Biochem chapter 2: Water, the solvent of life
The oxygen in H2O is more electronegative than hydrogen, what does this mean?
-The oxygen nucleus attracts electrons more strongly than does the hydrogen nucleus (a proton)-oxygen is more electronegative. -This means that the shared electrons are more often in the vicinity of the oxygen atom than of the hydrogen. The result of this unequal electron sharing is two electric dipoles in the water molecule, one along each of the H-O bonds.
In liquid water at room temperature and atmospheric pressure, water molecules are disorganized and in continuous motion, so that each molecule forms hydrogen bonds with an average of only _______ other molecules
3.4
Hydrogen bond
A weak electrostatic attraction between one electronegative atom (such as oxygen or nitrogen) and a hydrogen atom covalently linked to a second electronegative atom. is an intermolecular force
As a result of the electronegativity differences between H and O in H2O and the partial charges that are formed, what can happen between various water molecules?
As a result, there is an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another, called a hydrogen bond
Explain the dissociation of NaCl with H2O in terms of ΔG and entropy.
As a salt such as NaCl dissolves, the Na+ and Cl- ions leaving the crystal lattice acquire far greater freedom of motion. -The resulting increase in entropy (randomness) of the system is largely responsible for the ease of dissolving salt such as NaCl in water. -In thermodynamic terms, formation of the solution occurs with a favorable free-energy change: ΔG=ΔH-TΔS, where ΔH has a small positive vale and TΔS a large positive value; thus ΔG is negative
Why would macromolecules such as proteins, nucleic acids, and polysaccharides have far less effect on the osmolarity of a solution than an equal mass of their monomeric components would have?
Because the effect of solutes on osmolarity (a colligative property) depends on the number of dissolved particles, not their mass, macromolecules (proteins, nucleic acids, polysaccharides) have far less effect on the osmolarity of a solution than an equal mass of their monomeric components would have (higher number of dissolved particles).
Why might numerous weak noncovalent interactions bestow much greater molecular stability than would be expected intuitively from a simple summation of small binding energies?
Because to dissociate two biomolecules (such as an enzyme and its bound substrate) that are associated noncovalently through multiple weak interactions, all of these interactions must be disrupted at the same time. -Because the interactions fluctuate randomly, such simultaneous disruptions are very unlikely.
Explain why butane has a boiling point of only -0.5 C, whereas butanol has a relatively high boiling point of 117 C.
Butanol has a polar hydroxy group and thus can form intermolecular hydrogen bonds. Butane is a hydrocarbon capable of only forming van der waals intermolecular forces. -Molecules of butane are non-polar (they have a symmetrical distribution of electron density) and therefore exhibit only London dispersion forces.
Disruption of ordered water molecules is part of the driving force for binding of a polar substrate (reactant) to the complementary polar surface of an enzyme. Explain this phenomena in terms of entropy
Entropy increases as the enzyme displaces ordered water from the substrate and as the substrate displaces ordered water from the enzyme surface.
T/F Hydrogen bonds are unique to water.
F
T/F Alcohols, aldehydes, ketones, and compounds containing N-H bonds cannot form hydrogen bonds with water and are therefore not soluble in water.
F Alcohols, aldehydes, ketones, and compounds containing N-H bonds all form hydrogen bonds with water and are therefore water soluble.
T/F Hydrogen bonds are rather permanent, lasting for a prolonged period of time.
F At any given time, most of the molecules in liquid water are hydrogen-bonded, but the lifetime of each hydrogen bond is just 1 to 20 picoseconds. When one hydrogen bond breaks, another hydrogen bond forms, with the same partner or a new one, within 0.1ps.
T/F Hydrogen bonds and ionic, hydrophobic, and van der waals interactions are individually strong
F Hydrogen bonds and ionic, hydrophobic, and van der waals interactions are individually WEAK, but collectively they have a very significant influence on the three-dimensional structures of proteins, nucleic acids, polysaccharides, and membrane lipids.
T/F In biomolecules, it is the dielectric constant for the bulk solvent, not the high localized dielectric constant (as in a hydrophobic pocket of a protein) that determines the interaction of two polar moieties.
F In biomolecules, it is not the dielectric constant for the bulk solvent, but the highly localized dielectric constant, as in a hydrophobic pocket of a protein, that determines the interaction of two polar moieties.
T/F Plants do not use osmotic pressure to achieve mechanical rigidity.
F Plants do use osmotic pressure to achieve mechanical rigidity. The very high solute concentration in the plant cell vacuole draws water into the cell, but the nonexpandable cell wall prevents swelling; instead, the pressure exerted against the cell wall (turgor pressure) increases, stiffening the cell, the tissue, and the plant body. -When the lettuce in your salad wilts, it is because loss of water has reduced turgor pressure.
T/F The noncovalent interactions responsible for the strength and specificity of "recognition" among biomolecules are not influenced by water's properties as a solvent.
F The noncovalent interactions responsible for the strength and specificity of "recognition" among biomolecules are decisively influenced by water's properties as a solvent, including its ability to form hydrogen bonds with itself and with solutes.
T/F Water does not interact electrostatically with charged solutes
F Water does interact electrostatically with charged solutes
In ice, each water molecule is fixed in space and forms hydrogen bonds with a full complement of four other water molecules to yield a regular lattice structure. Hydrogen bonds account for the relatively high melting point of water, because much thermal energy is required to break a sufficient proportion of hydrogen bonds to destabilize the crystal lattice of ice. T/F When Ice melts or water evaporates, heat is released by the system
F When ice melts or water evaporates, heat is taken up by the system: ΔH2O (s) -----> H2O (l) ΔH = 5.9 kJ/mol H2O (l) -------> H2O (g) ΔH = 44.0kj/mol
T/F In addition to requiring an input of energy, dissolving hydrophobic compounds in water produces a measurable increase in entropy
F decrease in entropy. Water molecules in the immediate vicinity of a nonpolar solute are constrained in their possible orientations, as they form a highly ordered cagelike shell around each solute molecule to maximize solvent-solvent hydrogen bondings -the ordering of water molecules reduces entropy.
T/F weak noncovalent interactions are not crucial to macromolecular structure and function
F weak noncovalent interactions are crucial to macromolecular structure and function
The strength, or force (F), of ionic interactions in a solution depends on: (what's the equation?)
F= (Q1)(Q2)/εr^2 The strength, or force (F), of ionic interactions in a solution depends on the magnitude of the charges (Q), the distance between the charged groups (r), and the dielectric constant (ε, which is dimensionless) of the solvent in which the interactions occur
T/F a compound such as NaCl, which dissociates in solution, has an effect on osmotic pressure (a colligative property) that is the same of that of an equal number of moles of a nondissociating solute such as glucose.
False, colligative properties of a solution depend on the number of solute particles, not the solute identity. A compound such as NaCl dissociates in solution into Na+ and Cl- and such will have an effect on osmotic pressure that is twice that of an equal number of moles of a nondissociating solute such as glucose.
why would a cell store fuel as polysaccharides (glycogen or starch) rather than as glucose or other simple sugars?
Storing fuel as polysaccharides (glycogen/starch) rather than as glucose or other simple sugars avoids an enormous increase in osmotic pressure in the storage cell. Cells of liver and muscle store carbohydrate (energy) not as low molecular weight sugars, such as glucose or sucrose, but as the high molecular weight polymer glycogen. This allows the cell to contain a large mass of glycogen with a minimal effect on the osmolarity of the cytosol.
T/F A gram of a polysaccharide composed of 1,000 glucose units has about the same effect on osmolarity as a milligram of glucose.
T
T/F Although many of the solvent properties of water can be explained in terms of the uncharged H2O molecule, the small degree of ionization of water to hydrogen ions (H+) and hydroxide ions (OH-) must also be taken into account.
T
T/F By screening the electrical charges of ions and by increasing the entropy of the system, water dissolves crystals of ionizable solutes
T
T/F Enzymes, which catalyze all of the processes inside a cell, have evolved to function optimally at near-neutral (physiological) pH. However, enzymes that function in intracellular compartments of low or high pH show their greatest activity and stability at those pH values.
T
T/F Hydrogen bonding between water and polar solutes also causes an ordering of water molecules, but the energetic effect is less significant than with nonpolar solutes.
T
T/F Extended networks of hydrogen-bonded water molecules can form bridges between solutes (proteins and nucleic acids, for example) that allow the larger molecules to interact with each other over distances of several nanometers without physically touching
T
When an amphipathic compound is mixed with water, what happens?
When an amphipathic compound is mixed with water, the polar, hydrophilic region interacts favorably with the water and tends to dissolve, but the nonpolar, hydrophobic region tends to avoid contact with the water. The nonpolar regions of the molecules cluster together to present the smallest hydrophobic area to the aqueous solvent, and the polar regions are arranged to maximize their interaction with each other and with the solvent, a phenomenon called the hydrophobic effect.
How could you determine the stability of a noncovalent interaction, such as the hydrogen bonding of a small molecule to its macromolecular partner?
You could calculate the stability of this noncovalent interaction from the binding energy: the reduction in the energy of the system when binding occurs. -Stability, as measured by the equilibrium constant of the binding reaction, varies exponentially with binding energy.
Water molecules have a slight tendency to undergo reversible ionization to yield:
a proton (hydrogen ion) and a hydroxide ion H2O⇌H+ + OH-
Hydrogen bonds readily form between an electronegative atom, the hydrogen __________________ (usually oxygen or nitrogen) and a hydrogen atom covalently bonded to another electronegative atom, the hydrogen _______________ in the same or another molecule.
acceptor donor
________________ compounds contain regions that are polar (or charged) and regions that are nonpolar.
amphipathic
An aqueous solution of a weak acid and its salt makes a:
an aqueous solution of a weak acid and its salt makes a buffer that resists changes in pH in response to added acid or base. Biological systems are buffered to maintain a narrow pH range, in which their macromolecules retain their functional structure, which depends on their ionization state. Conditions that produce blood pH outside the range of 7.3 to 7.5 are life-threatening in humans.
Osmosis is an important factor in the life of most cells. T/F Plasma membranes are more permeable to water than to most other small molecules, ions, and macromolecules because proteins channels, ___________, in the membrane selectively permit the passage of water.
aquaporins
Nonpolar gases are __________ soluble in water a) very soluble b) poorly soluble
b: poorly soluble
nonpolar compounds force energetically _____________ changes in the structure of water a) favorable b) unfavorable
b:unfavorable
Water is a polar solvent. It ready dissolves biomolecules that are ______________________ Compounds that dissolve easily in water are _____________________________ Nonpolar solvents such as chloroform and benzene are poor solvents for __________ biomolecules but easily dissolve those that are ______________________:
charged or polar compounds hydrophilic polar hydrophobic: nonpolar molecules such as lipids and waxes
The very different ___________ of H and O make water a highly polar molecule, capable of forming ______________ with itself and with solutes.
electronegativities hydrogen bonds Hydrogen bonds are fleeting, primarily electrostatic, and weaker than covalent bonds
Like all reversible reactions, the ionization of water can be described by an _________ constant
equilibrium When weak acids are dissolved in water, they contribute H+ by ionizing; weak bases consume H+ by becoming protonated. These processes are also governed by equilibrium constants
To predict the state of ionization of solutes in water, we must take into account the relevant ______________ for each ionization reaction
equilibrium constants
The ionization behavior of water and of weak acids and bases dissolved in water can be represented by one or more ____________________
equilibrium constants Most biomolecules are ionizable; their structure and function depend on their ionization state, which is characterized by equilibrium constants
The nearly tetrahedral arrangement of the orbitals about the oxygen atom allows each water molecule to form hydrogen bonds with as many as _____________ neighboring water molecules.
four
Water molecules tend to move from a region of _______ water concentration to one of __________ water concentration.. Or in other words, water molecules tend to move from a region of _________ solute concentration to one of ________ solute concentration
higher lower lower higher
__________________ between water molecules provide the cohesive forces that make water a liquid at room temperature and a crystalline solid (ice) with a highly ordered arrangement of molecules at cold temperatures.
hydrogen bonds
Nonpolar (hydrophobic) compounds dissolve poorly in water; they cannot _____________ with the solvent, and their presence forces an energetically ___________ ordering of water molecules at their hydrophobic surfaces. To minimize the surface exposed to water, nonpolar and amphipathic compounds such as lipids form ___________ (micelles and bilayer vesicles) in which the hydrophobic moieties are sequestered in the interior, an association driven by the ________________, and only the more polar moieties interact with water
hydrogen-bond unfavorable aggregates hydrophobic effect
Many biomolecules are amphipathic; proteins, pigments, certain vitamins, and the sterols and phospholipids of membranes all have both polar and nonpolar surface regions. Structures composed of these molecules are stabilized by the _______________, which favors aggregation of the nonpolar regions.
hydrophobic effect
During melting or evaporation of water, the entropy of the aqueous system ________________
increases The entropy of the aqueous system increases as the highly ordered arrays of water molecules in ice relax into less orderly hydrogen-bonded arrays in liquid water or into the wholly disordered gaseous state.
Water has a higher melting point, boiling point, and heat of vaporization than most other common solvents. These unusual properties are a consequence of:
intermolecular forces between water molecules (hydrogen bonding)
Osmosis also has consequences for lab protocol. Mitochondria, chloroplasts, and lysosomes, for example, are enclosed by semipermeable membranes. In isolating these organelles from broken cells, biochemists must perform the fractionations in ___________ solutions to prevent excessive entry of water into the organelles and the swelling and bursting that would follow.
isotonic buffers used in cellular fractionations commonly contain sufficient concentrations of sucrose or some other inert solute to protect the organelles from osmotic lysis.
Solutions of osmolarity equal to that of a cell's cytosol are said to be _________ relative to that cell. Surrounded by an _______ solution, a cell neither gains nor loses water. In a ________ solution, one with higher osmolarity than that of the cytosol, the cell shrinks as water moves out. In a ___________ solution, one with a lower osmolarity than the cytosol, the cell swells as water enters.
isotonic isotonic hypertonic hypotonic
In their natural environments, cells generally contain higher concentrations of biomolecules and ions than their surroundings, so osmotic pressure tends to drive water into cells. If not somehow counterbalanced, this inward movement of water would distend the plasma membrane and eventually cause bursting of the cell called ______________
osmotic lysis In bacteria and plants, the plasma membrane is surrounded by a non-expandable cell wall of sufficient rigidity and strength to resist osmotic pressure and prevent osmotic lysis.
The total hydrogen ion concentration from all sources is experimentally measurable and is expressed as the _____ of the solution
pH
Each hydrogen atom bears a partial ___________ charge, and the oxygen atom bears a partial __________ charge in H2O.
positive negative Each hydrogen atom bears a partial positive charge (&+), and the oxygen atom bears a partial negative charge equal in magnitude to the sum of the two partial positives (2&-).
When two different aqueous solutions are separated by a ________________ membrane, water molecules diffusing from the region of higher water concentration to the region of lower water concentration produce ________________
semipermeable membrane osmotic pressure
Water at 25 C has a dielectric constant of 78.5, and for the very nonpolar solvent benzene, the dielectric constant is 4.6. The dependence of r^2 is such that ionic attractions or repulsions operate only very ___________ distance-in the range of 10 to 40 nm (depending on the electrolyte concentration)
short
The ordering of water molecules reduces entropy. The number of ordered water molecules, and therefore the magnitude of the entropy decrease, is proportional to:
the surface area of the hydrophobic solute enclosed within the cage of water molecules.
Three other gases, NH3, NO, and H2S, also have biological roles in some organisms. Are these gases soluble?
these gases are polar and dissolve readily in water.
Why do alcohols, aldehydes, ketones, and compounds containing N-H bonds tend to be soluble in water?
they are polar molecules and can form hydrogen bonds with water molecules.
The free-energy change for dissolving a nonpolar solute in water is thus ___________: ΔG=ΔH-TΔS, where ΔH has a _________ value, ΔS has a _______ value, and ΔG is __________
unfavorable positive negative positive
Regarding van der waals interactions, as the two nuclei draw closer together, their electron clouds begin to repel each other. At the point where the net attraction is maximal, the nuclei are said to be in:
van der waals contact
____________ interactions exist when two nearby nuclei induce dipoles in each other. The nearest approach of two atoms defines the _____________ radius of each
van der waals interactions van der waals radius
What are van der waal interactions?
van der waals interactions are weak interatomic attractions -When two uncharged atoms are brought very close together, their surrounding electron clouds influence each other. Random variations in the positions of the electrons around one nucleus may create a transient electric dipole, which induces a transient, opposite electric dipole in the nearby atom. The two dipoles weakly attract each other, bringing the two nuclei closer. These weak attractions are called van der waals interactions ( or London dispersion forces).
Each atom has a characteristic _______________, a measure of how close that atom will allow another to approach.
van der waals radius
What are colligative properties?
vapor pressure, boiling point, melting point (also freezing point), and osmotic pressure are the properties of a solution that depend on the number of solute particles per unit volume These are called colligative properties because the effect of solutes on all four properties has the same basis: the concentration of water is lower in solutions than in pure water. -The effect of solute concentration on the colligative properties of water is independent of the chemical properties of the solute; it depends only on the number of solute particles (molecules or ions) in a given amount of water.
_________, _______ interactions, in large numbers, decisively influence the folding of macromolecules such as proteins and nucleic acids. The most stable macromolecular conformations are those in which hydrogen bonding is maximize within the molecule and between the molecule and the solvent, and in which hydrophobic moieties cluster in the interior of the molecule away from the aqueous solvent
weak, noncovalent
These stable structures of amphipathic compounds in water, called ________, may contain hundreds or thousands of molecules. By clustering together, nonpolar regions of the amphipathic molecules achieve the greatest thermodynamic stability by minimizing the number of ordered water molecules required to surround hydrophobic portions of the solute molecules, increasing the _______________ of the system. A special case of this hydrophobic effect is the formation of _______________ in biological membranes
micelles entropy lipid bilayers
N2, O2, and CO2 are _________ and __________ soluble in water. NH3 and H2S are ionizable and therefore ____________ soluble in water
nonpolar poorly soluble very soluble
When two aqueous compartments are separated by a semipermeable membrane (such as the plasma membrane separating a cell from its surroundings), water moves across that membrane to equalize the _________ in the two compartments. This tendency for water to move across a semipermeable membrane produces the osmotic pressure.
osmolarity
osmolarity of solution
osmolarity=ic i=van't hoff factor c= solute's molar concentration
Water dissolves salts such as NaCl by:
Water dissolves salts such as NaCl by hydrating and stabilizing the Na+ and Cl- ions, weakening the electrostatic interactions between them and thus counteracting their tendency to associate in a crystalline lattice.
Why is water effective in screening the electrostatic interactions between dissolved ions?
Water is effective in screening the electrostatic interactions between dissolved ions because it has a high dielectric constant, a physical property that reflects the number of dipoles in a solvent.
osmosis
Water movement across a semipermeable membrane driven by differences in osmotic pressure. Bulk flow of water through a semipermeable membrane into another aqueous compartment containing solute at a higher concentration.
Macromolecules such as proteins, DNA, and RNA contain so many sites of potential hydrogen bonding, ionic, van der waals, or hydrophobic clustering that the cumulative effect can be enormous. For macromolecules, what is the native structure in terms of these weak interactions?
For macromolecules, the most stable structure is the native structure. This structure is usually that in which these weak interactions are maximized. -The folding of a single polypeptide or polynucleotide chain into its three-dimensional shape is determined by this principle. -The binding of an antigen to a specific antibody depends on the cumulative effects of many weak interactions. -The energy released when an enzyme binds noncovalently to its substrate is the main source of the enzyme's catalytic power. -The binding of a hormone or a neurotransmitter to its cellular receptor protein is the result of multiple weak interactions.
Hydrogen atoms covalently bonded to carbon atoms do not participate in hydrogen bonding, why?
Hydrogen atoms covalently bonded to carbon atoms do not participate in hydrogen bonding because the electronegativity differences between Carbon and Hydrogen is very slight and therefore the C-H bond is only very weakly polar. - The partial charges on Carbon and Hydrogen are not large enough to separate a distinct (&+) on the hydrogen atom that could be used in a hydrogen bond with an electronegative atom.
In aqueous solvent at 25 C, the available thermal energy can be of the same order of magnitude as the strength of the weak noncovalent interactions shown before, and the interaction between solute and solvent (Water) molecules is nearly as favorable as solute-solute interactions. What does this mean?
Hydrogen bonds and ionic, hydrophobic, and van der waals interactions (weak noncovalent interactions) are continually forming and breaking
Explain how the directionality of the hydrogen bond can influence electrostatic interaction/attraction
Hydrogen bonds are strongest when the bonded molecules are oriented to maximize electrostatic interaction, which occurs when the hydrogen atom and the two atoms that share it are in a straight line: when the acceptor atom is in line with the covalent bond between the donor atom and H. This arrangement puts the partial positive charge of the hydrogen directly between the two partial negative charges. Hydrogen bonds are thus highly directional and capable of holding two hydrogen-bonded molecules or groups in a specific geometric arrangement. Seen later, this property of hydrogen bonds confers very precise three-dimensional structures on protein and nucleic acid molecules, which have many intramolecular hydrogen bonds
The biologically important gases CO2, O2, and N2 are nonpolar molecules. Why?
In O2 and N2, electrons are shared equally by both atoms. In CO2, each C=O bond is polar, but the two dipoles are oppositely directed and cancel each other.
moiety
In organic chemistry, a moiety is a part of a molecule that is given a name because it is identified as a part of other molecules as well -A half of a molecule
Why do polar biomolecules dissolve readily in water? Why are nonpolar biomolecules poorly soluble in water? In aqueous solutions, nonpolar molecules tend to ___________________ together
Polar biomolecules dissolve readily in water because they can replace water-water interactions with energetically favorable water-solute interactions. Nonpolar biomolecules are poorly soluble in water because they interfere with water-water interactions but are unable to form water-solute interactions. Cluster together
What is osmotic pressure
Pressure generated by the osmotic flow of water through a semipermeable membrane into an aqueous compartment containing solute at a higher concentration osmotic pressure, π, is measured as the force necessary to resist water movement, and can be approximated by the van't hoff equation: π=icRT R=gas constant T=temp in kelvin i= van't hoff factor (a measure of the extent to which the solute dissociates into two or more ionic species) c= the solute's molar concentration In dilute NaCl solutions, the solute completely dissociates into Na+ and Cl-, doubling the number of solute particles, and thus i=2. For all nonionizing solutes, i=1. For solutions of several (n) solutes, π is the sum of the contributions of each species: π=(i1c1., i2c2., i3c3...)RT
T/F Hydrogen bonds and ionic, hydrophobic, and van der waals interactions are much weaker than covalent bonds.
T An input of about 350kJ of energy is required to break a mole of C-C single bonds, and about 410kJ is needed to break a mole of C-H bonds, but as little as 4kJ is sufficient to disrupt a mole of typical van der waals interactions. Interactions driven by the hydrophobic effect are also much weaker than covalent bonds, although they are substantially strengthened by a highly polar solvent (a concentrated salt solution, for example). Ionic interactions and hydrogen bonds are variable in strength, depending on the polarity of the solvent and the alignment of the hydrogen-bonded atoms, but they are always significantly weaker than covalent bonds.
T/F One consequence of the large size of enzymes and receptors (relative to their substrates or ligands) is that their extensive surfaces provide many opportunities for weak interactions
T At the molecular level, the complementarity between interacting biomolecules reflects the complementarity and weak interactions between polar and charged groups and the proximity of hydrophobic patches on the surfaces of the molecules.
T/F One consequence of the large size of enzymes and receptors (relative to their substrates or ligands) is that their extensive surfaces provide many opportunities for weak interactions.
T At the molecular level, the complementarity between interacting biomolecules reflects the complementarity and weak interactions between polar and charged groups and the proximity of hydrophobic patches on the surfaces of the molecules.
T/F Some organisms have water-soluble "carrier proteins" (hemoglobin and myoglobin, for example) that facilitate the transport of O2.
T Carbon dioxide forms carbonic acid (H2CO3) in aqueous solution and is transported as the HCO3- (bicarbonate) ion. -Either free, or bound to hemoglobin, bicarbonate is very soluble in water
T/F Although noncovalent interactions (hydrogen bonds, ionic, hydrophobic, and van der waals interactions) are individually weak relative to covalent bonds, the cumulative effect of many such interactions can be very significant.
T For example, the noncovalent binding of an enzyme to its substrate may involve several hydrogen bonds and one or more ionic interactions, as well as the hydrophobic effect and van der Waals interactions. The formation of each of these associations contributes to a net decrease in the free energy of the system.
T/f The solvent properties of water shaped the evolution of living things.
T Most small intermediates of metabolism, as well as nucleic acids and proteins, are soluble in water. Lipid bilayers, the likely forerunners of biological membranes, form spontaneously in water and are stabilized by their interaction with it. Although hydrogen bonds, ionic interactions, and the hydrophobic effect are individually weak, their combined effects powerfully influence the three-dimensional shape and stability of biological molecules and structures.
T/F The hydrophobic effect on interactions among lipids, and between lipids and proteins, is the most important determinant of structure in biological membranes. T/F The aggregation of nonpolar amino acids in protein interiors, driven by the hydrophobic effect, also stabilizes the three-dimensional structures of proteins.
T T
T/F In multicellular animals, blood plasma and interstitial fluid (the extracellular fluid of tissues) are maintained at an osmolarity close to that of the cytosol
T The high concentration of albumin and other proteins in blood plasma contributes to its osmolarity. Cells also actively pump out Na+ and other ions into the interstitial fluid to stay in osmotic balance with their surroundings.
T/F Hydrogen bonds are relatively weak.
T The hydrogen bonds present in water have a bond dissociation energy (the energy required to break a bond) of about 23kj/mol O-H bonds (covalent) in water have 470kj/mol
T/F The attractive forces between water molecules and the slight tendency of water to ionize are of crucial importance to the structure and function of biomolecules.
T The water molecule and its ionization products, H+ and OH-, profoundly influence the structure, self-assembly, and properties of all cellular components, including proteins, nucleic acids, and lipids.
T/F When the structure of a protein such as hemoglobin is determined by x-ray crystallography, water molecules are often found to be bound so tightly that they are part of the crystal structure; the same is true for water in crystals of RNA or DNA.
T These bound water molecules have properties that are distinctly different from those of the "bulk" water of the solvent. For example, the bound water molecules are not osmotically active. For many proteins, tightly bound water molecules are essential to their function.
T/F Water readily dissolves carboxylic acids (-COO-), protonated amines (-NH3+), and phosphate esters or anhydrides. Water replaces the _________________________ bonds linking these biomolecules to each other with ________________________ bonds, thus screening the electrostatic interactions between solute molecules
T solute-solute hydrogen bonds solute-water hydrogen bonds
T/F At room temperature, both the melting of ice and the evaporation of water occur spontaneously
T the tendency of the water molecules to associate through hydrogen bonds is outweighed by the energetic push toward randomness. ΔG (Free energy change) must be negative for a process to occur spontaneously:ΔG=ΔH -TΔS, where ΔG represents the driving force, ΔH the enthalpy change from making and breaking bonds, and ΔS the change in randomness. -Because ΔH is positive for melting and evaporation, it is clearly the increase in entropy (ΔS) that makes ΔG negative and drives these changes.
T/F All molecules or ions in aqueous solution interfere with the hydrogen bonds of some water molecules in their immediate vicinity
T, but polar or charged solutes (such as NaCl) compensate for lost water-water hydrogen bonds by forming new solute-water interactions. The net change in enthalpy for dissolving these solutes is generally small. Hydrophobic solutes, however, off no such compensation, and their addition to water may therefore result in a small gain of enthalpy; the breaking of hydrogen bonds between water molecules takes up energy from the system, requiring the input of energy from the surroundings.
Each hydrogen atom of a water molecule shares an electron pair with the central oxygen atom. The geometry of the molecule is dictated by: The H-O-H bond angle is 104.5, slightly less than the 109.5 of a perfect tetrahedron, why?
The geometry of the molecule is dictated by the shapes of the outer electron orbitals of the oxygen atom, which are arranged similar to the sp3 bonding orbitals of carbon, displaying a rough tetrahedron the bond angle is slightly less than 109.5 because of the crowding by the lone pairs occupying the nonbonding orbitals of the oxygen atom
Why are CO2, O2, and N2 not soluble?
The movement of molecules from the disordered gas phase into aqueous solution constrains their motion and the motion of water molecules and therefore represents a decrease in entropy. The nonpolar nature of these gases and the decrease in entropy when they enter solution combine to make them very poorly soluble in water.
Ionic interactions between dissolved ions are much stronger in less polar environments. Why is this?
This is because there is less screening of charges by the nonpolar solvent.
When water is mixed with benzene or hexane, what happens?
Two phases form because neither liquid is soluble in the other. Nonpolar compounds such as benzene and hexane are hydrophobic-they are unable to undergo energetically favorable interactions with water molecules, and they interfere with the hydrogen bonding among water molecules.
