Lehninger Principles of Biochemistry - Chapter 2: Water, the Solvent of Life

Vocabulary practice flashcards covering water's molecular geometry, hydrogen bonding, the hydrophobic effect, colligative properties, ionization equilibria, titration curves, and physiological buffer systems from Chapter 2 of Lehninger Principles of Biochemistry.

Hybrid Orbitals of Water

The four identical teardrop-shaped orbitals formed by the mixing of oxygen's single $s$ and three $p$ valence orbitals; they arrange in a rough tetrahedron that results in a V-shaped molecule with an $H\text{-}O\text{-}H$ bond angle of $104.5^\circ$.

Hydrogen Bond

An electrostatic attraction between an electronegative atom (acceptor) and a hydrogen atom covalently bound to another electronegative atom (donor); it is longer and weaker than a covalent bond and achieves maximum strength when the donor atom, hydrogen atom, and acceptor atom lie in a straight line.

Hexagonal Ice

The crystal lattice formed by solid water where each $H_2O$ molecule forms four fixed hydrogen bonds with four neighboring molecules, creating an open structure with lower density than liquid water.

Amphipathic Biomolecules

Compounds that contain regions of both polar (hydrophilic) and nonpolar (hydrophobic) character, such as phenylalanine or phosphatidylcholine.

Dielectric Constant ($\varepsilon$)

A dimensionless physical property that reflects the number of dipoles in a solvent ($\varepsilon = 78.5$ for water at $25^\circ\text{C}$, compared to $4.6$ for benzene); it reduces the electrostatic attraction force ($F$) between oppositely charged ions according to $F = \frac{Q_1 Q_2}{\varepsilon r^2}$.

Hydrophobic Effect

The thermodynamically driven aggregation of nonpolar groups in aqueous solution, which minimizes the exposed hydrophobic surface area, reduces the formation of ordered water cages, and increases system entropy ($\Delta S$).

Micelles

Spherical structures formed by amphipathic lipids in aqueous solution in which all hydrophobic groups are sequestered into the interior away from water, thereby minimizing the ordered shell of $H_2O$ molecules and maximizing entropy.

van der Waals Radius

A measurement of an atom's space-filling dimensions that defines how close another atom can approach before repulsive forces between their electron clouds dominate.

Cytochrome f Water Chain

A functional chain of five tightly bound, immobilized water molecules inside cytochrome $f$ that provides a structural conduit for proton translocation through the membrane.

Colligative Properties

Properties of a solution (including vapor pressure, boiling point, melting point/freezing point, and osmotic pressure) that depend strictly on the number of solute particles per unit volume of solvent, rather than on their chemical identity.

Osmosis

The spontaneous movement of water molecules across a semipermeable membrane from a region of higher water concentration to one of lower water concentration, driven by the thermodynamic tendency toward increased disorder.

Hypotonic Solution

A solution that has a lower solute osmolarity than the interior of a cell, driving water into the cell and generating outward hydrostatic pressure that can lead to cell swelling and bursting.

Proton Hopping

The extremely fast relay of protons across a network of hydrogen-bonded water molecules through sequential covalent bond and hydrogen bond reorganizations, giving $H_3O^+$ and $OH^-$ exceptionally high ionic mobilities.

Ion Product of Water ($K_w$)

The equilibrium expression describing the self-ionization of water, defined as $K_w = [H^+][OH^-] = (55.5\,\text{M})(K_{\text{eq}}) = 1.0 \times 10^{-14}\,\text{M}^2$ at $25^\circ\text{C}$.

Conjugate Acid-Base Pair

A proton donor and its corresponding proton acceptor related by the reversible transfer of a single hydrogen ion.

Acid Dissociation Constant ($K_a$)

The equilibrium constant describing the dissociation tendency of a weak acid ($HA \rightleftharpoons H^+ + A^-$), defined as $K_a = \frac{[H^+][A^-]}{[HA]}$; stronger acids possess larger $K_a$ and smaller $\text{p}K_a$ values.

Buffering Region

The relatively flat zone on a weak acid titration curve extending roughly $1\,\text{pH}$ unit above and below the $\text{p}K_a$, where an aqueous mixture of weak acid and its conjugate base maximally resists changes in pH.

Henderson–Hasselbalch Equation

The mathematical relationship between pH, $\text{p}K_a$, and the ratio of conjugate base to weak acid: $\text{pH} = \text{p}K_a + \log\left(\frac{[A^-]}{[HA]}\right)$.

Phosphate Buffer System

The primary intracellular buffering system active in the cytoplasm of all cells, utilizing the equilibrium $H_2PO_4^- \rightleftharpoons H^+ + HPO_4^{2-}$ with a $\text{p}K_a$ of $6.86$.

Bicarbonate Buffer System

The major physiological buffer system operating in human blood plasma, based on the equilibrium $H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$, which links with dissolved $CO_2$ regulated in the lungs.

Histidine Residue Buffering

The buffering provided near physiological neutral pH by protein-bound histidine residues due to their imidazole side chains, which have a $\text{p}K_a$ of $6.0$.

Water Molecule Geometry

A bent, $V$-shaped molecular geometry with an $H\text{-}O\text{-}H$ bond angle of $104.5^\bot$, resulting from oxygen's four $sp^3$ hybrid orbitals forming a rough tetrahedron with two corners occupied by hydrogen atoms and two by unshared electron pairs.

Hydrogen Bond

An electrostatic attraction between an electronegative atom (the hydrogen acceptor) and a hydrogen atom covalently bonded to another electronegative atom (the hydrogen donor) in the same or another molecule.

Hydrogen Bond Directionality

The structural property stating that hydrogen bonds are strongest when the acceptor atom is positioned in a straight line with the covalent bond between the donor atom and the hydrogen atom, thereby maximizing electrostatic interaction.

Hexagonal Ice

The solid lattice state of water in which every water molecule forms four hydrogen bonds with four neighboring water molecules, resulting in an open crystal structure that has a lower density than liquid water.

Dielectric Constant ($\varepsilon$)

A dimensionless physical property that reflects the number of dipoles in a solvent and measures its ability to screen electrostatic interactions between ions; for pure water, $\varepsilon = 78.5$ at $25^\circ\text{C}$, compared to $4.6$ for nonpolar benzene.

Force of Ionic Interactions ($F$)

The electrostatic force between two dissolved ions in solution governed by $F = \frac{Q_1 Q_2}{\varepsilon r^2}$, which indicates that solvents with a high dielectric constant ($\varepsilon$) substantially reduce the attractive force between oppositely charged ions.

Hydrophobic Effect

The clustering together of nonpolar, hydrophobic groups in an aqueous environment to minimize their exposed surface area, which reduces the number of ordered water molecules in surrounding cage structures and increases the overall entropy ($\Delta S > 0$) of the system.

Micelles

Spherical stable aggregates formed by amphipathic compounds in aqueous solution where all nonpolar hydrophobic regions are sequestered in the center away from water and hydrophilic heads form the outer shell, minimizing the shell of ordered water molecules.

Entropic Driving Force of Substrate Binding

The release of tightly ordered water molecules from the complementary interacting surfaces of an enzyme and substrate into bulk solvent, which increases system entropy ($\Delta S > 0$) and drives enzyme-substrate complex formation.

van der Waals Radius

A measurement describing the space-filling dimension of an atom that determines how close another unbonded atom can approach before repulsive forces between their overlapping electron clouds dominate.

Cytochrome $f$ Water Chain

A linear arrangement of five tightly bound, immobilized water molecules within cytochrome $f$ that provides a fixed hydrogen-bonded pathway for protons to move through the biological membrane.

Colligative Properties

Physical properties of a solution (including vapor pressure, boiling point, melting/freezing point, and osmotic pressure) that depend strictly on the concentration of solute particles (number of molecules or ions) in a given amount of water, regardless of their chemical identity or mass.

Osmosis

The spontaneous net diffusion of water molecules across a semipermeable membrane from a region of higher water concentration to an area of lower water concentration in accordance with the thermodynamic tendency toward disorder.

Osmotic Pressure ($\Pi$)

The exact mechanical force or pressure applied to a solution separated from pure solvent by a semipermeable membrane that is required to completely counteract and halt osmosis.

Hypotonic Solution

An extracellular solution containing a lower solute concentration than the inside of the cell, which causes water to move inward, creating outward osmotic pressure that causes the cell to swell and potentially burst.

Hydronium Ion ($H_3O^+$)

A hydrated hydrogen ion formed immediately when a proton released from the reversible ionization of a water molecule coordinates with an unshared electron pair of a neighboring neutral water molecule.

Proton Hopping

A relay mechanism wherein a series of proton transfers through hydrogen bonds connecting neighboring water molecules produces extremely rapid net translocations of $H^+$ and $OH^-$ over long distances, making aqueous acid-base reactions exceptionally fast.

Ion Product of Water ($K_w$)

The equilibrium constant expression given by $K_w = (55.5\,\text{M})(K_{\text{eq}}) = [H^+][OH^-]$, which equals exactly $1.0 \times 10^{-14}\,\text{M}^2$ at $25^\circ\text{C}$.

$$pH$$

The negative logarithm of the hydrogen ion concentration, defined by $pH = -\log[H^+]$, in which each single unit change represents a $10$-fold change in hydrogen ion activity.

Conjugate Acid-Base Pair

A proton donor and its corresponding proton acceptor related by the reversible gain or loss of a proton, such as acetic acid ($CH_3COOH$) and acetate ($CH_3COO^-$).

Acid Dissociation Constant ($K_a$)

The equilibrium constant for the reversible dissociation of an acid into a proton and its conjugate base, defined as $K_a = \frac{[H^+][A^-]}{[HA]}$, where larger values signify stronger acids.

$$pK_a$$

The negative logarithm of the acid dissociation constant, expressed as $pK_a = -\log K_a$, which corresponds to the $pH$ value at the exact midpoint of a weak acid's titration curve where $[HA] = [A^-]$.

Buffering Region

The relatively flat portion of a weak acid titration curve extending roughly $1$ unit of $pH$ above and below the $pK_a$, within which the mixture of weak acid and conjugate base exhibits maximum resistance to $pH$ changes.

Henderson-Hasselbalch Equation

The fundamental mathematical equation $pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)$ describing the quantitative relationship between $pH$, $pK_a$, and the ratio of conjugate base to weak acid in a buffer system.

Phosphate Buffer System

The primary intracellular biological buffer system operating in cell cytoplasm via the reversible equilibrium $H_2PO_4^- \rightleftharpoons H^+ + HPO_4^{2-}$, possessing a $pK_a$ of $6.86$.

Bicarbonate Buffer System

The principal physiological buffer system active in blood plasma, wherein dissolved carbon dioxide is converted via carbonic acid into bicarbonate ($CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$).

Histidine In Vivo Buffering

A biological buffering mechanism provided by protein histidine residues whose imidazole functional group exhibits a $pK_a$ of approximately $6.0$, allowing proteins to buffer effectively near neutral cellular $pH$.