Regulating cellular pH and volume

Question 1

What is pH a measure of?

Answer 1

The concentration of hydrogen ions.

Question 2

A change of what pH will double or halve the concentration of hydrogen ions?

Answer 2

0.3

Question 3

What is the plasma pH and what is the cytosol pH? Are there any changes to pH for organelles?

Answer 3

7.35-7.45
7.2, roughly.
There is a range of pH for organelles depending on their functions. Mitochondria have a pH of 8 and lysosomes have a pH of 4.7

Question 4

What ions are cytoplasmic buffers for pH?

Answer 4

HCO3- (bicarbonate), and PO4^2- (phosphate).

Question 5

What are the two bodily systems that act to buffer pH?

Answer 5

Pulmonary - removes CO2 rapidly, that is dissolved as carbonic acid; removing CO2 makes the blood more alkaline.
Renal - regulates the excretion of hydrogen and bicarbonate ions.

Question 6

How do proteins act to buffer pH?

Answer 6

Proteins have large negatively charged sites which can interact and temporarily take hydrogen ions out of solution.
The amino acid Ka can also determine whether it is protonated or deprotonated, which can help to buffer the pH.

Question 7

If the pH out of the cell is less than the pH in the cell, why do hydrogen ions leave the cell?

Answer 7

Despite the electrochemical gradient, the polarity of the cell membrane favours the outward movement of hydrogen ions.

Question 8

How does the NHE change pH, and what are its activators and inhibitors?

Answer 8

Firstly, an electrochemical gradient is set up by the sodium/ potassium ATPase, via primary active transport.
Then, the sodium electrochemical gradient is utilised to pull sodium ions into the cell, exchanging them with hydrogen ions, and pumping them out.
This decrease of hydrogen ions within the cell increases the pH.
An increase in pH will then decrease the activity of the NHE.
It is activated by growth factors.
It is inhibited by amiloride.

Question 9

Explain the effects of the sodium-driven chloride-bicarbonate exchanger: NDCBE.

Answer 9

The sodium electrochemical gradient pulls sodium ions into the cell as the energy source (secondary active transport). As this happens, 2 bicarbonate ions are drawn into the cell and a chloride ion is removed.
It can also be stated that one bicarbonate ion is drawn in and one hydrogen ions removed.
Either way, there is an increase in the pH within the cell and it remains electroneutral.

Question 10

What are the effects of the sodium/ bicarbonate co-transporter?

Answer 10

As the name suggests, the sodium electrochemical gradient is utilised to pull bicarbonate ions into the cell.
There are two forms, a 1:1 ratio, which is electroneutral, or a 1:2 where more bicarbonate ions are drawn into the cell, which is electrogenic, increasing the cytoplasmic membrane potential - making it more negative.
Both of these would increase the pH within the cell.

Question 11

Explain the effects of the chloride-bicarbonate exchanger/ anion exchanger.

Answer 11

This is where the chloride electrochemical gradient is used to pull chloride ions into the cell and the electrochemical gradient of bicarbonate ions to removes them.
This would decrease the pH of the cell.
NOTE: this makes it facilitated diffusion, not secondary active transport.

Question 12

Why is pH so tightly regulated?

Answer 12

Disrupts the electrostatic interactions and hydrogen bonding within a protein. This means that the structure and therefore the function of the protein may change.
pH also affects:
- Ca homeostasis; Ca-ATPase is pH sensitive.
- Metabolism; phosphofructokinase is pH sensitive.
- Cell communication.
- Apoptosis.

Question 13

How can ischaemia cause a pH change?

Answer 13

Anaerobic glycolysis with increase lactate production.
This will dissociate and increase the hydrogen ion concentration, decreasing pH.
The sodium-potassium ATPase will have insufficient ATP to generate the sodium electrochemical gradient, meaning that the NHE will not be able to be utilised.

Question 14

Why do we need transporters and not just buffers?

Answer 14

Buffers are can only limit the impact of acute pH changes and are insufficient in dealing with large pH changes.
The transporters prevent acidosis and alkalosis.

Question 15

What effect does the NHE, anion exchanger and sodium/ bicarbonate co-transport have on cell volume?

Answer 15

They all transport osmotically active ions into the cell. This means that water follows them in and they swell.

Question 16

When there is slight shrinkage or swelling, do the opposite methods of water transport stop?

Answer 16

No, water movement is active in both directions. It is the proportion of water movement which allows the cell to swell or shrink.

Question 17

What can dehydration biochemically result in?

Answer 17

Hypernatraemia. This is where there are elevated concentrations of sodium ions in the blood plasma. This is because the kidney has reabsorbed them to prevent further loss of water.
This is often seen in the elderly.

Question 18

What can marathon running or MDMA toxicity, associated with excess water drinking, biochemically, cause?

Answer 18

This can cause hyponatraemia. This is because there is excess water within the body after drinking so much. This means that the kidney reabsorbs less so that more water will leave the blood plasma in the urine.

Question 19

How can drug transport of weak acids or bases occur, passively?

Answer 19

If they are present in their neutral form then they can pass through the lipid bilayer more freely.
This is protonated in weak acids, and deprotonated in weak bases.

Question 20

Why is cell volume regulation so important?

Answer 20

Swelling or shrinking of the cell can lead to cytoskeleton interference.
Excessive swelling can lead to the membrane bursting.
Excessive shrinkage of the cell can lead to cell death.
Cellular function depends on correct hydration of proteins.

Question 21

How do hypertonic, isotonic and hypotonic solutions affect cell volume?

Answer 21

Hypertonic means that there is excessive solute in the solution outside the cell, leading to water moving out of the cell, into the interstitium, leading to cell shrinkage.
Isotonic means there is the same solute concentration inside and outside the cell, so the volume of the cell does not change.
Hypotonic solutions mean there is less solute outside the cell than inside the cell, so the water moves into the cell and the cell swells.

Question 22

What is the response to osmotic shrinkage called?

Answer 22

Regulatory volume increase.

Question 23

What is the response to cell swelling called?

Answer 23

Regulatory volume decrease.

Question 24

What are the short-term responses to osmotic shrinkage?

Answer 24

The short-term responses can occur over seconds or minutes to help regulate cell volume, and influx ions and water into the cell.
The sodium/ chloride co-transporters brings sodium and chloride ions into the cell, utilising the electrochemical gradients. This is non-electrogenic.
The sodium/ potassium/ 2 chloride co-transport utilises the sodium electrochemical gradient to bring potassium and 2 chloride ions into the cell. This is non-electrogenic.

Question 25

Explain how RVI can use pH change to drive ion influx.

Answer 25

The NHE brings sodium ions in and pumps hydrogen ions out. This itself will bring water into the cell as sodium is more osmotically active.
This alkalinisation would result in activation of the anion exchanger, where osmotically active chloride ions would be pulled in with water and bicarbonate ions lost.

Question 26

Explain the long-term response to cell shrinkage.

Answer 26

To increase the osmolality, the cell would influx and synthesise more cytoplasmic proteins and sugars. This would draw water into the cell.
The influx of these organic molecules is driven by secondary active transport of sodium ions.

Question 27

Explain the short-term response to cell swelling.

Answer 27

There are two stretch-activated ion channels, which allow facilitated diffusion:
- Potassium stretch activated channel.
- Chloride stretch activated channel.
This means that when the cell swells, these channels undergo a conformational change to allow the efflux of their ions, out of the cell.
There is also the potassium/ chloride co-transporter. This pumps both potassium and chloride ions out of the cell, bringing water with them.

Question 28

Explain how RVD can use secondary pH changes to drive ion efflux.

Answer 28

The potassium/ hydrogen exchanger (KHE) can be used. This is where potassium is pumped out of the cell and hydrogen ions pumped in. As K+ is more osmotically active, water follows the potassium ions out of the cell, acidifying the cell.
In response, the chloride/ bicarbonate exchanger could then pump bicarbonate ions into the cell and chloride ions out of the cell, to deal with this change. The very osmotically active chloride ions would then cause water to also efflux.
Furthermore, the bicarbonate ion could combine with the proton forming H2CO3. This could then dissociate into water and carbon dioxide, allowing more water to leave the cell.

Question 29

Explain the long-term response of RVD.

Answer 29

This is where there is an efflux of solutes such as amino acids and sugars, and there is a decrease in the synthesis of organic osmolytes, such as proteins.
This means that the osmotic pressure in the cell decreases and water flows out of the cell.