2 Water, the Solvent of Life

Vocabulary flashcards covering key terms related to water as the solvent of life, its interactions, buffering, and physiological pH concepts.

Hydrogen bond

Electrostatic attraction between a hydrogen atom covalently bonded to an electronegative atom (such as oxygen) and another electronegative atom; crucial for water’s properties.

Dielectric constant

A dimensionless property reflecting solvent dipoles; water has a high dielectric constant (~78.5 at 25°C), which screens electrostatic interactions.

Hydrogen bond strength and lifetime

Hydrogen bonds in liquid water are relatively weak (~23 kJ/mol) and fleeting (lifetime ~1–20 picoseconds).

Hydrophobic effect

Nonpolar regions cluster to maximize water–water hydrogen bonding, driving folding of biomolecules and formation of membranes and micelles.

Amphipathic

Molecules with both polar (hydrophilic) and nonpolar (hydrophobic) regions, such as phosphatidylcholine.

Hydrophilic

Describes substances that readily dissolve in water because they are polar or charged.

Hydrophobic

Describes substances that do not dissolve well in water because they are nonpolar.

Micelles

Thermodynamically stable aggregates formed by amphipathic molecules in water, with hydrophobic tails inward and polar heads outward.

Solvation/Hydration

Water’s ability to dissolve salts and charged biomolecules by screening electrostatic interactions.

Osmotic pressure

The force required to resist water movement across a semipermeable membrane; described by Π = i c R T.

Van’t Hoff factor (i)

The number of particles a solute dissociates into in solution; influences osmotic pressure (i = 1 for nonionizing solutes, i = 2 for salts).

Osmolarity

Product of i and c; a measure of osmotically active particle concentration.

Isotonic

Solutions with osmolarity equal to that of a cell’s cytosol; no net water movement.家具

Hypertonic

Solution with higher osmolarity than the cytosol; water moves out of the cell.

Hypotonic

Solution with lower osmolarity than the cytosol; water moves into the cell.

Ionization constant (Ka)

Equilibrium constant for HA ⇌ H+ + A−; larger Ka means a stronger acid.

pKa

Negative logarithm of Ka; pH at which the acid is half-dissociated.

Conjugate acid-base pair

An acid and its conjugate base that interconvert by donation/acceptance of a proton (e.g., acetic acid/acetate).

Henderson–Hasselbalch equation

pH = pKa + log([A−]/[HA]); relates pH to the ratio of conjugate base to acid.

Buffer

Aqueous system of a weak acid and its conjugate base that resists changes in pH upon addition of small amounts of acid or base.

Buffering region

Flat portion of a titration curve where pH ≈ pKa; maximal buffering capacity.

Phosphate buffer system

H2PO4− ⇌ H+ + HPO4^2−; most effective near pH ≈ 6.86 (effective ~5.9–7.9).

Bicarbonate buffer system

In blood plasma: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−; effective near pH ~7.4 and influenced by pCO2.

CO2 hydration (Kh)

CO2(aq) + H2O ⇌ H2CO3; Kh is the hydration constant linking dissolved CO2 to carbonic acid.

Combined buffer constant (Kcombined)

Product of Kh and Ka for the bicarbonate system (≈8.1 × 10^−7 M^2); pKcombined ≈ 6.1.

Neutral pH

pH at which [H+] = [OH−]; in pure water at 25°C, pH = 7.0.

Blood pH and acidosis

Normal blood pH ~7.35–7.45; acidosis is pH <7.35 and can be life-threatening if severe.

Untreated diabetes and acidosis

Diabetes can cause ketoacidosis due to β-hydroxybutyric and acetoacetic acids lowering blood pH.

Van der Waals interactions

Weak, distance-dependent attractions/repulsions between transient dipoles; weaker than covalent bonds.

Van der Waals radius

Atomic radius describing how close another atom can approach; differs from covalent radius.

Proton hopping

Movement of H+ through water by rapid transfer between adjacent water molecules via hydronium (H3O+).

pH scale

Logarithmic scale of hydrogen ion concentration; pH + pOH = 14 at 25°C.

Titration curve

Plot of pH versus added base; the midpoint corresponds to the acid’s pKa.

Histidine as a pH buffer

Histidine side chain has a pKa near 6.0, enabling buffering near neutral pH.

Lipid bilayers

Membranes that form spontaneously in water; stabilized by interactions with water and important for cellular compartments.

H2O solubility of salts

Water dissolves salts by screening electrostatic interactions; entropy gain drives dissolution.