L4: Maintenance of pH Flashcards
What is the ‘solubility of water’ (Kw)?
- In aqueous solutions the product of H+ and OH- concentrations is constant
- This equates to 10^-14 mole squared metres squared
What happens in an acidic solution to the Kw?
The H+ concentration increases and the OH- decreases
What happens in a basic solution to Kw?
The H+ concentration decreases and the OH- increases
Define pH
What would the H+ concentration be in a neutral solution?
10^-7
What is important to remember about proton and hydroxide concentrations?
The sum of the concentrations is not constant, but the product of them is
What is the range of pH values?
0-14
What is the H+ concentration in water?
- Water has an Mr of 18 - this is the molecular weight
- 1000g of water in a litre
- 1000/18 = 55 moles per litre = concentration of water
What pH range are most physiological processes within cells between?
5-7.6
How is pH measured?
- Litmus paper (most basic form)
- Universal indicator (range of indicators that change colour)
- Natural biological indicator is red cabbage
- pH metre (most sophisticated form)
How does a pH metre work?
- The reference electrode (usually a platinum wire immersed in silver or silver chloride
- This is immersed in potassium chloride solution
- Sensing membrane within the glass membrane
- Different amount of protons associated with the glass membrane depending on pH of the solution
- This determines the potential difference between the two electrodes
How is the voltage (potential difference) of the pH metre converted into a pH value?
- Have to calibrate the electrode
- Have to use 3 precision pH metres - one acidic, one neutral and one basic
- Displays the pH reading after
When can a pH metre not produce a reading? Why?
- At extreme pH readings below 0 and above 14
- Process becomes non-linear
- There is normally a linear charge of protons associated with a glass membrane - restrict reading to within the normal 0-14 range
Define a strong and weak acid
- Strong acids will fully dissociate in an aqueous solution e.g. HCl
- Weak acids will partially dissociate in an aqueous solution e.g. carboxylic acids
What is the conjugate acid and what is the conjugate base in this equilibrium equation?
- HA is the conjugate acid
- A- is the conjugate base
What is the Henderson-Hasselbach equation?
Describe the graph for lactate
- Start point at pH 0
- Initially the addition of the base means a quick rise in pH from 0-2
- Linear range in pH with very small change in the mid-section
- Lactate acts as a pH buffer in the middle - keeps the pH of the solution constant
- This is the same for the ammonia solution as well (but at different pHs)
What is a good buffer around pH 7, which is more physiological?
Phosphate buffer
What does the pK value (where 50% of the base is added) tell you?
Half of the weak acid or base is dissociated and half is not dissociated
What does a high and low pH for an acid tell you from the graph?
- More dissociated weak acid for a high pH
- Less dissociated weak acid for a low pH
When is the charged or uncharged version of the lactate shown on the graph?
- The protonated version is uncharged
- Above the pK value (50%) for lactate is the charged version
- Below the pK value (50%) for lactate is the uncharged version
When is the charged or uncharged version of the ammonia shown on the graph?
- The protonated version is charged
- Above the pK value (50%) for ammonia is the uncharged version
- Below the pK value (50%) for ammonia is the charged version
What is an example of a biological molecule which can be both basic and acidic?
- Amino acids
- Have both an amine and carboxylic acid group
What has to happen to amino acids in order to demonstrate pH dependency on a graph?
Have to add 2 base equivalents because there are 2 sites which need to be protonated or deprotonated
What is the plateau on the graph for an amino acid demonstrating?
- The isoelectric point
- The point where the overall charge of all the species is zero
At what point is the charge positive and negative on the graph?
- At the very bottom is a negative charge - both the amine and carboxyl group are protonated (1)
- At the very top is a positive charge - the carboxyl group is deprotonated whilst the amine group os still protonated (3)
- At the isoelectric point the carboxyl group is deprotonated and the amine is protonated - net charge is zero (2)
What is the pH of the stomach, plasma, and pancreatic juice?
- Stomach = pH 2 (1-3)
- Plasma = pH 7.4
- Pancreatic juice = pH 8.8
What is the main way pH can be regulated in the human body?
Lots of protons in a compartment and use a proton pump
What enzyme facilitates the conversion of carbon dioxide and water into carbonic acid?
Carbonic anhydrase
What will carbonic acid spontaneously dissociate into?
Bicarbonate ion (HCO3-) and H+
What does carbonic anhydrase buffer?
Major buffer in the body for circulation and in tissue - large function for the dissociation of oxygen in erythrocytes
How do parietal cell in the gastric glands of the stomach produce stomach acid?
- Use of a proton pump
- Convert carbon dioxide in the blood into carbonic acid aided by carbonic anhydrase
- This dissociates into the bicarbonate ion and hydrogen, which is transported into the bloodstream, and a chloride ion enters the cell (antiporter)
- The chloride ion and hydrogen are transported into the gastric duct and a potassium ion is removed into the bloodstream from the gastric duct (antiporter)
What is the pH of blood?
7.4
What can the pH of blood rise or fall too under strenuous conditions or disease?
pH 6.8-7.8
How can the pH of the blood be regulated under strenuous or disease conditions?
- Leads to the dissociation of protein side chains to produce H+ to bring the pH level down
- Leaves behind negatively charged proteins (mainly albumin in the blood)
If not treated, what can the production of albumin, as a result of a high blood pH, lead to?
- Albumin captures calcium ions, so the calcium ions are not available for muscle cells and contraction
- This can lead to tetanic cramps and death
What pH of blood is known as acidosis?
7.35
Why can a low blood pH cause issues?
- Less bicarbonate ions available to buffer, so lower buffering capacity, which can lead to accidental changes in proton concentration, which can lead to higher pH changes
What are the 3 components to the alteration of acid-base balance?
1) Buffering (and H+ pumping)
2) Alteration in arterial pCO2
3) Alteration in HCO3- excretion
What can the ionic state of different compartments dictated towards drugs?
- Ionic states can dictate how a drug can be administered and the bioavailability
- Determines how a drug can be taken orally or injected
How does pK affect the administration of a drug?
- Drugs with a high pK value have to be administered intravenously
- Drugs with a lower pK value can be administered orally
- Molecules will only cross plasma membranes in the uncharged form
Where can various forms of carbonic anhydrase be found?
Brain interstitial space
How can seizures be treated with relation to pH?
- pH shifts accompany neural activity associated with a seizure
- Find a drug that can target the change in pH and prevent this, therefore preventing the seizures
How does pH influence protein interactions?
Increases protein stability and allows salt bridges between amino acids
Given the salt bridge :
Over what range of pH will this salt bridge be stable ?
(Assume that side chains must be at least
50% charged in order for salt bridge to be stable.)
- pKa value for lysine is 10
- pKa value for glutamic acid is 4.2
- If the pH of the solution is the same as the pKa value, the protonated and deprotonated form are both at 50%
- If the pH is above the pKa value the deprotonated form is dominant (i.e. >50%), if the pH value is lower it is the other way around.
-Have to determine, whether the protonated or the deprotonated form is
charged, i.e. whether the pH value needs to be higher or lower than the pka
value for the salt bridge to be stable. - Glutamic acid is ionic in its
deprotonated form, whereas lysine is ionic in its protonated form - Therefore, the lysine:::::glutamic acid salt bridge should be stable between pH values
of 4.2 and 10
Given this salt bridge:
Over what range of ph will this salt bridge be stable?
- pKa value for tyrosine is 10
- pKa value for arginine is 12
- If the pH of the solution is the same as the pKa value, the protonated and deprotonated form are both at 50%
- If the pH is above the pKa value the deprotonated for is dominant (i.e. >50%), if the pH value is lower it is the other way around.
-Have to determine, whether the protonated or the deprotonated form is
charged, i.e. whether the pH value needs to be higher or lower than the pka
value for the salt bridge to be stable. Tyrosine is ionic in its
deprotonated form, whereas arginine is ionic in its protonated form - Therefore the tyrosine:::::arginine salt bridge will be stable between pH values of 10 and 12
DNA is acidic, but the overall nucleus is basic Why is this?
There are other molecules in the nucleus e.g. histone proteins which are basic
- Histones prevent elongation and allow tight packaging of the DNA
Why do mitochondria have a proton gradient?
- Intra-membrane space has a higher proton concentration than the mitochondrial matrix - difference of 1pH unit
- Electrons in NADH and FADH2 donated to the electron transport chain and these are passed between electron centres (lose some energy each time)
- The proton is pumped from the inner mitochondrial matrix to the intra-membrane space
- Allows oxidative phosphorylation and production of ATP - proton channel to allow protons back into inner mitochondrial matrix
- This pH regulation is known as partitioning
