Biological Macromolecules I: Structure, Chemistry, and Function

Biological Macromolecules

Introduction to Macromolecules

Biological macromolecules are large, complex molecules essential for life. They are constructed from smaller subunits and play critical roles in cellular structure and function. The four major classes are carbohydrates, lipids, proteins, and nucleic acids.

• Carbohydrates: Energy storage and structural support

• Lipids: Membrane structure, energy storage, signaling

• Proteins: Catalysis, structure, transport, signaling

• Nucleic acids: Information storage and transfer

Monomers and Polymers

Macromolecules are typically polymers, assembled from repeating monomer units. The process of polymerization involves dehydration reactions, while breakdown occurs via hydrolysis.

• Monomer: A repeating unit that serves as a building block for polymers.

• Polymer: A long molecule consisting of many monomers joined together.

Polymerization Reaction:

• Dehydration synthesis: Removes a water molecule to form a new bond between monomers.

• Hydrolysis: Adds a water molecule to break a bond between monomers.

Equations:

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

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

Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as the monomers for more complex carbohydrates. They are characterized by the presence of a carbonyl group (C=O) and multiple hydroxyl groups (-OH).

• Structure: Can exist in straight-chain or ring forms.

• Classification: Based on the location of the carbonyl group (aldose or ketose) and the number of carbon atoms (triose, pentose, hexose).

• Examples: Glucose, Fructose

Disaccharides and Polysaccharides

Monosaccharides can be joined by glycosidic linkages to form disaccharides and polysaccharides.

• Disaccharide: Two monosaccharides joined by a glycosidic bond (e.g., sucrose, lactose).

• Polysaccharide: Long chains of monosaccharides; can be branched or unbranched.

Functions of Polysaccharides

• Storage: Starch (plants), glycogen (animals)

• Structure: Cellulose (plant cell walls), chitin (exoskeletons of arthropods, fungal cell walls)

Table: Carbohydrate Types and Functions

Component	Examples	Functions
Monosaccharides	Glucose, Fructose	Fuel; carbon sources for biosynthesis
Disaccharides	Lactose, Sucrose	Transport and energy
Polysaccharides	Starch, Glycogen, Cellulose, Chitin	Energy storage, structural support

Lipids

Structure and Diversity

Lipids are a diverse group of hydrophobic molecules, not true polymers. They include fats, phospholipids, and steroids.

• Fats (Triacylglycerols): Glycerol + three fatty acids; energy storage

• Phospholipids: Glycerol + two fatty acids + phosphate group; major component of cell membranes

• Steroids: Four fused carbon rings; signaling molecules (e.g., cholesterol, hormones)

Phospholipids and Membranes

Phospholipids are amphipathic, possessing hydrophilic heads and hydrophobic tails, which enables them to form bilayers in aqueous environments.

• Bilayer formation: Basis of biological membranes

• Example: Cell membrane structure

Table: Lipid Types and Functions

Component	Examples	Functions
Fats	Triacylglycerols	Energy storage
Phospholipids	Membrane phospholipids	Membrane structure
Steroids	Cholesterol, hormones	Signaling, membrane fluidity

Nucleic Acids

Structure and Assembly

Nucleic acids are polymers of nucleotides, which consist of a nitrogenous base, a pentose sugar, and phosphate groups. The two main types are DNA and RNA.

• Nucleotide: Monomer unit; composed of a nitrogenous base (purine or pyrimidine), a sugar (ribose or deoxyribose), and phosphate group(s).

• Polynucleotide: Chain of nucleotides joined by phosphodiester bonds.

DNA and RNA

• DNA: Double-stranded; stores genetic information

• RNA: Single-stranded; involved in gene expression and information transfer

Base Pairing: Hydrogen bonds between complementary bases provide structure and enable information transfer.

• DNA base pairs: Adenine (A) with Thymine (T), Guanine (G) with Cytosine (C)

• RNA base pairs: Adenine (A) with Uracil (U), Guanine (G) with Cytosine (C)

Table: Nucleic Acid Types and Functions

Component	Examples	Functions
DNA	Deoxyribonucleic acid	Genetic information storage
RNA	Ribonucleic acid	Gene expression, information transfer

Summary of Learning Objectives

• Identify and describe macromolecules in terms of chemistry, structure, and function

• Describe the assembly, structure, and function of simple and complex carbohydrates

• Explain how lipid structure relates to function, with examples

• Describe the structures of nucleic acids and their assembly into functional polynucleotides (DNA and RNA)

• Describe each macromolecule as monomers and polymers as they appear in the cell

Additional info: Proteins are mentioned as a major class but not detailed in these notes; they will be covered in subsequent lectures.