Water: Polarity and Significance

• Water's significance includes its role as the most abundant molecule in living organisms (approximately 70% of body mass), biochemical reactions, homeostasis, and influences on macromolecule structure and function. It is also significant in evolution and adaptation of life forms.

Molecular Structure and Polarity

• Water is a V-shaped molecule with a 104.5° bond angle.
• Oxygen's high electronegativity results in a partial negative charge (δ-) on the oxygen atom and partial positive charges (δ+) on hydrogen atoms.
• This formation of dipoles makes water a polar molecule.
• Atoms with different electronegativity values form polar bonds.
• Large electronegativity differences between a metal and a nonmetal result in highly polar ionic bonds.
• Slight differences between two nonmetals lead to polar covalent bond formation.
• Electrons in polar covalent bonds spend more time closer to one atom, leading to partial positive and negative charges.
• Molecular geometry affects molecule polarity.
• Though the covalent bonds between carbon and oxygen in carbon dioxide (CO2) are polar, CO2 is nonpolar due to its linear structure where partial charges cancel each other out, resulting in a net dipole moment of zero.
• Water has a bent geometry, with 104.5°.
• Water has a net dipole moment because the positive and negative charges aren't evenly distributed and do not cancel each other out.
• Water's bent geometry results from the two lone electron pairs on the oxygen atom.
• The electronic structure of oxygen is 1s2 2s2 2p4.
• A hydrogen atom contributes one electron to fill the valence shell and give oxygen 1s2 2s2 2p6.
• This means four electrons (two pairs) in the 2p shell don't participate in chemical bonds.
• The lone electron pairs have the same negative electrical charge, repelling each other.
• These electron pairs are also repelled by the chemical bonds between the hydrogen and oxygen atoms (but not by the same amount).
• The hydrogen atoms repel each other, leading to a tetrahedral geometry.
• Water is a POLAR SOLVENT because the attraction between oxygen and hydrogen atoms of neighboring water molecules leads to hydrogen bond formation.
• Hydrogen bonds aren't as strong as covalent bonds; only about 20% of water molecules are free to interact with other chemical species at any given time, known as dissolving or hydration.

Hydrogen Bonding

• Water is called a "universal solvent" because it dissolves more substances than any other solvent; however, it only dissolves polar solutes.
• Water molecules form hydrogen bonds (H-bonding), leading to high cohesion.
• Each water molecule can form up to 4 hydrogen bonds, creating a dynamic network.
• Hydrogen bonds break and reform rapidly, giving water its fluidity.

Cohesion and Adhesion

• Cohesion describes water molecules sticking to each other due to hydrogen bonding, leading to high surface tension.
• Water molecules sticking to surfaces, or Adhesion, enables capillary action, crucial for water transport in plants and blood vessels.

High Specific Heat Capacity

• A large amount of energy is required to change water's temperature.
• The amount of heat needed to raise 1g of water by 1°C is 4.18 J/g°C.
• Water resists temperature fluctuations, maintaining stable cellular and environmental conditions.
• It helps regulate body temperature, preventing overheating from metabolic reactions.
• Water molecules (H₂O) can form four hydrogen bonds per molecule because of having two hydrogen atoms and two lone pairs of electrons on the oxygen.
• Alcohols (R-OH) can only form up to three hydrogen bonds per molecule.
• The extensive network of strong hydrogen bonds in water requires a lot of energy to break, leading to a high boiling point.
• Water molecules are relatively small, allowing them to pack closely together, strengthening the intermolecular forces, including hydrogen bonds.

High Heat of Vaporization

• Energy is required to convert liquid water to vapor at 540 cal/g.
• Evaporation is a cooling mechanism.
• Sweating in humans and panting in animals dissipate heat, preventing overheating in high temperatures.

Density and Ice Formation

• Water is less dense as a solid than as a liquid.
• Ice forms an open hexagonal structure, allowing it to float.
• Biological significance: Ice insulates water bodies, protecting aquatic life in winter.
• Water maintains seasonal water circulation, such as lake overturning.

Solvent Properties

• Water is the universal solvent due to its polarity.
• Hydrophilic (water-loving) substances include salts, sugars, and proteins with polar groups.
• Hydrophobic (water-fearing) substances include lipids and nonpolar molecules.
• Biological Significance:
  • It transports nutrients and waste in blood and cells.
  • It dissolves gases (O2, CO2) for respiration.

Role as a Reactant

• Hydrolysis reactions break bonds using water.
  • ATP hydrolysis → energy release.
  • Digestion of macromolecules (proteins → amino acids, polysaccharides → monosaccharides).
• Condensation (dehydration synthesis) reactions:
  • Formation of peptide bonds (proteins) and glycosidic bonds (carbohydrates).
• Micelles form spontaneously in water with amphiphiles.
• Amphiphiles have water-loving (hydrophilic) and water-fearing (hydrophobic) parts, acting as a head that likes water and a tail that hates it.

pH and Buffer Systems

• Dissociation of water: H₂O ⇌ H+ + OH- (self-ionization).
• The water ion product Kw = 1 × 10^-14 at 25°C.
• pH Scale: Acidic solutions are pH < 7, Neutral pH = 7, Basic solutions are pH > 7.
• Buffer systems in living organisms:
  • Bicarbonate (HCO3¯ / H2CO3) system maintains blood pH (~7.4).
  • Phosphate buffer system in intracellular fluid.

Causes and Effects of Dehydration

• Causes are excessive sweating, vomiting, diarrhea, and inadequate intake.
• Symptoms include fatigue, dizziness, confusion, and organ failure. Severe dehydration leads to hypovolemic shock.

Overhydration and Hyponatremia

• Excess water intake dilutes blood sodium levels.
• This leads to brain swelling, confusion, seizures, and coma.

Regulation of Water Intake and Output

• Hormones' role:
  • Antidiuretic hormone (ADH) increases water retention in the kidneys.
  • Aldosterone promotes sodium and water reabsorption.
• The thirst mechanism is regulated by the hypothalamus.