Organic Molecules: Structure, Function, and Biological Importance

Organic Molecules in Biology

Introduction to Organic Molecules

Organic molecules are the fundamental building blocks of cells and living organisms. They are large, complex molecules composed of smaller repeating subunits called monomers. The four main classes of organic molecules in biology are proteins, carbohydrates, lipids, and nucleic acids.

• Four elements that bond forming organic molecules: Carbon, Nitrogen, Oxygen, Hydrogen

• Four main classes of organic molecules: Proteins, Carbohydrates, Lipids, Nucleic acids (e.g., DNA, RNA, ATP)

Monomers and Polymers

Most organic molecules are polymers, which are long chains of monomers joined together. The process of joining monomers to form polymers is called polymerization, and in biological systems, this often occurs via dehydration synthesis.

• Monomer: A small molecule that can bind to others to form a polymer (e.g., amino acid).

• Polymer: A large molecule made up of repeating monomer units.

• Polymerization: The process of joining monomers via covalent bonds to form a polymer.

• Dehydration synthesis: A chemical reaction in which a water molecule is removed to form a covalent bond between monomers: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{H}_2\text{O}$

• Hydrolysis: The reverse process, where a water molecule is added to break a covalent bond, splitting a polymer into monomers.

Summary Table: Four Major Organic Molecules

The following table summarizes the main types of organic molecules, their monomers, covalent bonds, and functions:

Polymer (organic molecule)	Monomer	Covalent bond	Functions
Protein	Amino acid (20 types)	Peptide bond	Enzymes Structure (cellular, anatomical) Membrane proteins
Carbohydrate	Monosaccharide	Glycosidic linkage	Energy (e.g., ATP) Structure (cell wall in plants, chitin in arthropods/fungi)
Nucleic acids (DNA, RNA)	Nucleotide	Phosphodiester bond	Genetic information (heredity) Cell control (gene expression) Energy (ATP)
Lipids	Monomers can include hydrocarbon chains	Various covalent bonds	Waxes Hormones Adipose tissue (energy storage) Cholesterol Phospholipids (main component of plasma membrane)

Proteins

Amino Acids: Structure and Properties

Proteins are polymers made of amino acid monomers. There are 20 standard amino acids, each with a central carbon atom bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a variable side chain (R group).

• Amino group (NH2): Basic functional group

• Carboxyl group (COOH): Acidic functional group

• R group: Variable side chain that determines the properties of each amino acid

General structure of an amino acid:

$\mathrm{H_2N{-}CHR{-}COOH}$

Classification of Amino Acids

• Electrically charged side chains: Can be positive (basic) or negative (acidic)

• Polar uncharged side chains: Hydrophilic, form hydrogen bonds

• Nonpolar side chains: Hydrophobic, often found in the interior of proteins

• Special cases: Glycine, proline, cysteine (unique properties)

Peptide Bond Formation (Dehydration Synthesis)

Amino acids are joined by peptide bonds through dehydration synthesis, releasing a water molecule:

$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + \text{H}_2\text{O}$

Levels of Protein Structure

• Primary structure: Sequence of amino acids in a polypeptide chain

• Secondary structure: Local folding into alpha helices and beta sheets, stabilized by hydrogen bonds

• Tertiary structure: Overall 3D shape formed by interactions between R groups

• Quaternary structure: Association of multiple polypeptide chains to form a functional protein

Example: Hemoglobin is a quaternary protein made of four polypeptide subunits, responsible for oxygen transport in blood.

Nucleic Acids

Structure and Function

Nucleic acids, such as DNA and RNA, store and transmit genetic information. Their monomers are nucleotides, each composed of a phosphate group, a five-carbon sugar, and a nitrogenous base.

• DNA: Contains deoxyribose sugar; bases are adenine (A), thymine (T), cytosine (C), guanine (G)

• RNA: Contains ribose sugar; bases are adenine (A), uracil (U), cytosine (C), guanine (G)

Phosphodiester bonds link nucleotides together to form the nucleic acid polymer.

Functions of Nucleic Acids

• Storage and transmission of genetic information (DNA, RNA)

• Energy transfer (ATP)

• Cell signaling (cAMP)

Carbohydrates

Structure and Function

Carbohydrates are organic molecules composed of monosaccharide monomers. They serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose)

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

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

Glycosidic linkages are covalent bonds that join monosaccharides in carbohydrates.

Functions of Carbohydrates

• Short-term energy storage (glucose, glycogen)

• Structural support (cellulose in plants, chitin in fungi and arthropods)

Lipids

Structure and Function

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. Their monomers can include fatty acids and glycerol, and they are joined by various covalent bonds.

• Types of lipids: Fats, oils, waxes, phospholipids, steroids

• Phospholipids: Major component of cell membranes

• Functions: Energy storage, insulation, cell membrane structure, hormones

Example: Phospholipids form the bilayer of plasma membranes, providing a barrier and functional interface for cells.

Summary Table: Organic Molecules (Blank for Practice)

Polymer (organic molecule)	Monomer	Covalent bond	Functions

Key Terms and Concepts

• Monomer: Small subunit that can join with others to form a polymer

• Polymer: Large molecule made of repeating monomers

• Dehydration synthesis: Reaction that joins monomers by removing water

• Hydrolysis: Reaction that breaks polymers by adding water

• Peptide bond: Covalent bond between amino acids in proteins

• Glycosidic linkage: Covalent bond between monosaccharides in carbohydrates

• Phosphodiester bond: Covalent bond between nucleotides in nucleic acids

Additional info: This guide expands on the provided notes by including definitions, examples, and a more detailed explanation of the structure and function of each class of organic molecule, as well as the chemical processes involved in their synthesis and breakdown.