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 of Monosaccharides

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

• Key Point 1: In their linear forms, all monosaccharides contain a carbonyl group (either an aldehyde or ketone) and several hydroxyl functional groups.

• Key Point 2: Monosaccharides do not contain more than one sugar unit; they are single-molecule sugars.

• Key Point 3: In their ring forms, monosaccharides can be pentoses (5 carbons) or hexoses (6 carbons), but not all are pentoses.

• Definition: Aldose – a monosaccharide with the carbonyl group at the end of the carbon chain. Ketose – a monosaccharide with the carbonyl group in the middle of the carbon chain.

Example: Glucose (an aldose) and fructose (a ketose) are common monosaccharides.

Structural Variations in Monosaccharides

Monosaccharides vary in the placement of their carbonyl group and the number of carbon atoms.

• Key Point 1: The carbon skeleton can be arranged as straight chains, branched trees, or rings.

• Key Point 2: The most common monosaccharides in biology are pentoses (5 carbons, e.g., ribose) and hexoses (6 carbons, e.g., glucose, galactose).

• Key Point 3: Isomers such as glucose and galactose differ in the arrangement of hydroxyl groups around the carbon skeleton.

Example: Ribose contains 5 carbons; glucose and galactose contain 6 carbons.

Ring Structures of Monosaccharides

In aqueous solutions, monosaccharides often form ring structures through a reaction between the carbonyl group and a hydroxyl group on the same molecule.

• Key Point 1: The linear form of glucose can cyclize to form either α-glucose or β-glucose, depending on the orientation of the hydroxyl group at carbon 1.

• Key Point 2: The ring structure is more stable and predominant in biological systems.

Example: The difference between α- and β-glucose is the position of the hydroxyl group on carbon 1 (down in α, up in β).

Carbon Skeletons

Types of Carbon Skeletons

The backbone of monosaccharides consists of carbon atoms arranged in various configurations.

• Key Point 1: Chains – straight or branched arrangements of carbon atoms.

• Key Point 2: Rings – cyclic structures formed by internal reactions.

• Key Point 3: Branched trees – less common in simple sugars, but important in polysaccharide structure.

Example: Glucose can be represented as a straight chain or as a ring structure.

Specific Monosaccharides

Glucose and Galactose

Glucose and galactose are both hexoses but differ in the orientation of the hydroxyl group on carbon 4.

• Key Point 1: Both have the molecular formula $C_6H_{12}O_6$.

• Key Point 2: The difference in structure leads to different biological properties and recognition by enzymes.

Example: Glucose is the primary energy source for cells; galactose is found in milk sugar (lactose).

Ribose

Ribose is a pentose sugar, essential for the structure of RNA.

• Key Point 1: Ribose contains 5 carbon atoms.

• Key Point 2: The ring structure of ribose is important for nucleic acid formation.

Example: Ribose is a component of ribonucleotides in RNA.

Summary Table: Types of Monosaccharides

Name	Number of Carbons	Type of Carbonyl	Example
Glucose	6	Aldose	Blood sugar
Galactose	6	Aldose	Milk sugar component
Ribose	5	Aldose	RNA component
Fructose	6	Ketose	Fruit sugar

Additional info:

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

• Isomerism in monosaccharides leads to diversity in structure and function.

• Ring formation is a reversible process in aqueous solution, and the equilibrium favors the ring form.