Biomolecules: Structure, Function, and Classification

Biomolecules

Introduction to Biomolecules

Biomolecules are organic molecules essential for the structure and function of living organisms. There are four primary classes of biomolecules, each with distinct roles in biological systems.

• Carbohydrates

• Proteins

• Nucleic Acids

• Lipids

These molecules are often organized as polymers, which are long chains of repeating subunits called monomers.

Monomers and Polymers

Monomers are the individual building blocks that can be repetitively linked to form polymers. The type of monomer determines the type of polymer and its biological function.

• Carbohydrates: Monomer = Monosaccharide

• Proteins: Monomer = Amino acid

• Nucleic Acids: Monomer = Nucleotide

• Lipids: Do not have true monomers, but are often formed from fatty acids and glycerol.

Polymers are formed by covalently bonding monomers together, often through dehydration synthesis reactions.

Building and Breaking Down Polymers

• Dehydration Synthesis: Forms covalent bonds between monomers by removing a water molecule, resulting in polymer formation.

• Hydrolysis: Breaks covalent bonds in polymers by adding a water molecule, resulting in monomer release.

Example Equation:

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

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

Carbohydrates

Structure and Classification

Carbohydrates are carbon-based molecules hydrated with many hydroxyl groups (–OH). They are commonly referred to as sugars and have the general formula $(CH_2O)_n$.

• Monosaccharides: Single sugar units (e.g., glucose, fructose)

• Oligosaccharides: 2–20 covalently linked monosaccharides

• Polysaccharides: More than 20 covalently linked monosaccharides (e.g., starch, glycogen, cellulose)

Carbohydrates can be simple (monosaccharides) or complex (oligo- and polysaccharides).

Formation and Breakdown of Polysaccharides

• Dehydration Synthesis: Links monosaccharides together to form polysaccharides via glycosidic bonds.

• Hydrolysis: Breaks down polysaccharides into individual monosaccharides.

Example: Formation of maltose from two glucose molecules:

$\text{Glucose} + \text{Glucose} \xrightarrow{\text{Dehydration}} \text{Maltose} + H_2O$

Functions of Carbohydrates

Carbohydrates serve two main functions in living organisms:

• Structural Support: Building materials for cell walls and exoskeletons (e.g., cellulose in plants, chitin in arthropods).

• Energy Storage: Short- and long-term storage of energy (e.g., starch in plants, glycogen in animals).

Function	Polysaccharides in Plants	Polysaccharides in Animals
Structural Support	Cellulose (cell walls)	Chitin (exoskeletons of insects and crustaceans)
Energy Storage	Starch (storage form of glucose)	Glycogen (storage form of glucose)

Proteins

Structure and Formation

Proteins are polymers made of amino acid monomers. Amino acids are linked by covalent peptide bonds, forming polypeptide chains with directionality (N-terminal to C-terminal).

• Monomer: Amino acid

• Bond: Peptide bond

Example: Formation of a protein from amino acids:

$\text{Amino Acid}_1 + \text{Amino Acid}_2 \xrightarrow{\text{Dehydration}} \text{Dipeptide} + H_2O$

Amino Acid Structure

Each amino acid contains:

• A central carbon atom (alpha carbon)

• An amino group (–NH2)

• A carboxyl group (–COOH)

• A hydrogen atom

• A unique side chain (R group)

There are 20 different amino acids, each with a distinct R group, leading to diverse protein structures and functions.

Summary Table: Biomolecule Classes

Class	Monomer	Polymer	Bond Type	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Glycosidic bond	Energy storage, structural support
Proteins	Amino acid	Polypeptide	Peptide bond	Enzymes, structure, transport, signaling
Nucleic Acids	Nucleotide	DNA/RNA	Phosphodiester bond	Genetic information storage and transfer
Lipids	Fatty acids, glycerol	Triglycerides, phospholipids	Ester bond	Energy storage, membrane structure

Practice and Application

• Dehydration synthesis creates polymers from monomers; hydrolysis breaks polymers into monomers.

• Carbohydrates are classified by the number of sugar units: monosaccharides, oligosaccharides, polysaccharides.

• Polysaccharides serve as energy storage (starch, glycogen) or structural support (cellulose, chitin).

• Proteins are made of amino acids, each with a central carbon, amino group, carboxyl group, and unique R group.

Additional info: Nucleic acids (DNA and RNA) are also polymers, composed of nucleotide monomers, and are responsible for storing and transmitting genetic information. Lipids, while not true polymers, are essential for membrane structure and energy storage.