Lecture 11 Membranes and Gasses Flashcards
What can be said about the water transport of the small intestine compared to the kidney
The small intestine absorbs huge volumes of water on the same scale as the proximal tubule of the kidney
What is odd about water reabsorption in the small intestine
This is a very high osmolarity in the lumen of the intestine yet we’re getting water reabsorption that appears to be against the gradient. In addition there are no aquaporins present in the apical membrane of the cells in the small intestine so no way of water moving in
How is it that water is taken up from the lumen of the small intestine at the apical membrane of the cells
Some co-transporters also transport water as part of their normal operation
Give some examples of co-transporters that uptake water as part of their transport
KCC4 – 500 molecules of H2O hSGLT1 – 235 molecules of H2O GLUT2 – 40-100 molecules of H2O NKCC1 – 590 molecules of H2O
For every time that NKCC1 in the small intestine turns over it transports 500 molecules of H2O into the cell. Yet in the TAL of the kidney NKCC is also expressed but this region of the nephron is known to be impermeable to water. Why is this
The TAL expresses the NKCC2 isoform which itself cannot transport water unlike NKCC1
Give an example of some evidence proving that other channels can transport water as part of their normal function
Adding KCl to choroid plexus cells causes a swelling of the cells that is due to water uptake. These choroid plexus cells express the K+ 2Cl- transporter KCC4. Inhibition of KCC4 with furosemide alleviates this increase in volume when KCl is added extracellularly hence KCC4 is mediating this water uptake
What are the 3 mode of water transport
Osmosis co-transport and co-transport/osmosis
Give an example of a transporter that mediates water uptake via a join osmosis and cotransport pathway
The Na+/glucose cotransporter and glutamate-aspartate cotransporters mediate water uptake via a joint osmosis and cotransport pathway. Osmotic uptake occurs via a micro-gradient produced by the uptake of the various solutes that water can harness to move into the cell
Which channels mediate water reabsorption in the small intestine
GLUT2 and SGLT1 on the apical membrane bring water in and GLUT2 and KCC4 present in the basolateral membrane transport water out
Recall Overton’s Law
The permeability of a membrane to a solute is proportional to the oil/water partition coefficient for that solute
What is the logical extension over Overton’s Law
Gases such as oxygen and carbon dioxide have a high solubility in oil so biological membranes are hence freely permeable to gasses
What artefact of the original experiments carried out in experimental membranes may have lead to the wrong observation that all biological membranes are permeable to gasses
Early studies used decane to generate artificial membranes. This decane increased the permeability of experimental membranes to CO2
CO2 and NH3 are both gasses what feature of these two compounds make it easy to investigate their permeability in biological membranes
CO2 is a weak acid and NH3 is a weak base. This means that they cause a pH change when added to a cell
Outline the differential permeability of biological membranes to NH3 and NH4+
NH4+ is charged and doesn’t cross biological membranes easily. In contrast NH3 is uncharged and should cross biological membranes easily
Explain what happens when ammonium chloride is added to the extracellular solution of a cell
If you add NH4Cl to the extracellular solution it will dissociate giving a mixture of NH4+ and NH3 depending on the pH of the solution. The ammonia will then rapidly diffuse across the cell membrane where once inside it will combine with a H+ to form NH4+. This results in an alkaline shift in pH. A slower phase that occurs after adding the NH4Cl is that the NH4+ will be gradually taken up. This can be mediated by channels such as ROMK and NKCC2 where it substitutes for K+. NH4+ uptake will then result in a late phase acidification as the NH4+ dissociates in NH3 and H+
What channels mediate NH4+ uptake
This can be mediated by channels such as ROMK and NKCC2 where it substitutes for K+.
What confers the degree of dissociation of NH4Cl in solution
The pH of that solution
What ratio of NH4+ and NH3 is produced from the dissociation of NH4Cl in a solution at pH4
19:1
Outline the differential permeability of biological membranes to HCO3- and CO2
HCO3- is charged and doesn’t cross biological membranes easily. In contrast CO2 is uncharged and should cross biological membranes easily
Draw the expected pH curve following the addition of ammonium chloride to the extracellular fluid of a cell
See diagram below
Draw the expected pH curve following the addition of a CO2 containing solution to the extracellular fluid of a cell
See diagram below
Below is a diagram showing the pH shift of TAL cells following the addition of NH4Cl to the apical or basolateral membrane. What does this data show about the basolateral membrane and what does this suggest
As expected addition of ammonium chloride to the basolateral membrane causes an initial alkalinisation consistent with the uptake of protons by the newly entered ammonia. This is followed by a late stage acidification caused by the uptake of ammonium and its dissociation releasing protons.
Below is a diagram showing the pH shift of TAL cells following the addition of NH4Cl to the apical or basolateral membrane. What does this data show about the apical membrane and what does this suggest
Unusually following addition of ammonium chloride to the apical membrane of the TAL cells there is no alkalinisation that would be expected if the cells were freely permeable to ammonia. There is however still a later stage acidification consistent with ammonium uptake and dissociation to release protons. Overall this goes against the idea that all membranes are freely permeable to gasses as this is clear evidence to suggest that the TAL apical membrane is impermeable to NH3
The data below shows the acidification rate of Xenopus oocytes expressing AQP versus control under two conditions one where they are expressing the channel alone and the other where pCMBS has been added. What does this data tell us about the role of AQP1
As CO2 permeability is proportional to the acidification rate and in turn the water permeability this data shows that the increase in acidification rate that occurs due to the expression of AQP1 is entirely mediated by that channel alone. This is the case as the addition of pCMBS to oocytes expressing AQP1 reverses the increase in acidification rate and thus CO2 permeability to the levels seen in control cells not expressing AQP1. Hence AQP1 is the gas channel responsible for CO2 permeability as well as H2O
