Chapter 5: An Introduction to Carbohydrates – Structure, Function, and Biological Importance

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Chapter 5: An Introduction to Carbohydrates

Overview

This chapter introduces the structure, classification, and biological roles of carbohydrates, with a focus on their importance in cell structure, identity, and energy storage. It also highlights the field of glycobiology and its applications in health and disease.

Glycobiology and Carbohydrate Research

Glycobiology: The Study of Glycans

Glycobiology is the study of the structure, biosynthesis, and biological function of glycans (carbohydrate chains) and glycan-binding proteins.
Research in this field explores how glycosylated molecules on cell surfaces influence health and disease, such as in cancer therapy.
Example: Protein therapies can be designed to target cancer cells based on their unique surface glycosylation patterns.

Classification and Structure of Carbohydrates

Types of Carbohydrates

Monosaccharides – Simple sugars (e.g., glucose, ribose); basic monomer units.
Oligosaccharides – Short chains of monosaccharides (2–10 units).
Polysaccharides – Long chains of monosaccharides (hundreds to thousands); complex carbohydrates.

Molecular Formula and Functional Groups

General formula: where n is the number of carbon atoms.
Key functional groups: Carbonyl group (C=O) and hydroxyl groups (–OH).
Carbohydrates are classified as aldoses (carbonyl at end) or ketoses (carbonyl in middle).

Structural Variability in Monosaccharides

Location of carbonyl group: Aldose (end), Ketose (middle).
Number of carbon atoms: Triose (3C), Pentose (5C), Hexose (6C).
Spatial arrangement of hydroxyl groups affects function.
Monosaccharides can exist in linear or ring forms (ring forms predominate in aqueous solutions).

Formation and Structure of Polysaccharides

Glycosidic Linkages

Monosaccharides are joined by glycosidic linkages (covalent bonds formed via condensation reactions between hydroxyl groups).
Linkages can be broken by hydrolysis.
Common linkages: α-1,4-glycosidic and β-1,4-glycosidic (between C-1 and C-4 of adjacent sugars).
Geometry of linkage (α vs. β) determines structure and function.

Major Polysaccharides and Their Functions

Polysaccharide	Monomer	Linkage	Function	Location
Starch	α-glucose	α-1,4 and α-1,6	Energy storage	Plants
Glycogen	α-glucose	α-1,4 and α-1,6 (more branched)	Energy storage	Animals (liver, muscle)
Cellulose	β-glucose	β-1,4	Structural support	Plant cell walls
Chitin	N-acetylglucosamine (NAG)	β-1,4	Structural support	Fungi, exoskeletons
Peptidoglycan	Modified sugars + peptides	β-1,4 + peptide cross-links	Structural support	Bacterial cell walls

Biological Functions of Carbohydrates

Structural Support

Cellulose, chitin, and peptidoglycan form strong fibers or sheets, providing rigidity and protection.
β-1,4-glycosidic linkages are resistant to hydrolysis, making these polysaccharides durable.
Hydrogen bonding between strands increases strength and water exclusion.

Cell Identity and Recognition

Carbohydrates on cell surfaces act as identification tags.
Glycoproteins (proteins with attached carbohydrates) and glycolipids (lipids with attached carbohydrates) are key in cell-cell recognition and signaling.
Example: Immune system cells use glycoproteins to distinguish self from non-self.

Energy Storage and Release

Carbohydrates store chemical energy in C–H and C–C bonds, which have high potential energy.
Photosynthesis stores solar energy in carbohydrate bonds:

Starch (plants) and glycogen (animals) are easily hydrolyzed to release glucose for energy.
Enzymes: Phosphorylase (glycogen breakdown), amylase (starch breakdown).
Glucose is used to produce ATP, the cell’s main energy currency:

Summary Table: Key Carbohydrate Types and Functions

Type	Example	Main Function
Monosaccharide	Glucose, ribose	Energy source, building block
Disaccharide	Sucrose, lactose	Transport, energy
Polysaccharide	Starch, glycogen, cellulose	Storage, structure

Key Terms and Definitions

Glycan: A generic term for any carbohydrate, especially polysaccharides and oligosaccharides.
Glycosidic linkage: Covalent bond joining two monosaccharides.
Glycoprotein: Protein with one or more covalently attached carbohydrates.
Glycolipid: Lipid with one or more covalently attached carbohydrates.
Hydrolysis: Chemical breakdown of a compound due to reaction with water.