This lecture covers the second mechanism of water solubility, ion-dipole interactions, and how ions and dissociated carboxyl groups dissolve in water. It contrasts hydrophilic vs. hydrophobic molecules, explains why nonpolar molecules aggregate in water, and introduces water as a reactant in hydrolytic reactions.
1. Diagram/explain how ion-dipole interactions make ions soluble in water; Na+ and a carboxylic acid.
An ion-dipole interaction is the second way a solute dissolves in water (the first is hydrogen bonding). Because water is a dipole, its partially negative oxygen orients toward cations and its partially positive hydrogens orient toward anions. For Na⁺: the ion's full positive charge attracts the partially negative oxygen (δ–) ends of surrounding water molecules, forming a hydration shell. For a carboxylic acid: at physiological pH the carboxyl group loses its proton to become carboxylate (–COO⁻); this negative oxygen attracts the partially positive hydrogen (δ+) ends of water. These charge-based attractions between a full ionic charge and water's dipole make ions soluble.
2. Define a dipole. Explain why water is a dipole.
A dipole is a molecule that has ends bearing opposite partial charges. Water is a dipole because oxygen is more electronegative than hydrogen: the oxygen end carries a partial negative charge (δ–) and the two hydrogen ends carry partial positive charges (δ+).
3. Define acid and base. Is the carboxyl group in a carboxylic acid in the acid or base form at pH 7?
An acid is a chemical that releases (donates) protons; a base is a chemical that accepts protons. At pH 7 the carboxyl group is dissociated (deprotonated): it has kicked off its proton to become the carboxylate ion (–COO⁻). This deprotonated form is the base form (the conjugate base) of the carboxyl group. The protonated –COOH form is the acid form.
4. How does water participate in chemical reactions in the cell? What are these reactions called / literally mean?
Besides acting as a solvent, water serves as a reactant in cellular chemistry. Reactions in which water is added to split a molecule are called hydrolytic reactions (hydrolysis). The term literally means 'water splitting' (hydro = water, lysis = to split/break). This is an important role of water in cells.
5. Know the products when water is split. Diagram this reaction.
When water is split (dissociates), it yields a hydrogen ion (H⁺) and a hydroxide ion (OH⁻): H₂O → H⁺ + OH⁻. These are the products of water's ionization.
6. Define hydrophilic and hydrophobic; literal meanings; why misnomers; an example of each.
Hydrophilic literally means 'water-loving', a chemical that is soluble in / interacts favorably with water (e.g., an ionic compound or a carboxylic acid). Hydrophobic literally means 'water-fearing', a nonpolar chemical that is NOT soluble in water (e.g., pentane, a hydrocarbon). The terms are misnomers because molecules do not actually 'love' or 'fear/hate' water; solubility is determined by whether the molecule can form favorable charge/polar interactions with water, not by any emotion or force from the solute.
7. Diagram/explain what happens to hydrophobic molecules added to water.
When nonpolar (hydrophobic) molecules like pentane are added to water, water forces them to aggregate: to clump together, so that they present the minimal surface area in contact with water. Importantly, this driving force comes from water, not from any attraction among the nonpolar molecules themselves; water reorganizing around nonpolar surfaces pushes them together (a thermodynamic effect).
8. Define Specific Heat.
Specific heat is the amount of heat energy required to raise the temperature of 1 gram of a substance by 1°C.
9. Explain why water has a high Specific Heat.
Water has a high specific heat because of its extensive hydrogen bonding. Added heat energy is first used to break/disrupt hydrogen bonds rather than to increase the kinetic (molecular) motion that raises temperature, so a large amount of heat produces only a small temperature change.
10. Why is water's high Specific Heat a benefit to cells? Why do reactions release heat? How is water a 'heat buffer' contributing to homeostasis?
Many cellular reactions are exergonic and release heat as a byproduct. Because water has a high specific heat, it can absorb this released heat with only a small rise in temperature, acting as a heat buffer. This resistance to temperature swings keeps the intracellular environment stable, contributing to cell homeostasis.
11. Define Heat of Vaporization.
Heat of vaporization is the amount of heat energy required to convert 1 gram of a liquid into a gas (vapor).
12. Describe what happens when a liquid is converted to a gas (vapor).
During vaporization, molecules absorb enough energy to overcome the attractive forces holding them in the liquid; the highest-energy molecules break free from the surface and escape into the gas phase. For water, this requires breaking the hydrogen bonds between molecules.
13. Explain why water has a high Heat of Vaporization.
Water has a high heat of vaporization because a large amount of energy is needed to break the many hydrogen bonds holding water molecules together before individual molecules can escape into the vapor phase.
14. Thought Question: How does perspiration cool organisms, and how does it relate to water's high Heat of Vaporization?
When perspiration (sweat) evaporates, the water molecules that leave the skin carry away a large quantity of heat, because water's high heat of vaporization means each gram vaporized absorbs a lot of energy. That heat is drawn from the body/skin surface, cooling the organism.
15. Besides water, what other molecule is essential to life, and why?
Carbon (carbon-based/organic molecules) is considered essential to life. Carbon can form four stable covalent bonds, allowing it to build the large, diverse, complex molecules (proteins, nucleic acids, carbohydrates, lipids) on which life depends.
Which water end faces which ion (ion-dipole)
'Opposites attract': O(δ–) hugs the cation, H(δ+) hugs the anion.
Na⁺ pulls oxygen's negative end; COO⁻ pulls hydrogen's positive end.
Carboxyl form at pH 7
'Seven = Set free the proton' → deprotonated → BASE form (–COO⁻).
At pH 7 the acid has already released its proton, so you're looking at the conjugate base.
Hydrophilic vs hydrophobic
phiLic = Likes water; phoBic = Bails on water.
L for Likes/Loves, B for Bails/fear, both are misnomers since molecules feel nothing.
What water does to nonpolar molecules
Pentane party: water forces the loners to AGGREGATE (A-word, not 'separate').
Minimal surface area in contact with water; the push comes FROM water.
Hydrolysis meaning
HYDRO-LYSIS = water (hydro) + cut (lysis).
Water is the reactant that splits the molecule.