Building Biological Molecules: Monomers, Polymers, and Carbohydrates

Building Biological Molecules: Monomers and Polymers; Carbohydrates

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.

Elements in Biological Molecules

Major Elements

Biological molecules consist mostly of four elements: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). Phosphorus and sulfur are also important for nucleic acids and proteins, respectively.

• Carbon: Forms the backbone of organic molecules due to its ability to form four covalent bonds.

• Hydrogen: Commonly bonded to carbon and oxygen in organic molecules.

• Oxygen: Essential for water and many organic compounds.

• Nitrogen: Found in amino acids and nucleotides.

Cell Synthesis of Large Organic Molecules

Cells synthesize large organic molecules by assembling smaller subunits (monomers) into larger structures (polymers). These processes occur in an aqueous (water-based) environment, with water both inside and outside the cell.

• Nucleotides are assembled into DNA and RNA.

• Amino acids are assembled into proteins.

• Sugars are assembled into polysaccharides.

Categories of Biological Molecules

Small Organic Building Blocks and Macromolecules

CHON (carbon, hydrogen, oxygen, nitrogen) combine in various ways to make four main categories of small organic molecules:

• Sugars → Polysaccharides, glycogen, and starch (in plants)

• Fatty acids → Fats and membrane lipids

• Amino acids → Proteins

• Nucleotides → Nucleic acids

Small organic building blocks are linked together by covalent bonds to make large organic macromolecules.

Macromolecules: Polymers and Monomers

Definition and Examples

• Polymers: Long molecules consisting of many similar building blocks linked end-to-end by covalent bonds.

• Monomers: Smaller, repeating molecules that serve as building blocks for polymers.

Three categories of organic macromolecules are polymers:

• Carbohydrates

• Nucleic acids

• Proteins

Note: 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

Synthesis of Polymers (Dehydration Reactions)

A dehydration reaction (also called condensation) occurs when two monomers bond together through the loss of a water molecule. This process is endergonic (requires energy) and is catalyzed by enzymes.

• Dehydration reactions are used to create covalent bonds in carbohydrates, fats, proteins, and nucleic acids.

• Each reaction produces a single water molecule.

Equation:

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

Breakdown of Polymers (Hydrolysis Reactions)

Hydrolysis is the process by which polymers are broken down into monomers by the addition of a water molecule. This process is exergonic (releases energy) and is also catalyzed by enzymes.

• Hydrolysis is used to break covalent bonds in carbohydrates, fats, proteins, and nucleic acids.

• Each reaction consumes a single water molecule.

Equation:

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

Role of Enzymes

• Enzymes catalyze both dehydration and hydrolysis reactions, making them occur more efficiently.

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

Water in Biochemical Reactions

• Dehydration reactions produce a water molecule derived from the atoms of the two molecules undergoing covalent bond formation.

• Hydrolysis reactions consume a water molecule from the environment to break a covalent bond between two molecules.

Carbohydrates: Structure and Function

Types of Carbohydrates

• Monosaccharides: Simple sugars (e.g., glucose, C6H12O6), the monomers of carbohydrates.

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., maltose).

• Polysaccharides: Long chains of monosaccharides (e.g., glycogen, starch, cellulose).

Structure of Sugars and Polysaccharides

• Glucose is a common monosaccharide with the formula C6H12O6.

• Maltose is a disaccharide formed from two glucose units.

• Polysaccharides can be branched (e.g., glycogen) or unbranched (e.g., cellulose).

Functions of Sugars in Cells

• Nutrition, energy: Glucose is broken down via cellular respiration to make ATP. Storage polysaccharides (glycogen in animals, starch in plants) store glucose for later use.

• Structural polysaccharides: Cellulose forms plant cell walls; chitin forms insect exoskeletons.

• Glycoproteins/glycolipids: Sugars are attached to proteins or lipids on cell surfaces, important for cell recognition (e.g., blood types).

Comparison Table: Types of Carbohydrates

Type	Structure	Example	Function
Monosaccharide	Single sugar unit	Glucose	Energy source
Disaccharide	Two sugar units	Maltose	Energy transport
Polysaccharide	Many sugar units	Glycogen, Starch, Cellulose	Energy storage, structural support

Summary Table: Small Organic Building Blocks and Macromolecules

Small Organic Building Block	Larger Organic Molecule
Sugars	Polysaccharides, glycogen, starch
Fatty acids	Fats, membrane lipids
Amino acids	Proteins
Nucleotides	Nucleic acids

Additional info:

• Carbohydrates that function in energy storage (e.g., starch, glycogen) are typically branched and easily hydrolyzed, while those that provide structural support (e.g., cellulose, chitin) are unbranched and form rigid structures due to hydrogen bonding between chains.

• Glycosidic bonds are the covalent bonds that link monosaccharides in disaccharides and polysaccharides.