Water
>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
Water
Amphipathic - contains regions that are polar (charged) and regions that are nonpolar
Water
Amphipathic molecules
Water
Water
Water
Water
WATER
Water
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
