Structure and Function of Biological Macromolecules: Polymers and Carbohydrates

Chapter 5: The Structure and Function of Large Biological Molecules

Overview of Biological Macromolecules

Biological macromolecules are essential for life, serving as structural components, energy sources, and catalysts. The four major classes are carbohydrates, proteins, nucleic acids, and lipids. Except for lipids, these macromolecules are polymers built from monomer subunits.

• Carbohydrates: Provide energy and structural support. Monomer: monosaccharide (e.g., glucose).

• Proteins: Perform a wide range of functions including catalysis, transport, and structure. Monomer: amino acid.

• Nucleic acids: Store and transmit genetic information. Monomer: nucleotide.

• Lipids: Serve as energy storage, membrane structure, and signaling molecules. Lipids are not true polymers.

Macromolecules as Polymers

Definition and Structure

Three of the four classes of macromolecules—carbohydrates, proteins, and nucleic acids—form polymers, which are long molecules composed of many similar or identical building blocks called monomers. These monomers are connected by covalent bonds.

• Polymer: A chain-like molecule made up of repeating monomer units.

• Monomer: A small molecule that serves as a building block for polymers.

Synthesis and Breakdown of Polymers

Cells use similar chemical mechanisms to assemble and disassemble polymers, processes that are facilitated by enzymes—specialized macromolecules that speed up chemical reactions.

Synthesis: Dehydration Reaction

• Monomers are joined by covalent bonds through the loss of a water molecule.

• This process is called a dehydration reaction.

• One monomer provides a hydroxyl group (–OH), the other provides a hydrogen atom (–H).

• Energy is required for the reaction.

$\text{Monomer}_1{-OH} + \text{Monomer}_2{-H} \xrightarrow{\text{Enzyme}} \text{Polymer} + H_2O$

Breakdown: Hydrolysis Reaction

• Polymers are broken down into monomers by hydrolysis, the reverse of dehydration.

• Water is added to break the covalent bond between monomers.

• A hydrogen atom (–H) attaches to one monomer, and a hydroxyl group (–OH) to the adjacent monomer.

• Energy is released in this process.

• Example: Digestion of food polymers into monomers.

$\text{Polymer} + H_2O \xrightarrow{\text{Enzyme}} \text{Monomer}_1{-OH} + \text{Monomer}_2{-H}$

Summary Table: Polymer Assembly and Disassembly

Process	Bond Formation/Breakage	Water Involved	Energy
Dehydration	Formation	Water removed	Required
Hydrolysis	Breakage	Water added	Released

Carbohydrates: Structure and Function

Classification of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, and include sugars and their polymers. They serve as fuel and building material in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Double sugars formed by two monosaccharides joined by a covalent bond (glycosidic linkage).

• Polysaccharides: Polymers made of many monosaccharides.

Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as major nutrients for cells. Their general formula is a multiple of CH2O (e.g., glucose: C6H12O6).

• Contain a carbonyl group (>C=O) and multiple hydroxyl groups (–OH).

• Classified by the location of the carbonyl group: aldose (aldehyde) or ketose (ketone).

• Classified by carbon skeleton length: triose (3C), pentose (5C), hexose (6C).

• Isomers differ in spatial arrangement around asymmetric carbons (e.g., glucose vs. galactose).

Disaccharides

Disaccharides are formed by joining two monosaccharides via a dehydration reaction, resulting in a glycosidic bond.

• Maltose: Glucose + Glucose

• Sucrose: Glucose + Fructose

• Lactose: Glucose + Galactose

Polysaccharides

Polysaccharides are large polymers of monosaccharides, joined by glycosidic linkages. Their function depends on their structure and the type of monomers used.

Storage Polysaccharides

• Starch: Storage form of glucose in plants; composed of α-glucose monomers, mostly linked by 1-4 glycosidic bonds.

• Amylose: Unbranched form of starch.

• Amylopectin: Branched form of starch.

• Glycogen: Storage form of glucose in animals; highly branched, stored in liver and muscles.

Structural Polysaccharides

• Cellulose: Major component of plant cell walls; composed of β-glucose monomers, forming straight, unbranched chains. Hydrogen bonds between chains provide strength.

• Chitin: Found in exoskeletons of arthropods and cell walls of fungi; similar to cellulose but with nitrogen-containing appendages.

Comparison Table: Major Polysaccharides

Polysaccharide	Monomer	Linkage Type	Function	Branching
Starch (Amylose)	α-glucose	1-4 glycosidic	Energy storage (plants)	Unbranched
Starch (Amylopectin)	α-glucose	1-4 & 1-6 glycosidic	Energy storage (plants)	Branched
Glycogen	α-glucose	1-4 & 1-6 glycosidic	Energy storage (animals)	Highly branched
Cellulose	β-glucose	1-4 glycosidic	Structural (plants)	Unbranched, straight
Chitin	Modified glucose (with N)	β-1-4 glycosidic	Structural (arthropods, fungi)	Unbranched

Key Terms and Definitions

• Polymer: Long molecule made of repeating monomers.

• Monomer: Small molecule serving as a building block for polymers.

• Dehydration reaction: Chemical reaction that joins monomers by removing water.

• Hydrolysis: Chemical reaction that breaks polymers into monomers by adding water.

• Glycosidic bond: Covalent bond joining two monosaccharides.

• Aldose/Ketose: Classification of monosaccharides based on carbonyl group position.

Example Exam Questions

• Polymers are made of monomer subunits joined by what type of bonds? Covalent bonds

• Monomers are joined together to form polymers by dehydration reaction.

• Polymers are broken down to form monomers by hydrolysis reaction.

• Energy is required in dehydration reaction; energy is released in hydrolysis reaction.

• The bond that joins two monosaccharides is a glycosidic bond.

• Lactose is a disaccharide.

• Unbranched form of starch is amylose; highly branched storage polysaccharide in animals is glycogen.

• Nitrogen-containing polysaccharide is chitin.

Additional info: The notes also reference the importance of enzymes in facilitating these reactions and the role of carbohydrates in energy metabolism and structural support.