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BIOPHYSICAL ANDBIOPHYSICAL ANDBIOCHEMICALBIOCHEMICAL

PRINCIPLESPRINCIPLES

Molecular structure and properties of water

Expression of concentrationBuffers

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WATERWATER

Content: 80% of the mass of living

organisms

All the chemical reactions of life take

place in aqueous solution.

Is a reactant or product in manymetabolic reactions

General formula of water is H2O.

O

H H

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Water Molecule

Composed of:

1 Oxygen Atom

2 Hydrogen Atoms

Share electrons

forming strong

covalent bonds.

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CharacteristicsCharacteristics

made up of two atoms of hydrogen and one atom ofoxygen forming a triangular shape (an angle about 105o).

has asymmetrical distribution of charge: one end (the Oatom) carries a slightly negative charge (-) and theother ends of the molecule (the H atoms) carry slightlypositive charges (+).

leads to an unequal / uneven distribution of electrical

charge , thus, water is a polar molecule. atoms results in a water molecule with slight negative

and positive sides, called a dipole.

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What properties make water

such an amazing molecule?

The polarity of

water molecules:

allows them to form

hydrogen bonds with

each other.

contributes to thevarious properties

water exhibits.

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Water molecules form hydrogen bondsWater molecules form hydrogen bonds

The bonds enable water to dissolve may

organic biomolecules that contain

functional groups that can participate inhydrogen bonding.

O atoms of aldehydes, ketones and

amides provide pairs of electrons

(hydrogen acceptors) while alcohols andamines can serve as both hydrogen

acceptors and donors.

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Formation of hydrogen bondingFormation of hydrogen bonding

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In ice, each water molecule is surrounded by four other molecules in atetrahedral arrangement (left). This new result on liquid water shows that the

molecules are connected only with two others. This implies that most molecules

are arranged in strongly hydrogen-bonded rings (middle) or chains (right)

embedded in a disordered cluster network connected mainly by weak hydrogen

bonds. The oxygen atoms are red and the hydrogen atoms are gray in the water

(H2O) molecules. (Figure courtesy H. Ogasawara, SSRL).

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Molecular Structure ofWater

Hydrogen bonds,

weak bonds between

adjacent molecules,

give water its uniqueproperties.

public domain image via Wikipedia Creative Commons

http://www.edinformatics.com/

math_science/dimer.htm

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Properties of waterProperties of water

Thermal propertiesThermal properties

Heat fusionHeat fusion

Heat vaporizationHeat vaporization

Specific heatSpecific heat

DensityDensity

Solvent propertiesSolvent properties

Dissolve in ionic andDissolve in ionic and

polar due to its dipolarpolar due to its dipolar

and hydrogen bondand hydrogen bond

AmphipathicAmphipathic

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Thermal Properties ofWaterThermal Properties ofWater

1.1. DensityDensity

When water becomes a solid or converts to ice, itexpands.

Solid water or ice is less dense than liquid water andtherefore it floats on liquid water.

This phenomenon is due to hydrogen bonding.

Water has its maximum density at 4Water has its maximum density at 4 C where the colderC where the colderwater will freeze forming ice.water will freeze forming ice.

This provides insulation by preventing the water belowThis provides insulation by preventing the water belowfrom freezing and allows aquatic ecosystems to survivefrom freezing and allows aquatic ecosystems to surviveunder subunder sub--zero temperatures.zero temperatures.

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Molecular Structure of Ice

The lattice

structure of ice

allows it to floaton the waters

surface.

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Life in FreezingWaters

How do fish survive in super-cooledarctic seas?

They have a variety of antifreezes. Theantifreeze molecules allow ice-fish to livein subfreezing water by plugging gaps inexisting small ice crystals and preventing

the attachment of more ice molecules. Ice crystal growth is thus effectively

stopped.

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2.2. High Latent Heat of VaporisationHigh Latent Heat of Vaporisation

Heat of vaporization is the quantity of heat aliquid must absorb for 1 gram of it to beconverted from a liquid to a gas.

This means that water must absorb a lot of heatThis means that water must absorb a lot of heatto break hydrogen bonds between its neighborsto break hydrogen bonds between its neighborsso that it can convert liquid water into waterso that it can convert liquid water into watervapour (gaseous phase).vapour (gaseous phase).

This makes water as an effective coolant whereThis makes water as an effective coolant whereanimals sweat and pantanimals sweat and pant to cool themselvesto cool themselves) and) andplants undergo transpiration.plants undergo transpiration.

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3.3. High Specific Heat CapacityHigh Specific Heat Capacity

A great deal of energy is required to raiseA great deal of energy is required to raise

the temperature of 1 kg by 1the temperature of 1 kg by 1 C, whichC, which

means that sudden temperature changesmeans that sudden temperature changesdo not occur easily.do not occur easily.

Temperatures remain relatively constantTemperatures remain relatively constantmeaning water does not changemeaning water does not change

temperature very easily.temperature very easily.

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Because water has a high specific heat, it can

minimize temperature fluctuations to within limits thatpermit life.

This moderation occurs because:

Heat is absorbed when hydrogen bonds break (heat offusion whereby acceleration of water molecules occur).

Heat is released when hydrogen bonds form.

This means that water can absorb a lot of heat

before it changes its temperature by 1oC. It

also means that water has to lose a lot of heat

before it changes its temperature by 1oC.

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What gives water its high heat capacity?

As ice is warmed and begins to melt, these

hydrogen bonds are increasingly disrupted

causing the open spaces in the ice lattice to fill

in.

As a consequence, density increases; this effect

reaches its maximum at 4oC, the temperature atwhich pure water reaches its greatest density.

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4.4. Cohesion and Surface TensionCohesion and Surface Tension

Cohesion: intermolecular attractionAdhesion: attraction of water to solid

phase (cell wall),

Water molecules "stick together" and has

high cohesion.

Water molecules stick to one another =

cohesion and surface tension.

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Cohesion

is the bonding of a high percentage of the molecules

to neighbouring molecules involves attractive force between particles of the

same kind.

can results in high surface tension.

is due to hydrogen bonding

Adhesion

is the clinging of one substance to another of waterto the walls of cells.

results in capillary rise.

can results in high tensile strength.

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Cohesion and adhesion in plantsCohesion and adhesion in plants

long columns of water can be sucked uplong columns of water can be sucked up

through the microscopic vessels of plants by tallby tall

trees by transpiration stream to the leaves attrees by transpiration stream to the leaves at

the top of the plant without breaking.the top of the plant without breaking.

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Surface tension

Is a measure of how

hard it is to break the

surface of a liquid.

Is related to cohesion.

Is determined by

hydrogen bonding.

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Solvent Properties of waterSolvent Properties of water

Ability to dissolve ionic and polarAbility to dissolve ionic and polar

substances.substances.

Determined by:Determined by:(1) its dipolar structure and(1) its dipolar structure and

(2) its capacity to form hydrogen bonds.(2) its capacity to form hydrogen bonds.

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Water is a universal solvent

It forms electrostatic interactions with ions and keeps ions in solution.

Dissolution of an ionically bonded compound, sodium chloride, by water

molecules. Image from Purves et al., Life: The Science of Biology, 4th Edition,

by Sinauer Associates (www.sinauer.com) andWH Freeman

(www.whfreeman.com), used with permission.

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Water is a universal solvent

Waters bipolar moleculedissolves salts and holds insolution (salts include manycompounds that can be dissolvedinto +/-ions).

NaCl Na+ and Cl-

Water molecules separate bybreaking strong covalent bondbetween H and O to form new

compounds.

CO2 + HOH H2CO3

Because water can disruptcovalent and ionic bonds, it is

considered a universal solvent.

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Water is a universal solvent

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Ions of saltsIons of salts

NaNa++ and Cland Cl-- dissolves easily in waterdissolves easily in water

because dipolar water molecules arebecause dipolar water molecules are

attracted to these ions, forming shells ofattracted to these ions, forming shells of

water molecules known aswater molecules known as solvationsolvation

spheresspheres..

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Organic moleculesOrganic molecules

With ionizable groups (carboxyl, amino),With ionizable groups (carboxyl, amino),

polar functional groups (alcohol, aldehyde)polar functional groups (alcohol, aldehyde)

also dissolve in water due to ability of thealso dissolve in water due to ability of the

organic molecules to form hydrogen bondsorganic molecules to form hydrogen bonds

with water molecules.with water molecules.

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Hydrophilic and HydrophobicHydrophilic and Hydrophobic

Nonpolar molecules are

hydrophobic.

Polarand ionicmolecules have positive

and negative charges and

are therefore attracted to

water molecules because

water molecules are also

polar

Polar and ionic molecules

are hydrophilic.

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Interaction with water influencesInteraction with water influences

the structure of biomoleculesthe structure of biomolecules Covalent and noncovalent bonds stabilize

biological molecules.

These forces can be either attractive orrepulsive.

Biomolecules fold to position polar and

charges groups on their surface amphipathic.

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Most biomolecules are

amphipathic possessregions rich in charged or

polar functional groups as

well as regions withhydrophobic character.

This pattern increase the tendency for the

formation of energetically favorable charge-dipole,dipole-dipole and hydrogen bonding interactions

between polar groups on the biomolecule and

water and vice-versa with hydrophobic groups.

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Amphipathic moleculesAmphipathic molecules

Contain both polar andContain both polar andnonpolar groups (soaps, bilenonpolar groups (soaps, bilesalts and phospholipids).salts and phospholipids).

FormForm micellesmicelles in waterin waterwhere the nonpolar tailswhere the nonpolar tailsaggregate in the centeraggregate in the centeraway from water and theaway from water and thepolar heads form thepolar heads form thehydrogen bonds with thehydrogen bonds with thesurrounding watersurrounding watermolecules.molecules.

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Water is an excellentWater is an excellent

nucleophilenucleophile NucleophileNucleophile metabolic reactions involvemetabolic reactions involve

attack by lone pair of electrons onattack by lone pair of electrons on

electronelectron--rich moleculesrich molecules Do not possess formalDo not possess formal ve or +ve charge.ve or +ve charge.

Nucleophile attack by water results in theNucleophile attack by water results in the

cleavage of amide, glycoside or estercleavage of amide, glycoside or esterbonds that holds the biopolymers togetherbonds that holds the biopolymers together

(process(process hydrolysis)hydrolysis)

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Inorganic CompoundsInorganic Compounds

are small compounds that do not containare small compounds that do not contain

the atoms C and H.the atoms C and H.

most important ones in humans includemost important ones in humans includecarbon dioxide (COcarbon dioxide (CO22) water, salts, acids &) water, salts, acids &

bases.bases.

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Inorganic CompoundsInorganic Compounds

SaltsSalts dissociate (ionize) intodissociate (ionize) into

ionsions when dissolved inwhen dissolved in

water.water.

yy an anion is formed andan anion is formed andyy a cation is formed.a cation is formed.

yy Example = NaCl in water.Example = NaCl in water.

HH22OO

NaClNaCl ------------------> Na> Na++ + Cl+ Cl--

IonsIons electrolyteselectrolytes (charged(charged

particles) that must beparticles) that must be

maintained within a verymaintained within a very

narrow range in our bloodnarrow range in our bloodand tissues (i.e.and tissues (i.e.

homeostasis);homeostasis);

required for musclerequired for muscle

contraction, nervecontraction, nerveimpulses, etc.;impulses, etc.;

Examples include NaExamples include Na++,,

KK++, Cl, Cl--, Ca, Ca++, PO, PO44--; HCO; HCO33

--,,

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Inorganic CompoundsInorganic Compounds

AcidsAcids

dissociate (ionize) indissociate (ionize) in

water intowater into aa hydrogenhydrogen

catcationion,, HH++ and an anion.and an anion.yy Example = HClExample = HCl

(hydrochloric acid).(hydrochloric acid).

HH22OO

HClHCl ---------------------->> HH++ + Cl+ Cl--

BasesBases

dissociate (ionize) in waterdissociate (ionize) in water

intointo aa hydroxylhydroxyl ananionion,, OHOH--

andand a cation.a cation.yy Example = NaOH (sodiumExample = NaOH (sodium

hydroxide).hydroxide).

HH22OO

NaOHNaOH ----------------------> Na> Na++ ++ OHOH--

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Nonpolar organic moleculesNonpolar organic molecules

Cannot dissolveCannot dissolve in water (triglycerides,in water (triglycerides,

cholesterol esters).cholesterol esters).

However, in the presence of amphipathicHowever, in the presence of amphipathicmolecules, they can form anmolecules, they can form an emulsionemulsion

where the organic molecules arewhere the organic molecules are

surrounded by the amphipathic moleculessurrounded by the amphipathic molecules

that are in contact with water molecules.that are in contact with water molecules.

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Water molecules exhibit a slight butWater molecules exhibit a slight but

important tendency to dissociateimportant tendency to dissociateWater can act both as an acid and a baseWater can act both as an acid and a base

whereby its ionization involveswhereby its ionization involves

intermolecular proton transferintermolecular proton transfer formingforminga hydronium ion (Ha hydronium ion (H33OO

++) and a hydroxide) and a hydroxide

ion (OHion (OH--))

H2O + H2OH2O + H2O HH33OO++ + OH+ OH--

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pH of some common items. Image from Purves et al.,pH of some common items. Image from Purves et al., Life: The Science ofLife: The Science of

BiologyBiology, 4th Edition, by Sinauer Associates (, 4th Edition, by Sinauer Associates (www.sinauer.comwww.sinauer.com) andWH) andWH

Freeman (Freeman (www.whfreeman.comwww.whfreeman.com), used with permission.), used with permission.

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Expression of ConcentrationExpression of Concentration

The concentration of a solution representsThe concentration of a solution represents

the amount of solute dissolved in a unitthe amount of solute dissolved in a unit

amount of solvent or of solution.amount of solvent or of solution.

can be expressed in a variety of ways:can be expressed in a variety of ways:

--qualitatively;qualitatively;

--semi quantitatively; orsemi quantitatively; or--quantitative.quantitative.

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Qualitative Expressions ofQualitative Expressions of

ConcentrationConcentrationA solution can be qualitatively describedA solution can be qualitatively described

as:as:

1. dilute1. dilute: a solution that contains a small: a solution that contains a smallproportion of solute relative to solvent,proportion of solute relative to solvent,

oror

2. concentrated2. concentrated: a solution that contains a: a solution that contains alarge proportion of solute relative tolarge proportion of solute relative to

solvent.solvent.

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SemiSemi--QuantitativeQuantitative

Expressions of ConcentrationExpressions of ConcentrationA solution can be semiA solution can be semi--quantitatively describedquantitatively describedasas

1. unsaturated1. unsaturated: a solution in which more solute: a solution in which more solute

will dissolve, orwill dissolve, or

2. saturated2. saturated: a solution in which no more solute: a solution in which no more solute

will dissolve.will dissolve.

TheThe solubilitysolubilityof a solute is the amount of soluteof a solute is the amount of solute

that will dissolve in a given amount of solvent tothat will dissolve in a given amount of solvent to

produce a saturated solution.produce a saturated solution.

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Quantitative Expressions ofQuantitative Expressions of

ConcentrationConcentration There are a number of ways to express theThere are a number of ways to express therelative amounts of solute and solvent in arelative amounts of solute and solvent in a

solution which depends on convenience.solution which depends on convenience.

For example, it is sometimes easier to measureFor example, it is sometimes easier to measurethe volume of a solution rather than the mass ofthe volume of a solution rather than the mass of

the solution.the solution.

Some expressions for concentration areSome expressions for concentration are

temperaturetemperature--dependent and others are not.dependent and others are not.

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Expressions of ConcentrationExpressions of Concentration

Percent Composition (by mass)Percent Composition (by mass)

MolarityMolarity

MolalityMolality Mole FractionMole Fraction

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Percent CompositionPercent Composition

(by mass)(by mass) Can consider in 2 ways:Can consider in 2 ways: The parts of solute per 100 parts of solution.The parts of solute per 100 parts of solution.

The fraction of a solute in a solution multiplied byThe fraction of a solute in a solution multiplied by

100.100.

To calculate the percent by mass of a solute in aTo calculate the percent by mass of a solute in a

solution must know 1) the mass of the solute insolution must know 1) the mass of the solute in

the solution and 2) the mass of the solution.the solution and 2) the mass of the solution. Percent by mass =Percent by mass = mass of solutemass of solute x 100x 100

mass of solutionmass of solution

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MolarityMolarity

The number of moles of solute in exactly oneThe number of moles of solute in exactly one

liter of a solution.liter of a solution.

To calculate the molarity of a solute in a solutionTo calculate the molarity of a solute in a solution

must know 1) the moles of solute present in themust know 1) the moles of solute present in thesolution and 2) the volume of solution (in liters)solution and 2) the volume of solution (in liters)

containing the solute.containing the solute.

Molarity =Molarity = moles of solute_________moles of solute_________

volume of solution in litersvolume of solution in liters

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Molar solutionsMolar solutions ((MM) contain one gram) contain one grammole of the substance per liter.mole of the substance per liter.

MolarityMolarityof a solution is the number ofof a solution is the number of

moles of the substance per one liter ofmoles of the substance per one liter of

solution.solution.

MolarityMolarity((mol/Lmol/L) =) = concentration (g/L)

molecular weight (molecular weight (gg))

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MoleMole

MoleMole (abbrv.(abbrv. molmol) composed of 6.022 x) composed of 6.022 x

1023 objects known as1023 objects known as AvogradosAvogrados

numbernumber..

A mole of a certain substance is found inA mole of a certain substance is found in

one gram mole (g mole) of this substanceone gram mole (g mole) of this substance

by weighing the atomic / molecular weightby weighing the atomic / molecular weight

of this substance in grams.of this substance in grams.

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NormalNormal solutions (solutions (NN) contain one gram) contain one gram

equivalent (equivalent (g Eqg Eq) of the substance per) of the substance perliter.liter.

Equivalent weightEquivalent weight== Molecular WeightMolecular Weight

ValenceValence

Normality (eq/L) =Normality (eq/L) = Concentration (g/L)Concentration (g/L)

Equivalent weightEquivalent weight

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MolalityMolality

the number of moles of solute dissolved inthe number of moles of solute dissolved in

exactly one kilogram of solvent.exactly one kilogram of solvent.

to calculate the molality of a solute in ato calculate the molality of a solute in asolution must know 1) the moles of solutesolution must know 1) the moles of solute

present in the solution and 2) the mass ofpresent in the solution and 2) the mass of

solvent (in kilograms) in the solution.solvent (in kilograms) in the solution.

Molality =Molality = moles of solutes______moles of solutes______

mass of solvents in kgmass of solvents in kg

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MolalMolal solutions contain one mole of thesolutions contain one mole of thesubstance per 1000g of solvent characterized bysubstance per 1000g of solvent characterized bya fixed ratio between solute and solventa fixed ratio between solute and solventmolecules.molecules.

1000g of water = 55.5 moles1000g of water = 55.5 moles

Aqueous solution = 1:55.5Aqueous solution = 1:55.5

Molality (Molality (mol/kgmol/kg) =) = Concentration (Concentration (g/kgg/kg))

Molecular weightMolecular weight

Important:Important:

Molality determine the osmotic pressure ofMolality determine the osmotic pressure ofsolutionssolutions

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Dissociation of waterDissociation of water

1 mole (mol) of water weighs 18g1 mole (mol) of water weighs 18g

1 liter (L) (1000g) of water contains1 liter (L) (1000g) of water contains

10001000 18 = 55.56 mol/L18 = 55.56 mol/LPure water is 55.56 molarPure water is 55.56 molar

A hydrogen in pure water exist as hydrogenA hydrogen in pure water exist as hydrogen

ions is 1.8 x 10ions is 1.8 x 10--99

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Dissociation constant,Dissociation constant, KK

Molar conc. of H+ ions or OHMolar conc. of H+ ions or OH-- ions is 1.8 xions is 1.8 x

1010--99 multiply with molar conc. of water 55.56.multiply with molar conc. of water 55.56.

Result = 1.0 x 10Result = 1.0 x 10

--77

mol/Lmol/LTo calculate K for pure water:To calculate K for pure water:

KK== [H+] [OH[H+] [OH--]] == [10[10--77] [10] [10--77]] = 1.8 x 10= 1.8 x 10--1616 mol/Lmol/L

[H[H22O]O] [55.56][55.56]

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Ion product,Ion product, KKww

KK== [H+] [OH[H+] [OH--]] = 1.8 x 10= 1.8 x 10--1616 mol/Lmol/L

[H[H22O]O]

KKww= (K) [H2O] = [H+] [OH= (K) [H2O] = [H+] [OH--]]= (1.8 x 10= (1.8 x 10--1616 mol/L) (55.56 mol/L)mol/L) (55.56 mol/L)

= 1.00 x 10= 1.00 x 10--1414 (mol/L)(mol/L)22

Importance ofImportance ofKKww::

to calculate the pH of acidic and basic solutionsto calculate the pH of acidic and basic solutions

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pHpH

defined as is the negative log of the hydrogendefined as is the negative log of the hydrogen

ion concentrationion concentration

pH =pH = -- log [H+]log [H+]

To calculate pH:To calculate pH:

1)1) Calculate hydrogen ion conc.Calculate hydrogen ion conc.

2)2) Calculate the base 10 logarithm of H+.Calculate the base 10 logarithm of H+.3)3) pH is negative value.pH is negative value.

pH for pure water, pH=pH for pure water, pH= -- log 10log 10--77 == -- ((--7) = 7.07) = 7.0

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pHpH

pH Scale ranges from 0 to 14:pH Scale ranges from 0 to 14:

00 ------------------------------------77----------------------------------------1414

acidacid neutralneutral basicbasic[H[H++] > [OH] > [OH--]] [H[H++] = [OH] = [OH--] [H] [H++] < [OH] < [OH--]]

Physiologic pH = 7.4Physiologic pH = 7.4

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Importance of pHImportance of pH

Knowledge of the dissociation of weakKnowledge of the dissociation of weak

acids and bases thus is basic toacids and bases thus is basic to

understanding the influence of intracellularunderstanding the influence of intracellular

pH on structure and biologic activity.pH on structure and biologic activity.

ChargeCharge--based separations such asbased separations such as

electrophoresis and ion exchangeelectrophoresis and ion exchange

chromatography also are best understoodchromatography also are best understoodin terms of the dissociation behaviour ofin terms of the dissociation behaviour of

functional groupsfunctional groups

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Low pH valuesLow pH values high conc. of H+ ionshigh conc. of H+ ions

High pH valuesHigh pH values low conc. of H+ ionslow conc. of H+ ions

Many biochemicals possess functionalMany biochemicals possess functional

groups that are weak acids or bases.groups that are weak acids or bases.

Carboxyl groups, amino groups, andCarboxyl groups, amino groups, and

phosphate esters, whose secondphosphate esters, whose second

dissociation falls within the physiologicdissociation falls within the physiologicrange, are present in proteins and nucleicrange, are present in proteins and nucleic

acids, most coenzymes, and mostacids, most coenzymes, and most

intermediary metabolites.intermediary metabolites.

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Importance of dissociationImportance of dissociation

Knowing the dissociation of weak acidsKnowing the dissociation of weak acids

and basesand bases understanding the influenceunderstanding the influence

of intracellular pH on structure andof intracellular pH on structure and

biological activity.biological activity.

Methodology / techniques to studyMethodology / techniques to study

dissociation behavior of functional groups :dissociation behavior of functional groups :

electrophoresis and ion exchangeelectrophoresis and ion exchangechromatography.chromatography.

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Law of Mass ActionLaw of Mass Action

The law stating that the rate of any givenThe law stating that the rate of any given

chemical reaction is proportional to thechemical reaction is proportional to the

product of the activities (or concentrations)product of the activities (or concentrations)

of the reactants.of the reactants.

In other words,In other words, the rthe rate of chemicalate of chemical

reaction isreaction is directly proportionaldirectly proportional to theto the

product of the molecular concentration ofproduct of the molecular concentration ofthe reacting substances at a giventhe reacting substances at a given

temperature and set of conditions.temperature and set of conditions.

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Expression from theExpression from the

Law of Mass ActionLaw of Mass Action The relative concentrations of reactants andThe relative concentrations of reactants and

products in terms of a quantity calledproducts in terms of a quantity calledKKeqeq known asknown as

equilibrium constantequilibrium constantof the reaction indicating theof the reaction indicating the

ratio between the products and the reactants whenratio between the products and the reactants whenequilibrium is attained.equilibrium is attained.

The value of KThe value of Keqeq is only affected by temperatureis only affected by temperature

and must be positive.and must be positive.

When KWhen Keqeq > 1, at equilibrium the concentration of> 1, at equilibrium the concentration ofproduct(s) is far greater reactants.product(s) is far greater reactants.

When KWhen Keqeq < 1, at equilibrium the concentration of< 1, at equilibrium the concentration of

reactants is much greater than products.reactants is much greater than products.

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BUFFERSBUFFERS

Solutions that resist changing of pH in spiteSolutions that resist changing of pH in spite

of the addition of moderate acid or alkaliof the addition of moderate acid or alkali

Composed of either:Composed of either:

(1) weak acid and its salt with strong base(1) weak acid and its salt with strong base

e.g. acetic acid + sodium acetatee.g. acetic acid + sodium acetate

(2) weak base and its salt with strong acid(2) weak base and its salt with strong acid

e.g. ammonium hydroxide + ammoniume.g. ammonium hydroxide + ammonium

chloridechloride

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Behavior of weak acids andBehavior of weak acids and

buffersbuffers

Can be described quantitatively using theCan be described quantitatively using the

HendersonHenderson--Hasselbalch EquationHasselbalch Equation..

pH = pKpH = pKaa + log+ log [A[A--]_]_

[HA][HA]

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pH of bufferspH of buffers

Dissociation of weak acids to buffer systemDissociation of weak acids to buffer systemcontaining acids at equilibrium.containing acids at equilibrium.

HendersonHenderson--Hasselbalch equationHasselbalch equation shows thatshows thatthe pH of the buffer depends upon the ratiothe pH of the buffer depends upon the ratio

between the concentrations of salt and the acid.between the concentrations of salt and the acid.At ratio of 1:1, the pH of the buffer = pkAt ratio of 1:1, the pH of the buffer = pkaa The buffering power of the system would be atThe buffering power of the system would be at

maximum, the ratio between the salt and acidmaximum, the ratio between the salt and acid

being least affected by either changes.being least affected by either changes. The stronger the acid, the higher kThe stronger the acid, the higher kaa, the lower its, the lower its

pkpkaa

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It is important to know that many metabolicIt is important to know that many metabolic

reactions are accompanied by the releasereactions are accompanied by the release

or uptake of protons, most intracellularor uptake of protons, most intracellular

reactions are buffered.reactions are buffered.

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Buffering SystemsBuffering Systems

Definition:Definition:

Buffers are compounds added to solutions toBuffers are compounds added to solutions to

prevent abrupt change in pH.prevent abrupt change in pH.

yy usuallyusually weakweak acids;acids;yy function by donating Hfunction by donating H++ when needed and bywhen needed and by

accepting Haccepting H++ when in excess;when in excess;

yy very important in biological systemsvery important in biological systemsyy Example :Example : carbonic acidcarbonic acid (H(H22COCO33) buffering system.) buffering system.

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Biological buffersBiological buffers

Buffering is an important property inBuffering is an important property in

biological systems, for which rapid pHbiological systems, for which rapid pH

changes can have disastrouschanges can have disastrous

consequences.consequences.

An effective biological buffer must helpAn effective biological buffer must help

maintain blood in the "safe" range ofmaintain blood in the "safe" range ofpHpH

7.357.35--7.457.45 by resisting pH changes inby resisting pH changes ineither direction outside of this range.either direction outside of this range.

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Biological buffersBiological buffers

Excursions in the acid direction (i.e.,Excursions in the acid direction (i.e.,below 7.35) are particularly to bebelow 7.35) are particularly to be

feared, given the variations infeared, given the variations in

production of acids such as lacticproduction of acids such as lactic

acid, pyruvic acid, acetic acid, etc.,acid, pyruvic acid, acetic acid, etc.,by metabolism.by metabolism.

Production of such acids during peakProduction of such acids during peak

exercise can lower peripheral bloodexercise can lower peripheral blood

pH to well below 7.0.pH to well below 7.0.

The most important single buffer inThe most important single buffer in

human blood is the bicarbonate ion.human blood is the bicarbonate ion.

HCO3-

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Physiologic Buffer SystemsPhysiologic Buffer Systems

Enzyme is very sensitive to changes to pH.Enzyme is very sensitive to changes to pH.

As enzymes are important in controllingAs enzymes are important in controllingmetabolic reaction, thus, acid/base formedmetabolic reaction, thus, acid/base formedinside the body should be buffered to allowinside the body should be buffered to allow

reactions to proceed.reactions to proceed. This is the function of the physiological bufferThis is the function of the physiological buffer

system present in tissues and body fluids.system present in tissues and body fluids.

There are 3 systems:There are 3 systems:

(1) Bicarbonate system*(1) Bicarbonate system*(2) Phosphate system(2) Phosphate system

(3) Protein system(3) Protein system

Bi b t b ff i tBi b t b ff i t

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Bicarbonate buffering systemBicarbonate buffering system central to pH regulation in human blood and can respondcentral to pH regulation in human blood and can respond

to pH changes in several ways:to pH changes in several ways:

1.1. The bicarbonate ion is really the conjugate base ofThe bicarbonate ion is really the conjugate base of

carbonic acid .carbonic acid .

2.2. Carbonic acid is very rapidly interconverted with COCarbonic acid is very rapidly interconverted with CO22

and water by carbonic anhydrase, making it a transientand water by carbonic anhydrase, making it a transientspecies.species.

3.3. Water is easily absorbed into the system, while COWater is easily absorbed into the system, while CO22 cancan

be expelled by respiration which loss can be controlledbe expelled by respiration which loss can be controlled

via neurological mechanisms.via neurological mechanisms.

4.4. [HCO[HCO33--] and [H] and [H++] can be manipulated (slowly and] can be manipulated (slowly and

incompletely) by physiological mechanisms operating inincompletely) by physiological mechanisms operating in

the kidney.the kidney.

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Blood BuffersBlood Buffers

Include all the physiological buffer systems.Include all the physiological buffer systems.

All these buffer systems are operative to a certain extent.All these buffer systems are operative to a certain extent.

It should be noted that:It should be noted that:

(1) the hemoglobin and oxyhemoglobin systems are(1) the hemoglobin and oxyhemoglobin systems areresponsible for the buffering of most COresponsible for the buffering of most CO22 added to theadded to theblood by tissues.blood by tissues.

(2) the bicarbonate system is the most efficient for the(2) the bicarbonate system is the most efficient for thebuffering of all acids added to the blood other thanbuffering of all acids added to the blood other thancarbonic acid.carbonic acid.

(3) the buffer systems of the blood are so efficient that they(3) the buffer systems of the blood are so efficient that theykeep the blood pH within a very limited range (pH7.35keep the blood pH within a very limited range (pH7.35--7.45) in spite of the continuous addition*.7.45) in spite of the continuous addition*.

b i idb i id (H(H COCO ))

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carbonic acidcarbonic acid (H(H22COCO33))

buffering systembuffering systemwhen pH is risingwhen pH is rising

HH22COCO33 HCOHCO33-- ++ HH++

when pH is fallingwhen pH is falling

Carbonic acidCarbonic acid bicarbonate ion hydrogen ionbicarbonate ion hydrogen ion

(H+ donor)(H+ donor) (H+ acceptor)(H+ acceptor)

As physiologic pH = 7.4.As physiologic pH = 7.4. pH < 7.4 = acidosis; lethal below 7.0;pH < 7.4 = acidosis; lethal below 7.0;

pHpH > 7.4 = alkalosis; lethal above 7.8.> 7.4 = alkalosis; lethal above 7.8.

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Bicarbonate buffering systemBicarbonate buffering system

is best suited to deal with acidosis, in partis best suited to deal with acidosis, in part

because respiration expels CObecause respiration expels CO22 and therebyand thereby

diminishes Hdiminishes H++..

In metabolically active tissues, such as muscleIn metabolically active tissues, such as muscleduring intense exercise, production of COduring intense exercise, production of CO22 (and(and

acid, in the form of lactic acid) can lead to aacid, in the form of lactic acid) can lead to a

transient fall in local pH.transient fall in local pH.

The system in blood can respond quickly to mildThe system in blood can respond quickly to mildacidosis (between pH 7.15 and 7.35) byacidosis (between pH 7.15 and 7.35) by

adjusting the COadjusting the CO22 release rate in the lungs.release rate in the lungs.

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Continuous addition ofContinuous addition of

(1)(1) Large volumes ofLarge volumes ofCOCO22 due to oxidation ofdue to oxidation ofcarbohydrates, proteins and fats.carbohydrates, proteins and fats.

(2)(2) SulfuricSulfuric andand phosphoric acidsphosphoric acids due to oxidation ofdue to oxidation ofproteins, phosphoric acid and uric acids from oxidationproteins, phosphoric acid and uric acids from oxidation

of nucleic acids.of nucleic acids.(3)(3) Ketone bodiesKetone bodies from partial oxidation of fats by thefrom partial oxidation of fats by the

liver during starvation and more marked in diabetesliver during starvation and more marked in diabetesmellitus.mellitus.

(4)(4) Lactic acidLactic acid from the partial oxidation of glucose by thefrom the partial oxidation of glucose by the

red blood cells and muscles during exercisered blood cells and muscles during exercise(5)(5) Amino acids, fatty acidsAmino acids, fatty acids andand phosphoric acidsphosphoric acids duedue

to digestion and absorption of food from the intestines.to digestion and absorption of food from the intestines.

BIOLOGICAL IMPORTANCEBIOLOGICAL IMPORTANCE

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BIOLOGICAL IMPORTANCEBIOLOGICAL IMPORTANCE

OFWATEROFWATER

As aAs a universal solventuniversal solvent dissolvedissolve

substances; used for transport,substances; used for transport,

removal of waste and secretion.removal of waste and secretion. In metabolismIn metabolism hydrolysis reactionhydrolysis reaction

As a lubricantAs a lubricant -- to aid movement andto aid movement and

reduce frictionreduce friction

Giving support.Giving support.

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SUMMARYSUMMARY

Characteristics of waterCharacteristics of water

-- Dipolar and unevenDipolar and unevendistribution of chargedistribution of charge

-- Hydrogen bondingHydrogen bonding

Thermal Properties ofThermal Properties ofwaterwater

-- denser than icedenser than ice

-- high latent heat ofhigh latent heat of

vaporisationvaporisation-- high specific heathigh specific heat

capacitycapacity

-- high cohesionhigh cohesion

Importance of waterImportance of water

-- As a universalAs a universalsolventsolvent

-- In metabolismIn metabolism

-- As a lubricantAs a lubricant-- Giving supportGiving support

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SummarySummary

It has a high specific heat capacityIt has a high specific heat capacity soso

water can maintain a reasonably constantwater can maintain a reasonably constant

temperature (homeostasis).temperature (homeostasis).

It has a high latent heat of vaporisationIt has a high latent heat of vaporisation

so animals use water to cool themselves.so animals use water to cool themselves.

It is less dense as a solid (ice) as ice is aIt is less dense as a solid (ice) as ice is a

poor conductor.poor conductor.

Water is a good solvent.Water is a good solvent.

S f Bi b tS f Bi b t

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Summary of BicarbonateSummary of Bicarbonate

Buffering SystemBuffering System

COCO22 + H+ H22O [HO [H22COCO33] H] H++ + HCO+ HCO33

--

produced alwaysproduced always inin produced preproduced pre--existingexisting

byby availableavailable equilibrium with by bloodequilibrium with by blood

metabolismmetabolism species onspecies on metabolism buffermetabolism buffereither sideeither side

directly adjusteddirectly adjusted converted toconverted to adjusted in adjusted inadjusted in adjusted in

expelled in kidney via productsexpelled in kidney via products kidneykidney kidney orkidney or

in lung physiologic on left byin lung physiologic on left by or reacts reacts with H+or reacts reacts with H+

mechanisms carbonic with to makemechanisms carbonic with to makeunrelated anhydrase bicarbonate products onunrelated anhydrase bicarbonate products on

to pH (RBC)to pH (RBC) to make leftto make left

productsproducts

on lefton left