CHEM213 Lecture 2: The Importance of Water in Living Systems

Importance of Water in Living Systems

• Comprises over 70% of the weight in most organisms, essential for various biological functions.
• Regulates body temperature, moistens mucous membranes, lubricates joints, and aids in waste elimination.

Structure of Water & Hydrogen Bonds

• Water's dipolar nature is due to unequal sharing of electrons, creating electric dipoles.
• Oxygen's electronegativity leads to stronger attraction to electrons than hydrogen.
• Water molecules form hydrogen bonds, enabling liquid form at room temperature and solid form (ice) at lower temperatures.
• Water has a rough tetrahedron geometry with a bond angle of 104.5°, slightly less than the ideal 109.5° due to non-bonding electron repulsion.

Hydrogen Bonds Characteristics

• Each water molecule can form up to 3.4 hydrogen bonds when liquid and 4 when solid (ice).
• Hydrogen bonds are relatively weak: breaking requires 23 kJ/mol, whereas breaking covalent O-H bonds requires 470 kJ/mol.

Melting and Boiling Points

• High energy is required to melt ice due to structured hydrogen bonding in the solid phase.
• Enthalpy change (ΔH) is key in phase changes, with ice to liquid (ΔH = +5.9 kJ/mol) and liquid to gas (ΔH = +44.0 kJ/mol) requiring positive values.

Thermodynamic Principles

• Free-energy change (ΔG) determines spontaneity of processes: ΔG = ΔH - TΔS must be negative for spontaneity.
• Entropy (ΔS) reflects disorder, which tends to increase according to the 2nd Law of Thermodynamics.

Hydrogen Bonds in Other Molecules

• Hydrogen bonds can form with other electronegative atoms (N, O) and contribute to the boiling point differences in compounds (e.g., butanol vs. butane).
• Solubility in water is influenced by hydrogen bonding, aiding the dissolution of uncharged, polar biomolecules (e.g., glucose).

Polarity of Water

• Water is a polar solvent, promoting dissolution of charged/polar biomolecules (hydrophilic).
• Non-polar molecules (lipids, waxes) poorly dissolve in polar solvents.
• The dissolution of ionic compounds (e.g., NaCl) in water increases entropy and stabilizes ions, facilitating their dissociation from the crystalline lattice.

Explore the significance of water in living organisms, including its role in regulating body temperature, lubricating joints, and influencing cellular structure. Learn about the structure of water and hydrogen bonds.