The Chemistry of Living Things: Water, pH, and Organic Molecules

The Chemistry of Living Things

Water as a Biological Solvent

Water is the most important molecule for life, making up about 60% of human body weight. It acts as an excellent solvent, remains liquid at body temperature, absorbs and retains heat, and participates in essential chemical reactions.

• Solvent: A liquid in which other substances (solutes) dissolve. Water is a polar solvent at body temperature.

• Example: Table Salt (NaCl): In water, Na+ and Cl- ions are pulled from the crystal lattice and surrounded by water molecules, preventing them from re-crystallizing. Thus, salt remains dissolved.

Types of Molecules: Hydrophilic vs. Hydrophobic

• Hydrophilic molecules: Polar, attracted to water (e.g., ions, polar molecules).

• Hydrophobic molecules: Non-polar, do not interact with water (e.g., oils).

Water's Physical States and Biological Importance

Water exists as a liquid at body temperature (0–100°C), which is crucial for its role as a transport medium in the body. Its state depends on hydrogen bonding:

• Below 0°C: Ice (stable hydrogen bonds, rigid lattice)

• 0–100°C: Liquid (hydrogen bonds constantly form and break, molecules move freely)

• Above 100°C: Gas (hydrogen bonds broken, water vapor forms)

Water is the main component of all fluid-filled body spaces (intracellular, intercellular, urine, digestive fluids, eye fluids) and is essential for transporting oxygen, carbon dioxide, nutrients, and waste.

Water and Body Temperature Regulation

• Water absorbs large amounts of heat energy, minimizing rapid temperature changes.

• Evaporation (sweating) removes heat from the body, cooling the blood and lowering body temperature.

The Importance of Hydrogen Ions and pH

Acids, Bases, and the pH Scale

• Acids: Substances that donate H+ ions, increasing H+ concentration and lowering pH (e.g., vinegar, coffee, orange juice).

• Bases: Substances that accept H+ ions, decreasing H+ concentration and raising pH (e.g., baking soda, detergents).

• pH Scale: Measures H+ concentration, ranges from 0 (most acidic) to 14 (most alkaline). Pure water is neutral at pH 7 (10-7 mol/L).

• The scale is logarithmic: a difference of 1 pH unit equals a tenfold change in H+ concentration.

• Blood pH: Maintained at ~7.4 (slightly alkaline), crucial for homeostasis. Changes in pH affect molecular structure, reaction rates, and protein function.

Buffers and pH Homeostasis

• Buffers: Substances that minimize changes in pH by absorbing or releasing H+ ions. Essential for maintaining stable pH in blood and other fluids.

• Biological buffers exist as pairs: an acid form (donates H+) and a base form (accepts H+).

• Bicarbonate buffer system in blood: HCO3- (base) and H2CO3 (acid) maintain blood pH by reversible reactions:

$\mathrm{HCO_3^- + H^+ \leftrightarrow H_2CO_3}$

$\mathrm{H_2CO_3 \leftrightarrow HCO_3^- + H^+}$

The Organic Molecules of Living Organisms

Carbon: The Building Block of Life

• Organic molecules contain carbon, often bonded to hydrogen, oxygen, nitrogen, or other carbons.

• Carbon forms four covalent bonds, allowing for complex branching and ring structures.

• Macromolecules are large organic molecules made of thousands to millions of smaller subunits.

Macromolecule Synthesis and Breakdown

• Dehydration synthesis: Joins subunits by removing water, requires energy.

• Hydrolysis: Breaks macromolecules by adding water, releases energy.

• Four main classes: carbohydrates, lipids, proteins, nucleic acids.

Carbohydrates

• Composed of carbon, hydrogen, and oxygen in a 2:1 ratio (like water).

• Serve as energy sources (e.g., glucose) and structural support (e.g., cellulose in plants).

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

• Oligosaccharides: Short chains (e.g., disaccharides like sucrose, lactose).

• Polysaccharides: Long chains (e.g., glycogen in animals, cellulose in plants).

Lipids

• Insoluble in water, include triglycerides, phospholipids, and steroids.

• Triglycerides: Glycerol + 3 fatty acids, used for energy storage.

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

• Unsaturated fats: Double bonds, liquid at room temperature, kinked chains.

• Phospholipids: Glycerol + 2 fatty acids + phosphate group, main component of cell membranes (polar head, nonpolar tail).

• Steroids: Four carbon rings, e.g., cholesterol (membrane structure, hormone precursor).

Proteins

• Polymers of amino acids (20 types), joined by peptide bonds.

• Structure determines function:

  • Primary: Amino acid sequence

  • Secondary: Alpha helix or beta sheet (hydrogen bonding)

  • Tertiary: 3D folding (R-group interactions)

  • Quaternary: Association of multiple polypeptides

• Functions: structural support, muscle contraction, membrane transport, enzymes.

• Denaturation: Loss of structure and function due to heat or pH changes.

Enzymes

• Proteins that act as biological catalysts, speeding up reactions without being consumed.

• Enzyme activity depends on shape, which is influenced by temperature, pH, and ion concentration.

Nucleic Acids

• DNA: Genetic material, double helix, bases A, T, C, G.

• RNA: Single-stranded, bases A, U, C, G, involved in protein synthesis.

• Both are polymers of nucleotides (phosphate, sugar, nitrogenous base).

ATP: The Energy Carrier

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

• Structure: Adenine base, ribose sugar, three phosphate groups.

• Energy is released when the terminal phosphate bond is broken:

$\mathrm{ATP \rightarrow ADP + P + energy}$

• ATP is regenerated from ADP and phosphate using energy from food (glycogen, fat).