Chemistry of Life: Foundations for General Biology

Chemistry of Life

Introduction

The chemistry of life is fundamental to understanding biological processes. This section covers the chemical basis of life, focusing on water, carbon, isomers, functional groups, and the formation of biological macromolecules. These concepts are essential for grasping how molecular interactions underpin cellular structure and function.

Water: The Wonder Molecule

Structure and Properties

• Polar Covalent Bonds: Water molecules have a bent shape and unequal sharing of electrons, resulting in partial charges ($\delta^+$ on hydrogen, $\delta^-$ on oxygen).

• Hydrogen Bonds: Weak attractions between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.

Emergent Properties of Water

• Cohesion: Water molecules stick together due to hydrogen bonding, creating surface tension (e.g., insects walking on water).

• Adhesion: Water clings to other substances, aiding processes like capillary action in plants.

• Expansion Upon Freezing: At 0°C, water forms a crystalline lattice, making ice less dense than liquid water. This insulates aquatic environments.

• Moderation of Temperature: Water has a high specific heat capacity, absorbing or releasing heat with minimal temperature change. This stabilizes climates and internal body temperatures.

• Versatility as a Solvent: Water dissolves many substances due to its polarity, forming hydration shells around ions and polar molecules.

Example: Salt (NaCl) dissolves in water as Na+ and Cl- ions are surrounded by water molecules.

Carbon: The Backbone of Life

Unique Properties of Carbon

• Valence Electrons: Carbon has four valence electrons, allowing it to form up to four covalent bonds.

• Complex Skeletons: Carbon atoms can bond to each other, forming chains, rings, and branched structures.

Example: Glucose, DNA, and proteins all have carbon-based backbones.

The Arrangement Matters: Isomers

Types of Isomers

• Structural Isomers: Same molecular formula, different connectivity of atoms.

• Cis-Trans (Geometric) Isomers: Differ in spatial arrangement around a double bond (cis: same side, trans: opposite sides).

• Enantiomers: Mirror-image isomers, important in biological systems and pharmaceuticals.

Example: L- and D- isomers of amino acids; only L-forms are used in proteins.

Biological Implications of Isomers

• Different Properties: Isomers can have drastically different biological activities.

• Pharmaceutical Importance: Enantiomers may have different effects (e.g., ibuprofen, thalidomide).

Drug	Condition	Effective Enantiomer	Ineffective Enantiomer
Ibuprofen	Pain, inflammation	S-Ibuprofen	R-Ibuprofen
Thalidomide	Morning sickness (historical)	(R)-thalidomide	(S)-thalidomide (teratogenic)
Citalopram	Depression	Escitalopram	Citalopram (mixture)

Example: Thalidomide tragedy—one enantiomer caused birth defects, while the other was therapeutic.

Functional Groups

Role in Biological Molecules

• Predictable Chemical Reactions: Functional groups confer specific chemical properties and reactivity.

• Drastic Change in Function: Small changes in functional groups can alter biological activity (e.g., estradiol vs. testosterone).

Functional Group	Structure	Properties	Example
Hydroxyl	-OH	Polar, forms hydrogen bonds	Alcohols
Carbonyl	-C=O	Polar, reactive	Aldehydes, ketones
Carboxyl	-COOH	Acidic, donates H+	Amino acids
Amino	-NH2	Basic, accepts H+	Amino acids
Sulfhydryl	-SH	Forms disulfide bonds	Proteins
Phosphate	-PO4	Negative charge, energy transfer	ATP, DNA
Methyl	-CH3	Nonpolar, gene regulation	DNA methylation

Building Large Molecules: Macromolecules

Polymerization

• Monomers: Small organic molecules (e.g., amino acids, nucleotides, monosaccharides).

• Polymers: Large molecules formed by linking monomers (e.g., proteins, nucleic acids, polysaccharides).

• Dehydration Reaction (Condensation): Synthesizing a polymer by removing a water molecule to form a new bond.

• Hydrolysis: Breaking down a polymer by adding a water molecule, breaking a bond.

Equations:

• Dehydration: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

• Hydrolysis: $\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Example: Formation and breakdown of starch, proteins, and DNA.

Additional info: These foundational chemical principles are directly relevant to Ch. 2 (Water and Carbon: The Chemical Basis of Life) and Ch. 3 (Protein Structure and Function) in a General Biology curriculum.