Chapter 13: Carbohydrates – Structure, Classification, and Reactions

Carbohydrate Basics

Definition and Types of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the general formula Cn(H2O)n. They are essential biomolecules that serve as energy sources and structural components in living organisms.

• Monosaccharide: Also known as a simple sugar, it is the basic unit of carbohydrates (e.g., glucose, fructose).

• Polysaccharide: Also called a complex carbohydrate, it is a polymer made up of many monosaccharide units (e.g., starch, cellulose).

• Polysaccharides are formed by linking monosaccharides through glycosidic bonds.

• Functions of Carbohydrates:

  • Primary energy source (e.g., glucose in cellular respiration)

  • Energy storage (e.g., glycogen in animals, starch in plants)

  • Structural roles (e.g., cellulose in plant cell walls)

  • Cell recognition and signaling (e.g., glycoproteins)

Chemical Reactions of Carbohydrates

Dehydration Synthesis and Hydrolysis

Carbohydrates undergo key chemical reactions that are essential for their biological functions.

• Dehydration Synthesis (Condensation Reaction): A reaction in which two monosaccharides are joined together by removing a molecule of water, forming a glycosidic bond.

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

• Hydrolysis: The reverse of dehydration synthesis; a disaccharide or polysaccharide is broken down into its monosaccharide units by the addition of water.

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

Classification of Carbohydrates

Types and Structural Features

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

• Monosaccharide: Single sugar unit (e.g., glucose, galactose, fructose)

• Disaccharide: Two monosaccharide units joined by a glycosidic bond (e.g., sucrose, lactose, maltose)

• Polysaccharide: Many monosaccharide units (e.g., starch, glycogen, cellulose)

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

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

• Classification by Carbon Number:

  • Triose (3 carbons), Tetrose (4), Pentose (5), Hexose (6), etc.

Chirality and Isomerism in Carbohydrates

Chiral Centers and Stereoisomers

Many carbohydrates have chiral centers, leading to the existence of isomers with different spatial arrangements.

• Chiral Carbon: A carbon atom bonded to four different atoms or groups. Such carbons give rise to stereoisomerism.

• Achiral Carbon: A carbon atom that does not have four different groups attached.

• D- and L-Isomers: Monosaccharides can exist as D- (right-handed) or L- (left-handed) isomers, based on the configuration of the chiral carbon farthest from the carbonyl group.

• Chain and Ring Forms: Monosaccharides can exist in an open-chain (linear) form or a cyclic (ring) form in solution.

• Haworth Structure: A common way to represent the ring form of monosaccharides. The α- and β- isomers (anomers) differ in the orientation of the hydroxyl group on the anomeric carbon.

  • α-Anomer: The OH group on the anomeric carbon is trans (opposite side) to the CH2OH group.

  • β-Anomer: The OH group on the anomeric carbon is cis (same side) to the CH2OH group.

Glycosidic Bonds and Disaccharides

Formation and Types of Glycosidic Bonds

Disaccharides and polysaccharides are formed by linking monosaccharides through glycosidic bonds.

• Glycosidic Bond: A covalent bond formed between the anomeric carbon of one monosaccharide and a hydroxyl group of another.

• Types of Glycosidic Bonds:

  • α(1→4)-glycosidic bond: e.g., in maltose

  • β(1→4)-glycosidic bond: e.g., in lactose and cellulose

  • α,β(1→2)-glycosidic bond: e.g., in sucrose

• Reducing Sugar: A sugar that has a free anomeric carbon capable of acting as a reducing agent (e.g., maltose, lactose). Sucrose is a non-reducing sugar.

• Hydrolysis of Disaccharides: The breakdown of disaccharides into their monosaccharide components:

  • Maltose: Yields two glucose molecules

  • Lactose: Yields glucose and galactose

  • Sucrose: Yields glucose and fructose

Polysaccharides

Major Types and Biological Roles

Polysaccharides are large carbohydrate molecules with important storage and structural functions.

• Amylose: Unbranched polymer of D-glucose with α(1→4) glycosidic bonds; found in plants (component of starch).

• Amylopectin: Branched polymer of D-glucose with α(1→4) and α(1→6) glycosidic bonds; found in plants (component of starch).

• Glycogen: Highly branched polymer of D-glucose with α(1→4) and α(1→6) glycosidic bonds; found in animals (main storage form of glucose).

• Cellulose: Linear polymer of D-glucose with β(1→4) glycosidic bonds; found in plants (structural component of cell walls).

Polysaccharide	Monomer	Type of Glycosidic Bond	Branching	Source
Amylose	D-glucose	α(1→4)	No	Plants
Amylopectin	D-glucose	α(1→4), α(1→6)	Yes	Plants
Glycogen	D-glucose	α(1→4), α(1→6)	Yes (more than amylopectin)	Animals
Cellulose	D-glucose	β(1→4)	No	Plants