Organic Molecules and Macromolecules: Properties, Structure, and Function

Organic Molecules and Macromolecules

Introduction

Organic molecules are the foundation of biological systems, composed primarily of carbon atoms bonded with other elements. Macromolecules, which are large, complex molecules, play essential roles in the structure and function of living organisms. This guide covers the properties of organic molecules, the significance of carbon, functional groups, isomerism, and the major classes of biological macromolecules.

Properties of Molecules

Carbon and Organic Molecules

• Carbon is a versatile element that forms the backbone of organic molecules due to its ability to form four covalent bonds.

• Organic molecules are compounds containing carbon atoms bonded to hydrogen and often other elements such as oxygen, nitrogen, sulfur, and phosphorus.

• The molecular shape of organic molecules is determined by the arrangement of atoms and the types of bonds formed.

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity. They are critical in determining the behavior of organic molecules in biological systems.

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, some amino acids
Methyl	R–CH3	May be attached to DNA, proteins, carbohydrates
Phosphate	R–O–PO32−	Nucleic acids, ATP, attached to amino acids
Sulfate	R–SO4−	May be attached to carbohydrates, proteins, lipids
Sulfhydryl	R–SH	Proteins containing cysteine

Isomers

Isomers are molecules with the same molecular formula but different structures or spatial arrangements, resulting in different properties.

• Structural isomers: Differ in the covalent arrangement of atoms. Example: n-propyl alcohol and 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 their different interactions with enzymes and receptors.

Atomic Structure and Bonding

Electron Shells and Orbitals

• Atoms consist of a nucleus surrounded by electrons in energy shells (orbitals).

• The first shell holds up to 2 electrons; the second shell holds up to 8 electrons in various orbitals.

• Electron configuration influences the bonding behavior of atoms.

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, resulting in partial charges.

Macromolecules

Overview

Macromolecules are large, complex molecules essential for life. They are typically polymers, made by joining smaller units called monomers.

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

• Roles include energy storage, structural support, catalysis, transport, protection, regulation, homeostasis, movement, growth, development, 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 types of reactions.

Proteins

Structure and Function

• Proteins are polymers of amino acids linked by peptide bonds.

• Functions include catalysis (enzymes), structural support, transport, signaling, and defense.

• Each amino acid has an amino group, carboxyl group, hydrogen atom, and a unique side chain (R group) attached to a central carbon (α-carbon).

Amino Acids and R Groups

• There are 20 common amino acids, each with distinct properties based on their R groups.

• Amino acids can be classified as nonpolar, polar, or electrically charged (acidic or basic).

• Stereoisomerism: Most amino acids (except glycine) exist as L- and D- isomers; only L-amino acids are found in proteins.

Levels of Protein Structure

• Primary structure: Linear sequence of amino acids.

• Secondary structure: Local folding patterns such as α-helix and β-pleated sheet, stabilized by hydrogen bonds.

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

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

Protein Shape and Denaturation

• Protein function depends on correct folding.

• Denaturation is the loss of structure (and function) due to heat, pH, or chemicals.

• Chaperonins and heat shock proteins assist in proper folding.

Carbohydrates

Structure and Types

• Carbohydrates are composed of carbon, hydrogen, and oxygen, typically with the formula $(CH_2O)_n$.

• Categories: Monosaccharides (simple sugars), disaccharides, oligosaccharides, and polysaccharides.

Monosaccharides

• Single sugar units such as glucose, fructose, and galactose.

• Can exist in linear or ring forms; isomerism is common (e.g., α- and β-glucose).

• Pentoses (5C) and hexoses (6C) are common in biology.

Disaccharides and Oligosaccharides

• Disaccharides (e.g., sucrose, lactose, maltose) are formed by glycosidic linkages between two monosaccharides via condensation reactions.

• Oligosaccharides (3–20 units) often attach to proteins and lipids, playing roles in cell recognition (e.g., ABO blood groups).

Polysaccharides

• Long chains of monosaccharides joined by glycosidic bonds.

• Starch: Storage polysaccharide in plants (α-1,4 linkages).

• Glycogen: Storage polysaccharide in animals (branched).

• Cellulose: Structural polysaccharide in plants (β-1,4 linkages, unbranched).

• Functional group modifications (e.g., amino sugars) alter properties and functions.

Lipids

Structure and Types

• Lipids are hydrophobic molecules with nonpolar covalent bonds.

• Not true polymers, but diverse in structure and function.

• Aggregate away from water due to hydrophobic effect.

Fats and Oils (Triglycerides)

• Composed of three fatty acids and one glycerol molecule.

• 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

• Consist of two fatty acid tails (hydrophobic) and a phosphate-containing head (hydrophilic) attached to glycerol.

• Form bilayers in aqueous environments, fundamental to biological membranes.

Other Lipids

• Carotenoids: Pigments in plants, precursors to vitamin A in animals.

• Steroids: Four-ring structure; includes cholesterol (membrane component) and hormones.

Nucleic Acids

Structure and Function

• Nucleic acids (DNA and RNA) store and transmit genetic information.

• Polymers of nucleotides, each consisting of a nitrogenous base, a pentose sugar (ribose or deoxyribose), and a phosphate group.

• Nucleosides are nucleotides without the phosphate group.

Nitrogenous Bases

• Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA).

• Purines: Adenine (A), Guanine (G).

DNA Structure

• Double-stranded helix with antiparallel strands (5' to 3' and 3' to 5').

• Complementary base pairing: A with T, G with C.

• Backbone formed by phosphodiester linkages.

RNA Structure

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

• Three main differences from DNA: ribose vs deoxyribose, uracil vs thymine, single vs double strand.

Other Roles of Nucleotides

• ATP (adenosine triphosphate): Main energy currency of the cell.

• GTP (guanosine triphosphate): Involved in signaling and protein synthesis.

• cAMP (cyclic adenosine monophosphate): Second messenger in signal transduction.