Carbohydrates: Structure and Function (Chapter 5 Study Notes)

Carbohydrates

Introduction

Carbohydrates are a major class of biological molecules essential for energy storage, structural integrity, and cellular communication. This chapter focuses on the structure, classification, and function of carbohydrates, with emphasis on monosaccharides and their derivatives.

Monosaccharides

General Features

Monosaccharides are the simplest carbohydrates, commonly referred to as simple sugars. They serve as building blocks for more complex carbohydrates.

• Definition: Monosaccharides are single sugar molecules, such as glucose, fructose, and galactose.

• Functional Groups: In their linear forms, all monosaccharides contain a carbonyl group (either an aldehyde or ketone) and several hydroxyl groups.

• Classification: Monosaccharides are classified by the number of carbon atoms (triose, pentose, hexose) and the position of the carbonyl group (aldose or ketose).

• Example: Glucose (an aldohexose) and fructose (a ketohexose).

Aldoses vs. Ketoses

The position of the carbonyl group determines whether a monosaccharide is an aldose or a ketose.

• Aldose: Carbonyl group at the end of the carbon chain (e.g., glucose).

• Ketose: Carbonyl group in the middle of the carbon chain (e.g., fructose).

• Structural Example: See diagram for glucose (aldose) and galactose (aldose).

Isomerism in Monosaccharides

Monosaccharides with the same molecular formula can differ in the arrangement of atoms, leading to isomers.

• Structural Isomers: Glucose and galactose differ in the orientation of the hydroxyl group at carbon 4.

• Ring Formation: In aqueous solution, monosaccharides often form ring structures, such as α-glucose and β-glucose.

Ring Structures

Monosaccharides can cyclize to form ring structures, which are more stable in aqueous environments.

• α-Glucose: The hydroxyl group on carbon 1 is below the plane of the ring.

• β-Glucose: The hydroxyl group on carbon 1 is above the plane of the ring.

• Molecular Formula: Glucose has the formula $C_6H_{12}O_6$.

Carbon Skeletons

Carbon skeletons of organic molecules can be arranged in various forms, influencing the properties of the molecule.

• Chains: Linear arrangement of carbon atoms.

• Branched Trees: Carbon atoms form branches off the main chain.

• Rings: Carbon atoms form a closed loop, as seen in cyclic sugars.

• Example: Glucose can exist in both chain and ring forms.

Examples of Monosaccharides

Common monosaccharides include glucose, galactose, and ribose.

• Glucose: A six-carbon (hexose) sugar, primary energy source for cells.

• Galactose: An isomer of glucose, differs at carbon 4.

• Ribose: A five-carbon (pentose) sugar, important in RNA structure.

• Number of Carbons in Ribose: 5

Disaccharides and Oligosaccharides

Formation and Structure

Disaccharides and oligosaccharides are formed by linking monosaccharides via condensation reactions, which release water.

• Condensation Reaction: Two monosaccharides join, expelling a water molecule.

• Glycosidic Bond: The covalent bond formed between two sugar molecules.

• Example: Sucrose (glucose + fructose), lactose (glucose + galactose).

• Molecular Formula Example: Linking three glucose molecules: $C_6H_{12}O_6 imes 3 - 2 imes H_2O = C_{18}H_{32}O_{16}$

Glycosidic Linkages

The type and position of glycosidic bonds affect the structure and properties of the resulting carbohydrate.

• 1,4 Glycosidic Linkage: Bond between carbon 1 of one glucose and carbon 4 of another.

• 1,2 Glycosidic Linkage: Bond between carbon 1 of one sugar and carbon 2 of another.

• Structural Diversity: Sequence of monomers and geometry of linkages can vary, leading to different oligosaccharides.

Polysaccharides

Structure and Function

Polysaccharides are large polymers composed of many monosaccharide units. They serve structural and storage roles in cells.

• Examples: Starch, glycogen, cellulose.

• Starch: Storage polysaccharide in plants, composed of α-glucose units.

• Glycogen: Storage polysaccharide in animals, highly branched.

• Cellulose: Structural polysaccharide in plants, composed of β-glucose units.

Enzymatic Specificity

Enzymes that hydrolyze polysaccharides are specific to the geometry of glycosidic bonds.

• Starch vs. Cellulose: Enzymes that break down starch (α-1,4 linkages) cannot hydrolyze cellulose (β-1,4 linkages) due to differences in bond geometry and enzyme active site specificity.

• Biological Implication: Most animals cannot digest cellulose, which is why it serves as dietary fiber.

Glycoproteins

Definition and Function

Glycoproteins are proteins with carbohydrate groups attached, playing key roles in cell-cell recognition and signaling.

• Structure: Protein backbone with covalently attached oligosaccharide chains.

• Function: Found on cell surfaces, involved in immune response and cellular communication.

Summary Table: Types of Carbohydrates

Type	Structure	Example	Function
Monosaccharide	Single sugar unit	Glucose, Ribose	Energy source, building block
Disaccharide	Two sugar units	Sucrose, Lactose	Transport, energy
Oligosaccharide	3-10 sugar units	Glycoprotein oligosaccharides	Cell recognition
Polysaccharide	Many sugar units	Starch, Cellulose, Glycogen	Storage, structure

Key Equations

• Molecular Formula for Glucose: $C_6H_{12}O_6$

• Condensation Reaction (for linking n monosaccharides): $C_6H_{12}O_6 \times n - (n-1) \times H_2O$

Additional info: Academic context and explanations have been expanded for clarity and completeness. All diagrams referenced are described in text for accessibility.