Chapter 5: The Macromolecules of Life – Structure and Function

Macromolecules: The Building Blocks of Life

Introduction to Macromolecules

Within cells, small organic molecules are joined together to form larger molecules known as macromolecules. These are large molecules composed of thousands of covalently connected atoms. Most macromolecules are polymers, which are long molecules consisting of many similar or identical building blocks called monomers.

• Macromolecule: A very large molecule, often formed by polymerization of smaller subunits (monomers).

• Polymer: A long chain molecule made up of repeating units (monomers).

• Monomer: The basic building block of a polymer (e.g., amino acids for proteins).

Examples: Proteins (polymers of amino acids), nucleic acids (polymers of nucleotides), carbohydrates (polymers of sugars), and lipids (not true polymers, but large molecules).

Classes of Biological Macromolecules

• Carbohydrates: e.g., starch

• Proteins: e.g., enzymes

• Nucleic acids: e.g., DNA

• Lipids: e.g., fats (not true polymers, but large and important biological molecules)

The Synthesis and Breakdown of Polymers

Dehydration Reaction (Dehydration Synthesis)

A dehydration reaction occurs when two monomers bond together through the loss of a water molecule. This process builds polymers from monomers.

• Dehydration synthesis: Removal of a water molecule to form a new covalent bond between monomers.

Hydrolysis

Hydrolysis is the process by which polymers are disassembled to monomers. It is essentially the reverse of the dehydration reaction, involving the addition of a water molecule to break a bond.

• Hydrolysis: Addition of a water molecule to break a covalent bond between monomers.

Diagram: Synthesis and Breakdown of Polymers

• Dehydration Reaction: Short polymer + unlinked monomer → longer polymer + H2O (water released)

• Hydrolysis: Polymer + H2O → shorter polymer + monomer (water consumed)

Carbohydrates: Fuel and Building Material

Overview of Carbohydrates

Carbohydrates include sugars and the polymers of sugars. The simplest carbohydrates are monosaccharides (single sugars). Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks.

• Monosaccharide: The simplest form of carbohydrate (e.g., glucose, ribose).

• Polysaccharide: A polymer made of many monosaccharide units (e.g., starch, cellulose).

Monosaccharide Structure and Classification

• Monosaccharides have molecular formulas that are usually multiples of CH2O (e.g., glucose: C6H12O6).

• Glucose is the most common monosaccharide.

• Monosaccharides are classified by:

  • Location of the carbonyl group: Aldose (aldehyde group) or ketose (ketone group).

  • Number of carbons in the carbon skeleton: Trioses (3C), pentoses (5C), hexoses (6C), etc.

Table: Classification of Monosaccharides

Type	Number of Carbons	Example	Carbonyl Group Location
Triose	3	Glyceraldehyde	Aldose
Pentose	5	Ribose	Aldose
Hexose	6	Glucose	Aldose
Hexose	6	Fructose	Ketose

Key Points

• Carbohydrates are essential for energy storage and structural support in living organisms.

• Monosaccharides serve as the monomers for more complex carbohydrates.

• Polysaccharides such as starch and cellulose have different functions and structures based on their monomer composition and glycosidic linkages.

Example: Starch (a polysaccharide) is used by plants for energy storage, while cellulose (another polysaccharide) provides structural support in plant cell walls.

Additional info: Lipids, while not true polymers, are also crucial macromolecules in biology, serving as energy storage molecules, components of cell membranes, and signaling molecules.