic bonds.

• Definition: Polysaccharides are complex carbohydrates with hundreds to thousands of monosaccharide units.

• Examples: Cellulose, Starch, Glycogen

• Structure: Can be linear (cellulose) or branched (glycogen, starch).

• Function: Serve as energy storage (starch, glycogen) or structural components (cellulose).

Classification of Glycosidic Bonds

Glycosidic bonds are classified based on the carbons involved and the stereochemistry (α or β) of the linkage.

Bond Type	Example	Structure
α(1→4)	Maltose, Starch	Glucose units linked at C1 (α) to C4
β(1→4)	Cellulose, Lactose	Glucose units linked at C1 (β) to C4
α(1→6)	Glycogen, Amylopectin	Branch points in polysaccharides

Structural Examples

• Cellulose: A linear polymer of glucose with β(1→4) glycosidic bonds. Provides structural support in plants.

• Starch: A mixture of amylose (linear, α(1→4)) and amylopectin (branched, α(1→4) and α(1→6)). Main energy storage in plants.

• Glycogen: Highly branched polymer of glucose with α(1→4) and α(1→6) linkages. Main energy storage in animals.

Additional info:

• Some handwritten notes referenced "anoles" and "alycosidic bonds"; these are interpreted as "anomeric carbons" and "glycosidic bonds" respectively.

• References to "1-glucos.d" and "2-Cr eT" are interpreted as 1-glucoside and 2-carbon entities, likely referring to the positions involved in glycosidic linkages.

• "Sucre a dilacchaad€" is interpreted as "sucrose, a disaccharide".

• "Cellulose" and "polysd oun" are interpreted as "cellulose" and "polysaccharide".