Organic Molecules and Macromolecules: Properties, Structure, and Function

Organic Molecules and Macromolecules

Introduction

Organic molecules and macromolecules are fundamental to the structure and function of living organisms. This guide covers the properties of carbon-based molecules, the diversity of functional groups, isomerism, and the major classes of biological macromolecules.

Properties of Molecules

Carbon and Organic Molecules

• Carbon is the backbone of organic molecules due to its ability to form four covalent bonds, allowing for diverse molecular structures.

• Organic molecules are compounds primarily composed of carbon atoms bonded to hydrogen, oxygen, nitrogen, and other elements.

• Molecular shape is determined by the arrangement of atoms and the types of bonds formed, influencing the molecule's function.

Functional Groups

Functional groups are specific groups of atoms within molecules that confer characteristic chemical properties and reactivity.

Functional Group	Formula	Examples of Where Found
Amino	R–NH2	Amino acids (proteins)
Carbonyl (Ketone)	R–CO–R	Steroids, waxes, proteins
Carbonyl (Aldehyde)	R–CHO	Sugars, some proteins
Carboxyl	R–COOH	Amino acids, fatty acids
Hydroxyl	R–OH	Alcohols, carbohydrates
Methyl	R–CH3	DNA, proteins, carbohydrates
Phosphate	R–O–PO32−	Nucleic acids, ATP
Sulfate	R–SO4−	Carbohydrates, proteins, lipids
Sulfhydryl	R–SH	Cysteine (amino acid)

Isomers

Isomers are molecules with the same molecular formula but different structures or spatial arrangements.

• Structural isomers: Differ in the covalent arrangement of atoms. Example: n-propyl alcohol vs. isopropyl alcohol.

• Stereoisomers: Same covalent arrangement but differ in spatial orientation.

  • Geometric isomers: Differ in arrangement around a double bond (cis/trans).

  • Enantiomers: Mirror-image isomers, important in biological systems due to enzyme specificity.

Bond Polarity

• C–H and C–C bonds are generally nonpolar due to similar electronegativities.

• C–O bonds are polar because oxygen is more electronegative than carbon, leading to partial charges.

Macromolecules

Overview

Macromolecules are large, complex molecules essential for life, typically formed by polymerization of smaller subunits called monomers.

• Four major types: proteins, carbohydrates, lipids, nucleic acids

• Roles include energy storage, structural support, catalysis, transport, defense, regulation, homeostasis, movement, and heredity.

Polymerization and Depolymerization

• Condensation (dehydration) reactions: Join monomers by removing water, forming covalent bonds.

• Hydrolysis reactions: Break polymers into monomers by adding water.

• Enzymes catalyze both processes.

Proteins

Structure and Function

Proteins are polymers of amino acids with diverse structures and functions, including catalysis (enzymes), transport, and structural roles.

• Amino acids: Contain an amino group, carboxyl group, hydrogen atom, and variable R group attached to a central carbon.

• Peptide bonds: Formed by condensation between amino and carboxyl groups, linking amino acids into polypeptides.

• Levels of protein structure:

  • Primary: Linear sequence of amino acids.

  • Secondary: Local folding (α-helix, β-pleated sheet) stabilized by hydrogen bonds.

  • Tertiary: Three-dimensional shape formed by interactions among R groups.

  • Quaternary: Association of multiple polypeptide subunits (e.g., hemoglobin).

• Denaturation: Loss of structure and function due to environmental changes (e.g., heat, pH).

• Chaperonins: Proteins that assist in proper folding of other proteins.

Carbohydrates

Structure and Types

Carbohydrates are carbon-based molecules with hydrogen and hydroxyl groups, serving as energy sources and structural components.

• General formula: $C_n(H_2O)_n$

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

• Disaccharides: Two monosaccharides joined by glycosidic linkage (e.g., sucrose, lactose, maltose).

• Oligosaccharides: Short chains (3–20) of monosaccharides, often attached to proteins/lipids (e.g., ABO blood groups).

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

Isomerism in Carbohydrates

• Structural isomers: Different arrangement of atoms (e.g., glucose vs. galactose).

• Stereoisomers: Same formula, different spatial arrangement (e.g., α- and β-glucose).

Glycosidic Linkages

• Formed by condensation reactions between hydroxyl groups of monosaccharides.

• Types: α-1,4 (starch, glycogen), β-1,4 (cellulose).

Lipids

Structure and Function

Lipids are hydrophobic molecules with diverse structures, not true polymers, and serve as energy storage, membrane components, and signaling molecules.

• Fats and oils (triglycerides): Glycerol + three fatty acids.

• Saturated fatty acids: No double bonds, straight chains, solid at room temperature.

• Unsaturated fatty acids: One or more double bonds, kinks in chains, liquid at room temperature.

• Phospholipids: Glycerol, two fatty acids, phosphate group; form bilayers in membranes due to hydrophilic heads and hydrophobic tails.

• Other lipids: Steroids (e.g., cholesterol), carotenoids (e.g., β-carotene), and waxes.

Nucleic Acids

Structure and Function

Nucleic acids store and transmit genetic information. Two main types are DNA and RNA.

• Nucleotides: Monomers composed of a nitrogenous base, pentose sugar (ribose or deoxyribose), and phosphate group.

• DNA: Double-stranded, antiparallel, with complementary base pairing (A–T, G–C), right-handed double helix.

• RNA: Single-stranded, contains ribose and uracil instead of thymine.

• Phosphodiester bonds: Link nucleotides in a chain.

• Other nucleotide roles: ATP (energy), GTP (signaling), cAMP (regulation).

Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Double-stranded	Single-stranded

Summary

• Organic molecules are based on carbon and exhibit diverse structures due to functional groups and isomerism.

• Macromolecules—proteins, carbohydrates, lipids, and nucleic acids—are essential for life and have unique structures and functions.

• Understanding the chemistry of these molecules is foundational for studying biology at the molecular and cellular levels.