Life and Aqueous Solution espyr1 S1

LSM1401 – Fundamentals of Biochemistry Summary notes (1) Lim Fang Jeng

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Life and Aqueous Solution

Definition of Acid and Bases

- Acid – donate H⁺ - Base – accept H⁺

Equilibrium Constant, K

’

�

⁼

2

� = −log⁡[�

^]

For the dissociation of weak acid

� =

Henderson-Hasselbalch Equation

� = � + log _�� ^�

- Used widely in calculating the pH of weak acid

Ratio

��

Implication

<1 Conjugate base predominates

>1 Acid predominates

=1 Equal dissociation, [A

]=[HA]

Titration Curves

- Equivalence point – Point at which enough acid is added to neutralize the base

- For diprotic acid, it has two inflection point o At this point,

[Acid]=[Conjugate base]

o Near inflection point, the slope and pH change is small, hence it is used as a buffering region

- End point, all acid converted to conjugate resulting the acid to be neutralized

Buffers

- Solution that resists the change of pH upon addition of either more acid or more base. - Consists of weak acid and its salt (conjugate base)

LSM1401 – Fundamentals of Biochemistry Summary notes (1) Lim Fang Jeng

© Not for distribution purpose - Reactions involved

�

⇌ �

⁺ �

(Dissociation of water)

�� ⇌ �

⁺ �

(Dissociation of acid)

- Buffering action

o _{When OH}

_{is added}

 H

O is formed, equilibrium shift towards left, loss of H

 Acid dissociates to restore the loss of H

o _{When H}

is added (Only involve the dissociation of acid)

 Conjugate base (A

)reacts with H

to form acid

- Effective buffer range

� − 1 < � ( � � � ^{) <} � + 1

- It is related to the concentration of weak acid and its conjugate base

- Greater [HA] and [A

], greater buffering capacity

Biological Buffers

- Intracellular – Phosphate buffer (�2 4^−/� 4²⁻) in proteins - Extracellular – Bicarbonate (�2� 3^/�� 3⁻⁾

- Physiological pH of most cells = 6.9 ~ 7.4

- _pKa (7.2) of phosphate system is near physiological pH, so it is suitable for us to use as an intracellular buffer

- The reason why phosphate buffer is used in cell is because phosphate is abundant in cells. - When acid is added, [H⁺] increases, the following reaction take place

HPO₄²⁻+ H⁺_{↔ H2}PO₄⁻

- When alkali is added, [OH^-] increases, the following reaction takes place HPO₄²⁻_{↔ H2}PO₄⁻+ OH⁻ Bicarbonate buffer

- _pKa=6.35

- The buffering action consists of three reactions o Bicarbonate/ carbonic acid couple

H₂CO_{3(aq ) ↔ H aq} ⁺ + HCO₃⁻ _aq o _CO2 dissolved

CO_{2 aq} +_{�2 ↔ �2� 3} o Gaseous CO^{2 in lungs}

CO_{2 g ↔ � 2} - _{When [H}⁺] increases, gaseous CO2 is produced

- _{When [H}⁺] decreases, less gaseous CO2 is produced  more aqueous CO^{2 in lungs}

LSM1401 – Fundamentals of Biochemistry Summary notes (1) Lim Fang Jeng

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Acidosis Alkalosis

[H

] level in blood Increases Decreases

pH in blood Decreases Increases

Respiratory symptoms

Respiratory Acidosis

Unable to expire CO2

Hypoventilation

Respiratory Alkalosis

Removal of large amount of CO2

Hyperventilation

Metabolic symptoms

Metabolic Acidosis

Overproduction of acids (High protein, low-fat diets)

Increase in lactic acid level during exercise

Metabolic Alkalosis

Overproduction of salts

Severe vomiting  loss of gastric juice

Laboratory Buffers

- Must suitable ionic strength, pK^a and does not interfere to biochemical reaction. Examples

- TRIS, HEPES, MOPS, PIPES