Structures and Functions of Biological Molecules: Carbohydrates

Structures and Functions of Biological Molecules

Overview of Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life. The four major classes are carbohydrates, proteins, nucleic acids, and lipids. Except for lipids, these are polymers made from repeating monomer units.

• Carbohydrates: Polymers of sugars, such as starch and cellulose.

• Proteins: Polymers of amino acids, with diverse functions including catalysis and structure.

• Nucleic Acids: Polymers of nucleotides, such as DNA and RNA, responsible for genetic information storage and transfer.

• Lipids: Not true polymers; include fats, phospholipids, and steroids, important for energy storage and membrane structure.

Monomer refers to the single, repeating unit that makes up a polymer.

Carbohydrates

Monosaccharides: Structure and Properties

Monosaccharides are the simplest carbohydrates, commonly known as simple sugars. The most important monosaccharides in biology are glucose and fructose, both hexoses (six-carbon sugars).

• Glucose: C6H12O6, a primary energy source for cells.

• Fructose: Also C6H12O6, found in fruits and honey.

• Monosaccharides can exist in linear or ring forms. In aqueous solutions, the ring form predominates.

Example: In water, glucose is about 3% linear and 97% ring form; fructose is about 8% linear and 92% ring form.

Disaccharides: Formation and Examples

Disaccharides are formed when two monosaccharides join via a dehydration reaction, creating a glycosidic linkage (covalent bond).

• Maltose: Formed from two glucose molecules.

• Sucrose: Formed from glucose and fructose.

Dehydration Reaction Equation:

• $\text{Monosaccharide}_1 + \text{Monosaccharide}_2 \xrightarrow{\text{dehydration}} \text{Disaccharide} + H_2O$

Polysaccharides: Structure and Function

Polysaccharides are long chains of monosaccharide units. They serve as energy storage or structural components in cells.

• Starch: Storage polysaccharide in plants, composed of glucose monomers. Exists as amylose (unbranched) and amylopectin (branched).

• Glycogen: Storage polysaccharide in animals, highly branched, stored in liver and muscle cells.

• Cellulose: Structural polysaccharide in plant cell walls, composed of glucose monomers linked by β-1,4 glycosidic bonds.

The structure and function of a polysaccharide depend on:

• The type of sugar monomers

• The type of glycosidic linkages

Comparison of Major Polysaccharides

Polysaccharide	Monomer	Linkage Type	Function	Location
Starch (Amylose/Amylopectin)	Glucose	α-1,4 and α-1,6	Energy storage	Plants
Glycogen	Glucose	α-1,4 and α-1,6 (more branched)	Energy storage	Animals
Cellulose	Glucose	β-1,4	Structural support	Plant cell walls

Metabolism and Health Implications of Fructose

Fructose is metabolized differently from glucose. Excessive intake, especially from processed foods and high-fructose corn syrup, is linked to health issues.

• Fructose is processed mainly in the liver.

• High intake can lead to fatty liver disease (NAFLD) and increased bad cholesterol.

• Fructose does not stimulate leptin or insulin, hormones that regulate appetite and satiety.

Example: Many processed foods contain high-fructose corn syrup, contributing to metabolic disorders.

Digestion of Polysaccharides

Animals can digest starch but not cellulose due to the absence of enzymes that break β-1,4 linkages. Some animals rely on symbiotic microbes to digest cellulose.

• Amylase: Enzyme that hydrolyzes starch.

• Cellulase: Enzyme (not present in animals) that hydrolyzes cellulose; found in some bacteria and fungi.

Additional info: Cellulose is the most abundant organic compound on Earth and is a major component of dietary fiber.

Summary Table: Carbohydrate Types and Functions

Type	Structure	Function	Examples
Monosaccharide	Single sugar unit	Immediate energy	Glucose, Fructose
Disaccharide	Two sugar units	Transport, energy	Sucrose, Maltose
Polysaccharide	Many sugar units	Storage, structure	Starch, Glycogen, Cellulose