Water

transcript

>70% of weight of most organismsPolar molecules - hydrophilicNonpolar molecules - hydrophobicHydrogen bonds are ______ interactions compared to covalent bondsEach water (H2O) molecule can form H-bonds with 4 other waters

WaterHydrogen bonding

Amphipathic - contains regions that are polar (charged) and regions that are nonpolar

Amphipathic molecules

Water chain in cytochrome f

Ionization of Water

H2O H+ + OH-

H+ is a hydrogen ionOH- is a hydroxyl ion

Actually2H2O H3O+ + OH-

H3O+ is a hydronium ion

Ionization of Water

H2O H+ + OH-

Keq = [H+][OH-] / [H2O]

[H2O] ~ 55.5 mol/LKeq = 1.8 x 10-16

Kw = (Keq) [H2O]Kw = (1.8 x 10-16) (55.5)

So Kw = [H+][OH-] = 10-14 (mol/L)2 at 25˚

In pure water, [H+] = [OH-] = 10-7, a neutral solution

pH = log (1/[H+]) = -log[H+]

A neutral solution has a pH = 7.0When [H+] > [OH-], pH < 7.0 (acidic)When [H+] < [OH-], pH > 7.0 (basic)

Acid/Base Pairs

Acid Conjugate base

HA+ H+ + A

HA H+ + A-

HA - H+ + A2-

For a strong acid, dissociation is complete so [H+] = [acid] and pH = -log [H+]

For weak acid, dissociation is incomplete so Ka = [H+][A-] / [HA]

Ka is acid dissociation constant

pH is function of [acid] and Ka [H+] = Ka [HA] / [A-]

-log[H+] = -logKa + log [A-] / [HA]

pKa = -logKa

pH = pKa + log [A-] / [HA] (Henderson-Hasselbach)

The pK of an acid (base) is the pH at which the acid (base) is half dissociated

pH of aqueous fluids

Conjugate acid-base pairs

Titration Curveacetic acid

Titration Curve

pK1 = a carboxyl grouppK2 = a amino group

Buffering against pH changesBuffers: mixtures of weak acids and their conjugate basesResist changes in pH when small amounts of acid/base are added

Maintenance of a specific and constant pH, near 7.0 is needed by cells and organisms - small changes in pH can have a large impact on rate of cellular processes

Buffering against pH changesamino acid histidine - weak acid

pKa = 6.0

Protein enzymes have an optimum pH for activity

Biological buffers - bicarbonate (phosphate too)Exerciselactic acid (H+) produced (pH of blood plasma lowered)reaction pushed to produce more CO2 - exhale

Protein catabolismammonia (NH3) produced and pH of blood plasma is raised Reaction pushed to create more H+ more CO2 from lungs dissolved in blood plasma

RATE OF BREATHING keeps BLOOD PH CONSTANT

Water as a reactant

HYDROLYSIS

CONDENSATION

Water

Documents