Cell biology lecture 22

Acids, bases and buffers

LOs

• Biological importance of acidity. ​

• Water dissociation. ​

• pH as a measure of H+ concentration. ​

• Weak acids, acid dissociation constant, ​

• pKa. ​

• Buffers.​

• Henderson-Hasselbalch equation

Why should we care about pH?​

• pH measures the acidity/ basicity of a solution.​

• Living organisms are very sensitive to small pH changes

Water dissociation

• H3O+, OH- ions are highly reactive​

• ​

Note:​

• H3O+ is commonly just called H+.​

• Free H+ does not exist in water, it’s just a convenient shorthand.​

Water acts as an equilibrium with H+ and OH-

How much does water dissociate - Kw?​

Kw equation

Acid

produces H+ ions by dissociation

Base 

releases OH- ions

What do acids and bases do?

change the amount of h+ available to react with other species

pH scale

measures conc of H+

pH scale vs H+ concentration​

what do buffers do?

stabilize the pH of a solution​

Visual explanation of pH buffering​

Buffer + strong base

Buffer and strong acid

Ka 

pKa

expresses acidity of weak acids​

Henderson-Hasselbalch: Calculating buffer pH​

Biological buffers

Buffer pH example​

Buffer practice 2 

Answer = 40

pH = pKa + log A/HA

7.7=6.1 + logA-/HA

1.6 = logA-/HA

10 to the 1.6 = a-/HA

39.8 = ratio

So answer is 40

pKa of amino acids​ if pH equals pKa

pKa of amino acids​ if pH is larger than pKa

pKa of amino acids if pH is less than pKa

State of molecules at a given pH 

Why is pH important for AAs and enzymes ?

• Consider an enzyme with a carboxyl group. The structure of that group will depend on the pH, If the enzyme needs to be protonated in order to be active, then the enzyme will only work in the pH range in which the majority of the enzyme molecules have their carboxyl group protonated.​

Henderson Hasselbach practice