Building Biological Molecules: Monomers, Polymers, and Carbohydrates

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Building Biological Molecules: Monomers and Polymers

Introduction to Biological Macromolecules

Cells are composed of large organic molecules known as macromolecules, which are essential for structure and function. These macromolecules are primarily constructed from a small set of elements and are organized into polymers built from repeating monomer units.

Key Elements: Biological molecules consist mostly of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). Phosphorus (P) and sulfur (S) are also important for nucleic acids and proteins.
Macromolecules: The four main families are carbohydrates, lipids, proteins, and nucleic acids.
Monomers: Small, repeating units that serve as building blocks for polymers.
Polymers: Long molecules consisting of many similar building blocks linked end-to-end by covalent bonds.

Major Categories of Biological Macromolecules

CHON elements combine in various ways to make four main categories of small organic molecules, which are then assembled into larger macromolecules.

Small Organic Building Block	Larger Organic Molecule
Sugars	Polysaccharides, Glycogen, Starch (in plants)
Fatty Acids	Fats, Membrane Lipids
Amino Acids	Proteins
Nucleotides	Nucleic Acids

Additional info: Fatty acids are not polymerized to make fats and lipids in the same "end-to-end" fashion as the other monomers.

Synthesis and Breakdown of Biological Macromolecules

Polymerization: Dehydration Reactions

Polymerization is the process by which monomers are joined to form polymers. This typically occurs through dehydration reactions, which remove a water molecule to form a covalent bond between monomers.

Dehydration Reaction: A chemical reaction that joins two monomers by removing a water molecule.
Enzymes: Biological catalysts that accelerate dehydration reactions, making them more efficient.
Equation:

Note: Matter is not created or destroyed during bond formation; atoms are simply rearranged.

Breakdown: Hydrolysis Reactions

Macromolecules are broken down into monomers by hydrolysis reactions, which are the reverse of dehydration reactions. Hydrolysis adds a water molecule to break covalent bonds.

Hydrolysis Reaction: A chemical reaction that breaks a covalent bond in a polymer by adding a water molecule.
Enzymes: Catalyze hydrolysis reactions, increasing their efficiency.
Equation:

Note: Hydrolysis reactions are exergonic but require enzymatic assistance to occur efficiently.

Role of Water in Biochemical Reactions

Water molecules are consumed and produced in many common biochemical reactions, especially during the synthesis and breakdown of polymers.

Dehydration: Produces a water molecule from the atoms of the two molecules undergoing covalent bond formation.
Hydrolysis: Consumes a water molecule from the environment to break a covalent bond between two molecules.

Carbohydrates: Structure and Function

Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen. They exist as monosaccharides (simple sugars), disaccharides (two monosaccharides joined), and polysaccharides (long chains of monosaccharides).

Monosaccharides: The simplest carbohydrates, such as glucose (C6H12O6).
Disaccharides: Formed by joining two monosaccharides, e.g., maltose (glucose + glucose).
Polysaccharides: Long chains of monosaccharides, e.g., glycogen, starch, and cellulose.

Structure of Sugars and Polysaccharides

The structure of carbohydrates determines their function in cells. Monosaccharides can be linked to form oligosaccharides and polysaccharides, which may be branched or unbranched.

Glucose: A common monosaccharide used for energy.
Maltose: A disaccharide formed from two glucose units.
Polysaccharides: Can be branched (e.g., glycogen) or unbranched (e.g., cellulose).

Functions of Sugars in Cells

Sugars serve multiple roles in cellular metabolism and structure.

Nutrition/Energy: Glucose is broken down via cellular respiration to make ATP. Storage polysaccharides include glycogen (animals) and starch (plants).
Structural Polysaccharides: Cellulose forms plant cell walls; chitin forms insect exoskeletons.
Glycoproteins/Glycolipids: Sugars are often attached to proteins or lipids, playing roles in cell recognition and signaling (e.g., blood types).

Example: Glycogen is a highly branched polysaccharide that serves as a major energy storage molecule in animals.

Comparison of Carbohydrates

Type	Structure	Function	Example
Monosaccharide	Single sugar unit	Energy source	Glucose
Disaccharide	Two sugar units	Transport, energy	Maltose
Polysaccharide	Many sugar units	Storage, structure	Glycogen, Starch, Cellulose

Summary Table: Biological Macromolecules

Macromolecule	Monomer	Polymer	Main Function
Carbohydrate	Sugar (monosaccharide)	Polysaccharide	Energy, structure
Protein	Amino acid	Polypeptide	Catalysis, structure, signaling
Nucleic Acid	Nucleotide	DNA/RNA	Genetic information
Lipid	Fatty acid	Fats, membrane lipids	Energy storage, membranes

Key Terms and Definitions

Monomer: A small molecule that can join with others to form a polymer.
Polymer: A large molecule made up of repeating monomer units.
Dehydration Reaction: A chemical reaction that joins two molecules by removing water.
Hydrolysis Reaction: A chemical reaction that breaks a molecule by adding water.
Enzyme: A protein that speeds up chemical reactions in cells.
Carbohydrate: An organic molecule composed of carbon, hydrogen, and oxygen; includes sugars and polysaccharides.

Learning Objectives

Compare the monomer subunit, bond responsible for polymerization, and important biological function(s) observed in proteins, nucleic acids, and carbohydrates.
Describe dehydration and hydrolysis reactions in the synthesis and breakdown of polymers.
Recognize the general structure of a monosaccharide, disaccharide, and polysaccharide and give an example of each.
Explain the fundamental differences between carbohydrates that function in energy storage and those that provide structural support.