Small Molecules and the Chemistry of Life

Introduction to Chemical Elements in Living Systems

Living organisms are composed of a limited set of chemical elements, with only a few making up the majority of biological matter. These elements interact through chemical bonds to form the molecules essential for life.

• Key Elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S) are the most abundant in living systems.

• Trace Elements: Elements such as iron (Fe), magnesium (Mg), and zinc (Zn) are required in smaller amounts but are vital for biological processes.

• Periodic Table: The arrangement of elements highlights their abundance and roles in biological systems.

Atoms, Molecules, and Macromolecules

Atoms are the fundamental units of matter, which combine to form molecules. Biological macromolecules are large, complex molecules essential for life, constructed from smaller subunits.

• Atoms: Consist of protons, neutrons, and electrons.

• Molecules: Formed by chemical bonds between atoms.

• Macromolecules: Include proteins, nucleic acids, carbohydrates, and lipids.

Chemical Bonds in Biological Molecules

Types of Chemical Bonds

Chemical bonds hold atoms together in molecules and determine the structure and function of biological compounds.

• Covalent Bonds: Atoms share electrons. Can be non-polar (equal sharing) or polar (unequal sharing).

• Ionic Bonds: Electrons are transferred from one atom to another, creating charged ions.

• Hydrogen Bonds: Weak attractions between a hydrogen atom and an electronegative atom (e.g., O or N).

• Van der Waals Interactions: Weak, transient interactions due to temporary dipoles.

• Hydrophobic Interactions: Nonpolar molecules aggregate to avoid water.

Polar and Nonpolar Covalent Bonds

The polarity of a covalent bond depends on the difference in electronegativity between the bonded atoms.

• Nonpolar Covalent Bonds: Electrons are shared equally (e.g., H2).

• Polar Covalent Bonds: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

Hydrophilic and Hydrophobic Compounds

Interactions with Water

Compounds are classified based on their affinity for water:

• Hydrophilic (Water-loving): Polar or charged molecules that dissolve easily in water.

• Hydrophobic (Water-fearing): Nonpolar molecules that do not dissolve in water and tend to aggregate.

Water: Structure, Properties, and Biological Importance

Structure and Polarity of Water

The water molecule (H2O) is polar due to the difference in electronegativity between hydrogen and oxygen, and its bent shape allows for hydrogen bonding.

Hydrogen Bonding in Water

Hydrogen bonds between water molecules are responsible for many of water's unique properties.

• Cohesion: Water molecules stick together, aiding transport in plants.

• High Specific Heat: Water resists temperature changes, stabilizing environments.

• High Heat of Vaporization: Evaporation of water cools surfaces.

• Lower Density of Ice: Ice floats, insulating aquatic life in winter.

• Excellent Solvent: Dissolves many substances, facilitating biochemical reactions.

Acids, Bases, and pH

Acids, Bases, and the pH Scale

Acids increase the concentration of hydrogen ions (H+) in solution, while bases decrease it. The pH scale measures the acidity or basicity of a solution.

• pH Formula:

• Neutral Solution: [H+] = [OH-] = M, pH = 7

• Acidic Solution: pH < 7

• Basic Solution: pH > 7

• Relationship:

Buffers

Buffers are substances that minimize changes in pH by absorbing or releasing H+ ions. They are crucial for maintaining stable pH in biological systems.

Functional Groups and Isomerism in Biomolecules

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical properties and reactions of those molecules.

Isomers

Isomers are molecules with the same molecular formula but different structures, leading to different properties.

• Structural Isomers: Differ in the covalent arrangement of atoms.

• Cis-Trans Isomers: Differ in spatial arrangement around double bonds.

• Optical Isomers (Enantiomers): Mirror images due to chiral carbons.

Biological Macromolecules

Overview and Classification

Macromolecules are large, complex molecules essential for life, including carbohydrates, proteins, nucleic acids, and lipids. Their structure and function are determined by their monomeric subunits and functional groups.

Formation and Breakdown of Macromolecules

Macromolecules are synthesized by condensation (dehydration) reactions and broken down by hydrolysis.

• Condensation: Monomers join, releasing water.

• Hydrolysis: Polymers are split into monomers by adding water.

Proteins: Structure and Function

Amino Acids and Peptide Bonds

Proteins are polymers of amino acids linked by peptide bonds. Each amino acid contains an amino group, a carboxyl group, a hydrogen atom, and a variable R group attached to a central carbon.

• Ionization: At physiological pH, amino acids exist as zwitterions (dipolar ions).

• Classification: Amino acids can be hydrophobic or hydrophilic based on their R groups.

Levels of Protein Structure

Proteins have four levels of structure:

• Primary: Sequence of amino acids.

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

• Tertiary: Overall 3D shape, stabilized by various interactions (hydrogen bonds, disulfide bridges, hydrophobic interactions).

• Quaternary: Association of multiple polypeptide chains.

Protein Denaturation

Denaturation is the loss of a protein's native structure due to external stress (e.g., heat, pH changes), resulting in loss of function. Primary structure remains intact, but secondary, tertiary, and quaternary structures are disrupted.

Carbohydrates: Structure and Classification

Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates are organic molecules with the general formula (CH2O)n. They are classified based on the number of sugar units:

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

• Disaccharides: Two monosaccharides linked by glycosidic bonds (e.g., sucrose, lactose).

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

Aldoses and Ketoses

Monosaccharides are further classified by the presence of an aldehyde (aldose) or ketone (ketose) group.

Lipids: Structure and Types

Major Classes of Lipids

Lipids are hydrophobic molecules with diverse structures and functions.

• Triacylglycerols (Triglycerides): Main energy storage molecules, composed of glycerol and three fatty acids.

• Phospholipids: Major components of cell membranes, with hydrophilic heads and hydrophobic tails.

• Steroids: Four fused carbon rings; include cholesterol and hormones.

Saturated vs. Unsaturated Fatty Acids

Saturated fatty acids have no double bonds and are solid at room temperature; unsaturated fatty acids have one or more double bonds and are liquid at room temperature.

Nucleic Acids: DNA and RNA

Structure and Function

Nucleic acids store and transmit genetic information. They are polymers of nucleotide subunits, each consisting of a sugar, phosphate group, and nitrogenous base.

• DNA: Double-stranded, contains deoxyribose, bases A, T, C, G.

• RNA: Single-stranded, contains ribose, bases A, U, C, G.

• Base Pairing: Complementary bases are held together by hydrogen bonds.