Lecture #02: Properties of Water: pH

Question 1

Alterations of pH

Answer 1

• Alterations of pH can drastically affect the internal electrostatic environment of an organism, which can alter the weak bonds that maintain the structure of biomolecules, which usually means loss of function.

Question 2

Answer 2

• Very small amounts of water dissociate and form hydronium and hydroxyl ions, with the concentration ion of each being 10-7 M.

○ The top arrow is shorter than the bottom one, signifying that the extent to which water dissociates into H+ and OH- is very small.

Question 3

Answer 3

○ The numerical value is 1.8 x 10-16. This value is extremely small, which is further proof that water does not dissociate into ions to a large extent.
§ Since Keq is such a small number, the concentration of water remains unchanged. So you can use the density of water (1000 g/L) to find the M concentration of it, 55.5 M. Multiply this number by the Keq and…[H+][OH-] = 1 x 10-14 M2

Question 4

Ion Product of Water

Answer 4

• The ion product of water, Kw, is a constant. Kw = [H+][OH-] = 1 x 10-14 M2
○ This property of water is central to its role as a biological solvent.
○ At equilibrium, [H+] and [OH-] concentrations will be the same.

Question 5

pH Equation

Answer 5

• pH is defined as -log [H+]. Note that the pH scale is logarithmic–a difference of 1 pH unit means that a solution has 10x [H+] than the other.

Question 6

Production of Weak Acids Each Day

Answer 6

• Metabolism results in 13-22 moles of weak organic acids produced/day. If dissolved in water, pH would be <1. This is why regulation is key.

Question 7

Extreme pH

Answer 7

• If pH is too extreme, it’ll cause proteins to denature (fold). If protein is not in correct form, it will not function properly.

Question 8

pH as a Signaling Mechanism

Answer 8

• pH as a Signaling Mechanism
○ pH increase of 0.2 or 0.3 units promotes cell proliferation, migration, assembly of action filaments.
○ Physiological changes in pH regulate specific proteins (pH sensors)
§ Change in binding affinity, activity, recruitment to membranes.
§ Examples: ion channels, pumps, enzymes

○ Association with disease (intracellular)
§ If more basic, it’ll confer advantage for cancer cells to survive, will affect permeability of drugs.
§ If a decrease in pH, several neurodegenerative diseases.

Question 9

Organic Acids

Answer 9

• Organic acids are prominent biomolecules. These acids will ionize to produce a proton and a base. Stronger the acid, the more completely they will ionize.

Question 10

Acid vs. Base

Answer 10

• An acid is a substance that can release hydrogen ions. A base is a substance that can accept hydrogen ions.

Question 11

Answer 11

• Ka, the equilibrium constant for an acid, represents the extent of ionization.
○
○ The larger the value of Ka, the stronger the acid. It’s the acid dissociation constant or equilibrium constant for dissociation.

Question 12

Henderson-Hasselbalch Equation

Answer 12

• pH = pKa + log [A-]/[HA]. This is the Henderson-Hasselbalch equation. pKa is simply the pH at which the acid is half dissociated.

This equation describes the extent of dissociation of a weak acid at any pH and can be plotted as a titration curve.
○ Each point on the curve represents a different ratio of dissociated to undissociated weak acid.

When [HA] = [A-], the acid is 50% dissociated abd pH = pKa, the inflection point on the curve.