Biomolecules: Structure, Function, and Classification

Biomolecules

Introduction to Biomolecules

Biomolecules are organic molecules essential for life, forming the structural and functional basis of all living organisms. There are four primary classes of biomolecules:

• Carbohydrates

• Proteins

• Nucleic Acids

• Lipids

Each class has unique structures and functions, contributing to the complexity and diversity of life.

Monomers and Polymers

Monomers and Polymerization

Most biomolecules are polymers, which are long chains of repeating units called monomers. The process of linking monomers to form polymers is called polymerization.

• Monomer: The basic building block that can be joined to others to form a polymer.

• Polymer: A large molecule composed of many monomers linked together.

• Different biomolecule classes use different monomers:

  • Carbohydrates: Monosaccharides

  • Proteins: Amino acids

  • Nucleic Acids: Nucleotides

  • Lipids: Do not form true polymers, but are often composed of fatty acids and glycerol.

Building and Breaking Down Polymers

• Dehydration (Condensation) Reaction: Forms covalent bonds between monomers by removing a water molecule, resulting in polymer formation.

• Hydrolysis Reaction: 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 rich in hydroxyl groups (–OH) and are commonly referred to as "sugars." Their general formula is often $C_n(H_2O)_n$, though actual ratios may vary.

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

• Oligosaccharides: Short chains of 2–20 monosaccharides.

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

Formation and Breakdown of Polysaccharides

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

• Hydrolysis: Breaks glycosidic bonds, releasing 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:

• Structural Support: Building materials for cells (e.g., cellulose in plants, chitin in arthropods).

• Energy Storage: Short- and long-term energy reserves (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. The covalent bonds linking amino acids are called peptide bonds. Proteins have directionality, with an amino (N-) terminal and a carboxyl (C-) terminal.

Example: Formation of a protein from amino acids:

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

Amino Acids

Amino acids are the monomers of proteins. Each amino acid contains:

• A central carbon atom (α-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, giving rise to protein diversity.

Component	Description
Central Carbon	α-carbon atom
Amino Group	–NH2 (N-terminal)
Carboxyl Group	–COOH (C-terminal)
Hydrogen Atom	Attached to α-carbon
R Group	Variable side chain

Summary Table: Classes of Biomolecules

Class	Monomer	Polymer	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structural support
Proteins	Amino acid	Polypeptide/protein	Enzymatic, structural, transport, regulatory
Nucleic Acids	Nucleotide	DNA/RNA	Genetic information storage and transfer
Lipids	Fatty acids, glycerol	Not true polymers	Energy storage, membrane structure, signaling

Practice and Application

• Dehydration reactions build polymers by removing water; hydrolysis reactions break polymers by adding water.

• Glycosidic bonds link monosaccharides in carbohydrates; peptide bonds link amino acids in proteins.

• Cellulose provides structural support in plants; chitin does so in arthropods.

• Starch and glycogen are energy storage polysaccharides in plants and animals, respectively.

Additional info: Nucleic acids and lipids are mentioned in the overview but not detailed in the provided content. For a complete study, students should also review the structure and function of DNA, RNA, and various lipid types (e.g., phospholipids, triglycerides).