Chapter 7: Carbohydrates – Structures, Properties, and Biological Functions

Carbohydrates: Overview

Introduction to Carbohydrates

Carbohydrates are a diverse class of biomolecules essential for life, named for their general formula $C_n(H_2O)_n$. They are produced in plants via photosynthesis from carbon dioxide and water. Carbohydrates range in size from small molecules like glyceraldehyde to large polymers such as amylopectin.

• Energy source and storage: Carbohydrates are primary sources of energy and are stored as glycogen or starch.

• Structural components: They form cell walls (cellulose in plants) and exoskeletons (chitin in arthropods).

• Informational molecules: Carbohydrates participate in cell-cell signaling.

• Glycoproteins and proteoglycans: Carbohydrates can be covalently linked to proteins, forming glycoproteins and proteoglycans.

Classification of Carbohydrates

Main Types of Carbohydrates

Carbohydrates are classified based on the number of monomeric units:

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

• Disaccharides: Two monosaccharides linked by a glycosidic bond (e.g., sucrose, lactose).

• Oligosaccharides: Short chains of monosaccharides (e.g., maltotriose).

• Polysaccharides: Long chains, often thousands of units (e.g., starch, cellulose, glycogen).

Monosaccharides: Structures and Nomenclature

Aldoses and Ketoses

Monosaccharides are classified by the type of carbonyl group they contain:

• Aldose: Contains an aldehyde group (e.g., glyceraldehyde).

• Ketose: Contains a ketone group (e.g., dihydroxyacetone).

Trioses are the simplest monosaccharides with three carbon atoms:

• Glyceraldehyde: An aldotriose.

• Dihydroxyacetone: A ketotriose.

Open-Chain and Ring Forms

Monosaccharides can exist in open-chain (linear) or cyclic (ring) forms. The ring form is predominant in aqueous solutions.

• Hexose Models: D-Glucose (an aldohexose), D-Fructose (a ketohexose).

• Pentose Models: D-Ribose (component of RNA), 2-Deoxy-D-ribose (component of DNA).

Glycoproteins vs. Proteoglycans

Structural and Functional Differences

Carbohydrates can be covalently attached to proteins, forming glycoproteins and proteoglycans, which differ in composition and function.

Feature	Glycoprotein	Proteoglycan
Composition	Primarily protein	Primarily carbohydrate
CHO Chains	May be negatively charged	Always negatively charged
Location	Cell surface	Connective tissue

Key Monosaccharides and Their Biological Roles

Common Monosaccharides

• Glucose: The most abundant monosaccharide, primary energy source.

• Fructose: A ketohexose found in fruits.

• Ribose: A pentose sugar in RNA.

• 2-Deoxy-D-ribose: A pentose sugar in DNA.

Example: D-Ribose is a component of ribonucleic acid (RNA), and deoxy-D-ribose is a component of deoxyribonucleic acid (DNA).

Structural Representation of Monosaccharides

Fischer Projections and Perspective Formulas

Monosaccharides are often depicted using Fischer projections, which show the stereochemistry of chiral centers. Horizontal bonds point toward the viewer, vertical bonds point away.

• Fischer projection: Standard for representing chiral carbohydrates.

• Perspective formula: Shows three-dimensional arrangement.

Summary of Chapter Topics

• Structures and names of monosaccharides

• Open-chain and ring forms of monosaccharides

• Structures and properties of disaccharides

• Biological function of polysaccharides

• Biological function of glycoconjugates

Additional info: The notes cover foundational biochemistry topics relevant for college-level study, including carbohydrate classification, structure, and biological roles. The provided images and text are consistent with standard biochemistry curriculum.