The Structure and Function of Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Overview: The Molecules of Life

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These molecules, known as macromolecules, are essential for the structure and function of cells. Macromolecules are large, complex molecules made up of thousands of covalently connected atoms, and their structure is closely linked to their function.

• Macromolecules: Large molecules composed of many atoms, often forming complex structures.

• Structure and Function: The specific arrangement of atoms in a macromolecule determines its biological role.

Concept 5.1: Macromolecules are Polymers Built from Monomers

Most biological macromolecules are polymers, which are long chains made from repeating units called monomers. The process of building and breaking down polymers is fundamental to cellular function.

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The small, repeating unit that serves as the building block of a polymer.

• Three major classes of biological polymers: carbohydrates, proteins, and nucleic acids.

Synthesis and Breakdown of Polymers

• Dehydration Reaction: A chemical reaction in which two monomers are covalently bonded to each other with the removal of a water molecule. This process builds polymers.

• Hydrolysis: A chemical reaction that breaks bonds between monomers by adding a water molecule, thereby disassembling polymers into monomers.

Equation for Dehydration Reaction:

• $\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + H_2O$

Equation for Hydrolysis:

• $\text{Polymer} + H_2O \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

Diversity of Polymers

Cells contain thousands of different macromolecules, which vary among cells, species, and even individuals. This diversity arises from the arrangement of a small set of monomers into countless unique polymers.

• Macromolecules vary among cells and species.

• A small set of monomers can create a vast array of polymers.

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Carbohydrates are organic molecules that include sugars and their polymers. They are essential for energy storage and structural support in cells.

• Monosaccharides: The simplest carbohydrates, also known as single sugars.

• Polysaccharides: Carbohydrate macromolecules composed of many sugar building blocks.

Monosaccharides

Monosaccharides have molecular formulas that are usually multiples of $CH_2O$. Glucose ($C_6H_{12}O_6$) is the most common monosaccharide. Monosaccharides are classified by the location of their carbonyl group and the number of carbons in their skeleton.

• Classification by Carbonyl Group:

  • Aldose: Carbonyl group at the end of the carbon skeleton.

  • Ketose: Carbonyl group within the carbon skeleton.

• Classification by Carbon Number:

  • Triose: 3 carbons (e.g., glyceraldehyde)

  • Pentose: 5 carbons (e.g., ribose)

  • Hexose: 6 carbons (e.g., glucose, galactose, fructose)

Type	Example	Structure
Aldose (Aldehyde Sugar)	Glucose	Carbonyl group at end
Ketose (Ketone Sugar)	Fructose	Carbonyl group within chain
Triose	Glyceraldehyde	3 carbons
Pentose	Ribose	5 carbons
Hexose	Glucose, Galactose, Fructose	6 carbons

• Example: Glucose ($C_6H_{12}O_6$) is a hexose aldose and a primary energy source for cells.

Additional info: Monosaccharides can exist in linear or ring forms, especially in aqueous solutions. They serve as fuel for cellular respiration and as raw material for building other molecules.