Water (H2O): The medium for life, also known as the universal solvent.
Importance: Facilitates enzyme activity and biochemical reactions; constitutes about 70% of the human body weight.
Protection: Water offers UV protection.
Transportation: Eases movement of nutrients and chemicals within organisms.
Crucial for Proteins and Nucleic Acids: Essential for the structure and function of proteins, nucleic acids, and cell membranes.
Structure of Water Molecule
Composition: One oxygen atom and two hydrogen atoms (H2O).
Electron Pairs: Four electron pairs around the oxygen atom.
Bonding: Two pairs are covalently bonded to hydrogen atoms; other two pairs are lone pairs.
Polarity: Water is a polar molecule, having partial positive and negative charges.
Hydrogen Bonds: Formed due to polarity; allows water molecules to bond with each other.
Dipole Moment and Electronegativity
Dipole Moment: Arises from the electronegativity difference between oxygen and hydrogen.
Electronegativity: Oxygen's high electronegativity induces a dipole moment, allowing water to engage in hydrogen bonding.
Hydrogen Bonding: Strongest when three atoms are aligned. Water can act as both an acid and a base due to hydrogen bonding.
Properties of Water
Boiling and Melting Points: High due to strong hydrogen bonds.
Surface Tension: Allows small insects to walk on water due to strong cohesion among water molecules.
Solvent Properties: Dissolves polar substances well; like dissolves like principle (polar substances dissolve in polar solvents).
Weak Hydrogen Bonding: Individually weak but collectively strong due to cooperation among many bonds.
Interaction with Nonpolar Substances
Poor Solvent for Nonpolar Substances: Nonpolar substances do not form hydrogen bonds with water, leading to phenomena like oil not mixing with water.
Examples: Nonpolar gases, aromatic rings, and aliphatic chains.
Amphipathic Molecules
Amphipathic: Contains both hydrophobic (nonpolar) and hydrophilic (polar) parts.
Example: Phenylalanine - has both nonpolar hydrocarbon chains and polar groups.
Solubility of Polar and Nonpolar Solutes
Polar Solutes: Dissolve well in water; e.g., amino acids, peptides, carbohydrates.
Nonpolar Solutes: Poor solubility in water; e.g., hydrocarbons, nonpolar gases.
Ion Dissolution: Water can dissolve salts, reducing system energy and stabilizing ions.
Ionization and pH of Water
Self-Ionization: Water dissociates into hydrogen (H+) and hydroxide ions (OH-).
Equilibrium Constant (Keq): Dependents on temperature; typically low, indicating minimal dissociation.
pH Scale: Measures acidity/alkalinity; 7 is neutral, <7 is acidic, >7 is basic.
pH Calculation
Definition: Negative logarithm of hydrogen ion concentration (-log[H+]).
Relationships: pH + pOH = 14.
Neutral Solution: [H+] = [OH-] = 10^-7 M, leading to pH of 7.
Buffers and Buffering Capacity
Buffer Systems: Resist changes in pH upon addition of acids or bases.
Buffers in the Body: Maintain pH for enzyme activity and metabolic processes; examples include bicarbonate and phosphate buffers.
Buffering Region: Optimal buffering occurs when pH ≈ pKa.
Biological and Laboratory Buffers
In Vivo Buffers: Such as bicarbonate in blood plasma, histidine in intracellular fluids.
In Vitro Buffers: Examples include phosphate buffers used in lab settings.
Summary
Water’s unique properties are due to its molecular structure and ability to form hydrogen bonds. Its role as a solvent facilitates numerous biological processes. Understanding water's behavior, pH balance, and buffer systems is crucial for comprehending biochemical reactions.