Chapter 2: Water: The Medium of Life Flashcards
- a major chemical component of the earth’s surface
- only liquid that most organisms ever encounter
water
Life originated, evolved, and thrives in the
seas
Typically, organisms are __ water
70% to 90%
normal metabolic activity can occur only when cells are at least __.
65% H2O
dependency of life on water is not a simple matter, but it can be grasped by considering the unusual __.
chemical and physical properties of H2O
critical determinants of the structure and function of many biomolecules, including amino acids and proteins, nucleotides and nucleic acids, and even phospholipids and membranes
water and its ionization products
- hydrogen ions (H+)
- hydroxide ions (OH-)
an indirect participant—a difference in the concentration of hydrogen ions on opposite sides of a membrane represents an energized condition essential to biological mechanisms of energy transformation.
water
Water has a substantially higher __, __, __, and __. These properties suggest that intermolecular forces of attraction between H2O molecules are __. Thus, the internal cohesion of this substance is __.
- boiling point
- melting point
- heat of vaporization
- surface tensions
- high; high
water has an unusually high __, its maximum density is found in the __, and it has a negative volume of melting (that is, the __, occupies more space than the __).
- dielectric constant (a measure of its ability to store electrical energy)
- liquid (not the solid) state
- solid form, ice; liquid form, water
the unrivaled ability of water to __ is the crucial fact to understanding its properties.
form hydrogen bonds
The two hydrogen atoms of water are linked __ to oxygen, each sharing an __, to give a __ arrangement. This __ structure of the H2O molecule has an enormous influence on its properties.
- covalently
- electron pair
- nonlinear
- “bent”
If H2O were linear, it would be a __.
nonpolar substance
- In the bent configuration, the electronegative O atom and the two H atoms form a __ that renders the molecule distinctly polar.
- this structure is ideally suited to __. Water can serve as both an __ and __ in H-bond formation.
- dipole (occur when electrons are shared unequally between atoms in the same molecule due to a high difference in the electronegativity of the atoms involved. A dipole is a molecule or covalent bond that has a separation of charges. e.g. The oxygen side of the molecule carries a net negative charge, while the side with the two hydrogen atoms has a net positive electrical charge.)
- H-bond formation
- H donor; and an H acceptor
The potential to form four H bonds per water molecule is the source of the __ that endow this substance with its anomalously high boiling point, melting point, heat of vaporization, and surface tension.
- strong intermolecular attractions
an H-bonded water molecule serving as an acceptor is a better H-bond donor than an unbonded molecule (and an H2O molecule serving as an H-bond donor becomes a better H bond acceptor). Thus, participation in H bonding by H2O molecules is a phenomenon of __. The H bonds between neighboring molecules are __ relative to the H–O covalent bonds (420 kJ/mol). As a consequence, the hydrogen atoms are situated __ between the two oxygen atoms along the O-O axis. There is never any ambiguity about which O atom the H atom is chemically bound to, nor to which O it is H bonded.
- mutual reinforcement
- weak (23 kJ/mol each)
- asymmetrically
In ordinary ice, the common crystalline form of water, each H2O molecule has __ to which it is hydrogen bonded: Each H atom __ an H bond to the O of a neighbor, and the O atom serves as an __ from H atoms bound to two different water molecules. A local __ results.
- four nearest neighbors
- donates
- H-bond acceptor
- tetrahedral symmetry
- In ice, the hydrogen bonds form a __. These bonds are __ and __; that is, the H atom lies on a __ between the two O atoms. This linearity and directionality mean that the H bonds in ice are __.
- the directional preference of the H bonds leads to an __ structure
- space-filling, three-dimensional network
- directional; straight
- direct line
- strong
- open lattice
In ice, the __ in ice hold the water molecules apart.
H bonds
Melting involves breaking some of the __ that maintain the crystal structure of ice so that the molecules of water (now liquid) can actually __. Thus, the density of ice is __ than that of water. Ice __, a property of great importance to aquatic organisms in cold climate
- H bonds
- pack closer together
- slightly less
- floats
In liquid water, the rigidity of ice is replaced by __ and the crystalline periodicity of ice gives way to __. The H2O molecules in liquid water form a __ H-bonded network, with each molecule having an average of 4.4 close neighbors situated within a center-to-center distance of 0.284 nm (2.84 Å). At least half of the hydrogen bonds have __
- fluidity
- spatial homogeneity
- disordered
- nonideal orientations (that is, they are not perfectly straight)
Liquid H2O lacks the __ structure of ice. The space about an O atom is not defined by the presence of four hydrogens but can be occupied by other water molecules __ oriented so that the local environment, over time, is essentially uniform.
- regular lattice-like
- randomly
The present interpretation is that water structure at the molecular level is, at any instant, __, consisting of local fluctuations between patches of near tetrahedral, ordered arrays of water molecules and more asymmetrical ensembles of water molecules linked together through __. The participation of each water molecule in an average state of H bonding to its neighbors means that each molecule is connected to every other in a fluid network of H bonds.
- inhomogeneous
- distorted H bonds
The average lifetime of an H-bonded connection between two H2O molecules in water is __. Thus, about every 10 psec, the average H2O molecule __, __, and __ with new neighbors
- 9.5 psec (picoseconds, where 1 psec = 10−12 sec)
- moves; reorients; interacts
In summary, pure liquid water consists of H2O molecules held in a __ that has a local preference for __, yet contains a large number of __ hydrogen bonds. The presence of strain creates a kinetic situation in which H2O molecules can __; fluidity ensues.
- disordered, three-dimensional network
- tetrahedral geometry
- strained or broken
- switch H-bond allegiances
Because of the water’s highly polar nature, water is an excellent solvent for __ such as __; __ such as __, __, and __; and __ such as __ and __
- ionic substances (e.g. salts)
- nonionic but polar substances (e.g. sugars, simple alcohols, and amines)
- carbonyl-containing molecules (e.g. aldehydes and ketones)
sodium chloride is dissolved because __ water molecules participate in __ with the Na+ and Cl− ions, leading to the formation of __ surrounding these ions
- dipolar
- strong electrostatic interactions
- hydration shells
Although hydration shells are stable structures, they are also __. Each water molecule in the inner hydration shell around a Na+ ion is replaced, on average, __ by another H2O. Consequently, a water molecule is trapped only __ by the electrostatic force field of an ion than it is by the H-bonded network of water. (Recall that the average lifetime of H bonds between water molecules is about 10 psec.)
- dynamic
- every 2 to 4 nsec (nanoseconds, where 1 nsec = 10−9 sec)
- several hundred times longer
The attractions between the water molecules interacting with, or hydrating, ions are much __ than the tendency of oppositely charged ions to __. Water’s ability to surround ions in dipole interactions and diminish their attraction for each other is a measure of its __. Indeed, ionization in solution depends on the __; otherwise, the strongly attracted positive and negative ions would unite to form __
- greater
- attract one another
- dielectric constant, D
- dielectric constant of the solvent
- neutral molecules.
The strength of the dielectric constant is related to the force, F (give the formula)
- The strength of the dielectric constant is related to the force, F, experienced between two ions
of opposite charge separated by a distance, r
F = e1e2/Dr2 - where e1 and e2 are the charges on the two ions.
In the case of nonionic but polar compounds
such as sugars, the excellent solvent properties of water stem from its ability to readily form __ with the __ on these compounds, such as __, __, and __. These polar interactions between solvent and solute are __ than the intermolecular attractions between solute molecules caused by __ and __. Thus, the solute molecules readily dissolve in water.
- hydrogen bonds
- polar functional groups (hydroxyls, amines, and carbonyl)
- stronger
- van der Waals forces
- weaker hydrogen bonding
Nonpolar solutes (or nonpolar functional groups on biological macromolecules) do not readily H bond to H2O, and as a result, such compounds tend to be only __ in water. The process of dissolving such substances is accompanied by __ of the water surrounding the solute so that the response of the solvent water to such solutes can be equated to __. Because nonpolar solutes must __, the random H-bonded network of water must __ to accommodate them. At the same time, the water molecules participate in as many H-bonded interactions with one another as the temperature permits.
- sparingly soluble
- significant reorganization
- “structure making”
- occupy space
- reorganize
In structure making, the H-bonded water network rearranges toward the formation of a local cagelike (__) structure surrounding each solute molecule. This fixed orientation of water molecules around a hydrophobic “solute” molecule results in a __. A major consequence of this rearrangement is that the molecules of H2O participating in the cage layer have markedly reduced options for orientation in __.
- clathrate
- hydration shell
- three-dimensional space
Water molecules tend to __ the nonpolar solute such that two or three tetrahedral directions (H-bonding vectors) are __ to the space occupied by the inert solute. __ allows the water molecules to retain their H-bonding possibilities because no H-bond donor or acceptor of the H2O is directed toward the caged solute. The water molecules forming clathrates are involved in __. That is, __ is accompanied by the significant ordering of structure or negative entropy
- straddle
- tangential
- “Straddling”
- highly ordered structures
- clathrate formation
Multiple nonpolar molecules tend to cluster together because their __ involves __ and thus fewer ordered water molecules than in their separate cages. It is as if the nonpolar molecules had some __ for one another. This apparent affinity of nonpolar structures for one another is called __
- joint solvation cage
- less total surface area
- net attraction
- hydrophobic interactions
In actuality, the “attraction” between nonpolar solutes is an __ process due to a net decrease in order among the H2O molecules. To be specific, hydrophobic interactions between nonpolar molecules are maintained not so much by __ between the inert solutes themselves as by the increase in entropy when the water cages coalesce and reorganize. Because interactions between nonpolar solute molecules and the water surrounding them are of __ and do not share the __ implicit in chemical bonding, the term hydrophobic interaction is more correct than the misleading expression hydrophobic bond
- entropy-driven
- direct interactions
- uncertain stoichiometry
- equality of atom-to-atom participation
Compounds containing both strongly polar and strongly non-polar groups are called __ (from the Greek amphi meaning “__” and philos meaning “loving”). Such compounds are also referred to as __(from the Greek pathos meaning “__”).
- amphiphilic molecules
- both
- amphipathic molecules
- passion
Example of amphiphilic molecule
salts of fatty acids (a long nonpolar hydrocarbon
tail and a strongly polar carboxyl head group)
Amphipathic molecules’ behavior in aqueous solution reflects the combination of the contrasting polar and nonpolar nature of these substances. The ionic carboxylate function __, whereas the long hydrophobic tail is __. Nevertheless, sodium palmitate and other amphiphilic molecules readily disperse in water because the hydrocarbon tails of these substances are joined together in hydro-phobic interactions as their polar carboxylate functions are hydrated in a typical hydrophilic fashion. Such clusters of amphipathic molecules are termed __;
- hydrates readily
- intrinsically insoluble
- micelles
The presence of dissolved substances __, thereby changing its __. The dynamic H-bonding interactions of water must now __ the intruding substance. The net effect is that solutes, regardless of whether they are polar or nonpolar, fix nearby water molecules in __. Ions, by establishing __ through interactions with the water dipoles, create local __. Hydrophobic substances, for different reasons, make __ within water. To put it another way, by limiting the orientations that neighboring water molecules can assume, solutes give order to the solvent and diminish the dynamic interplay among H2O molecules that occurs in pure water.
- disturbs the structure of liquid water
- properties
- accommodate
- a more ordered array
- hydration shells
- order
- structures
influence of the solute on water is reflected in a set of characteristic changes in behavior termed __, or properties related by a common principle.
colligative properties
In colligative properties, these alterations in solvent properties are related in that they all depend only on the __ and not on the __. These effects include __ (4)
- number of solute particles per unit volume of solvent
- chemical nature of the solute
- freezing point depression
- boiling point elevation
- vapor pressure lowering
- osmotic pressure effects
boiling points of solutions are higher than that of the pure solvent. This effect is directly proportional to the molality of the solute.
boiling point elevation
- the pressure difference needed to stop the flow of solvent across a semipermeable membrane
- proportional to the molar concentration of the solute particles in the solution.
osmotic pressure effects
The freezing points of solutions are all lower than that of the pure solvent. The freezing point depression is directly proportional to the molality of the solute.
freezing point depression
The vapor pressure of a solvent in a solution is always lower than the vapor pressure of a pure solvent. The vapor pressure lowering is directly proportional to the mole fraction of the solute.
vapor pressure lowering
A difference in the __ represents an energized condition essential to biological mechanisms of energy transformation.
concentration of hydrogen ions on opposite sides of a membrane
occur when two atoms in a molecule have substantially different electronegativity: One atom attracts electrons more than another, becoming more negative, while the other atom becomes more positive.
permanent dipoles
What are the properties of water?
(1) Water has a high dielectric constant
(2)Water Forms H Bonds with Polar Solutes
(3)Hydrophobic Interactions - apparent affinity of nonpolar
structures for one another
(4) Interaction with amphiphilic molecules (Compounds
containing both strongly polar and strongly nonpolar
groups)
The __ of water is the pressure at which the water will transition from a __. Specifically, the __ is the point at which the water is in a __, with the same number of water molecules transitioning from __ and from __.
- vapor pressure
- liquid to a gas (vapor)
- vapor pressure
- state of equilibrium
- liquid to gas; gas to liquid
The pressure necessary to push water back through the membrane at a rate exactly equaled by the water influx is the __
osmotic pressure of the solution
To minimize the osmotic pressure created by the contents of their cytosol, __such as amino acids and sugars in __. For example, a molecule of glycogen or starch-containing 1000 glucose units exerts only 1/1000 the osmotic pressure that 1000 free glucose molecules would.
- cells tend to store substances
- polymeric form
Water shows a small but finite tendency to __. This tendency is demonstrated by the __ of pure water, a property that clearly establishes the presence of charged species (ions). Water ionizes because the larger, strongly electronegative oxygen atom __ the electron from one of its hydrogen atoms, leaving the proton to __ (give the formula)
- form ions
- electrical conductivity
- strips
- dissociate
- H–O–H –> H+ + OH−
H–O–H –> H+ + OH−
Two ions are thus formed: __ (2)
Free protons are immediately hydrated to form __:
H+ + H2O –> H3O+
Indeed, because most hydrogen atoms in liquid water are __ to a neighboring water molecule, this __ is an __, and the ion products of water are __ and __:
(1) protons or hydrogen ions, H+;
(2) hydroxyl ions, OH−
- hydronium ions, H3O+
- hydrogen-bonded
- protonic hydration
- instantaneous process
- H3O+ ; OH−
states that when equilibrium is disturbed, the rates of the forward and reverse reactions change to relieve that stress and reestablish equilibrium.
Le Châtelier’s principle
Micelle formation by amphiphilic molecules in aqueous solution. Because of their negatively charged surfaces, neighboring micelles __ and thereby __ in solution.
- repel one another
- maintain relative stability
The solvent properties of water derive its polar nature (4)
1) water has a high dielectric constant
2) Water forms H bonds with polar solutes
3) hydrophobic interactions
4) interaction with amphiphilic molecules
To avoid the cumbersome use of negative exponents to express concentrations that range over 14 orders of magnitude, __, a Danish biochemist, devised the pH scale by defining pH as the __
- Søren Sørensen
- negative logarithm of the hydrogen ion concentration
pH = −log10 [H+]
The Swedish chemist __ proposed the first definition of acids and bases “Acids are substances that dissociate in water to produce __ and bases are substances that dissociate in water to produce __”
- Svante Arrhenius
- H+ ions
- OH- ions
__ and __ revised Arrhenius’s acid-base theory to include other solvents besides water. They defined acids and bases as follows: “An acid is a hydrogen-containing species that __. A base is any substance that __.”
- Johannes Brønsted
- Thomas Lowry
- donates a proton
- accepts a proton
Strong acids ionize __ and weak ones do not.
100%
A __ is used to represent the ionization of strong acids.
single arrow
__ are used to represent ionization of weak acids because an equilibrium is created
double arrows
Strong Acids (7)
HCl (hydrochloric acid)
HNO3 (nitric acid)
H2SO4 (sulfuric acid)
HBr (hydrobromic acid)
HI (hydroiodic acid)
HClO3 (chloric acid)
HClO4 (perchloric acid
List of Strong Bases (8)
LiOH (lithium hydroxide)
NaOH (sodium hydroxide)
KOH (potassium hydroxide)
Ca(OH)2 (calcium hydroxide)
RbOH (rubidium hydroxide)
Sr(OH)2 (strontium hydroxide)
CsOH (cesium hydroxide)
Ba(OH)2 (barium hydroxide)
List of Weak Acids
Formic Acid (HCOOH)
Acetic Acid (CH3COOH)
Benzoic Acid (C6H5COOH)
Hydrofluoric Acid (HF)
Phosphoric Acid (H3PO4)
Sulfurous Acid (H2SO3)
Carbonic Acid (H2CO3)
Nitrous Acid (HNO2)
Hydrocyanic Acid (HCN)
Hydrosulfuric Acid (H2S)
Citric Acid (C6H8O7)
List of Weak Bases
Ammonium Hydroxide (NH4OH)
Aniline (C6H5NH2)
Ammonia (NH3)
Methylamine (CH3NH2)
Ethylamine (CH3CH2NH2)
Aluminum hydroxide (Al(OH)3)
Magnesium Hydroxide (Mg(OH)2)
Pyridine (C5H5N)
Sodium Bicarbonate (NaHCO3)
Strong acids __ into their ions when they are mixed with water.
completely dissociate
__ are bases that completely dissociate in water into the cation and OH-(hydroxide ion).
Strong bases
Substances that are almost completely dissociated to form ions in solution are called __. The term __ describes substances capable of generating ions in solution and thereby causing an increase in the electrical conductivity of the solution.
- strong electrolytes
- electrolyte
Substances with only a slight tendency to dissociate to form ions in solution are called __.
weak electrolytes
The __ Describes the Dissociation of a Weak Acid In the Presence of Its Conjugate Base
Henderson–Hasselbalch Equation
__ is the analytical method used to determine the amount of acid in a solution.
Titration
__ are solutions that tend to resist changes in their pH as acid or base is added.
Buffers
indicates whether an acid is a strong acid or a weak acid.
pKa
indicates whether a system is acidic or alkaline.
pH
The buffer systems selected reflect both the need for a __ and the __
- pKa value near pH 7
- compatibility of the buffer components with the metabolic machinery of cells
Two buffer systems act to maintain intracellular pH essentially constant
the phosphate (HPO42−/H2PO4−) system and the histidine system
pH of the extracellular fluid that bathes the cells and tissues of animals is maintained by the __
bicarbonate/carbonic acid (HCO3−/H2CO3) system
