Membranes and gases

Question 1

The small intestine - why is it unusual? Features that allow for high h2o reabs?

Answer 1

Apical side has NO AQPS (which allow for the drive for the movement of water)
Large reabs of water occurs at the level of the small intestine due to the hypertonic conditions created by the enzymatic activity in the stomach.
BUT CAN STILL ABSORB WATER

Question 2

Wet transport proteins - what is meant but this?

Answer 2

Co transport proteins present in the small intestine - allow movement of water along with other solutes against the osmotic gradient

Question 3

Examples of wet transport proteins

Answer 3

GLUT2 - glucose transporter
hSGLT1 - Na+/glucose transporter
KCC4 - K+/CL- contransporter
NKCC1 - Na+/Cl-/2Cl- contransporter

Question 4

Transport models - examples and how do they operate?

Answer 4

AQPs - provide a pore in the membrane
KCC - K+/Cl- moved along with H20
SGLT1/EAAT1 - mixed mode (use pore type mechanism and cotransport process via microgradients to allow absorption of nutrients)

Question 5

Molecular turnover model - outline

Answer 5

• Na+ binds to the transporter.
• Glucose surrounding the protein also enters cleft (h2o is trapped along with glucose)
• a confirmational change occurs -> H20 and glucose released on other side of membrane into the IC environement

Question 6

Small intestine model - proteins on apical membrane? what does this happen off the back of

Answer 6

GLUT2 - brings glucose and water into cell from SI lumen - NOTE this protein is inserted into the apical membrane during digestion
SGLT2 - Na+, glucose and H20 brought into cell from lumen of SI
Eating food contains hypertonic conditions in the lumen

Question 7

Overton’s law for membranes and solutes?

Answer 7

‘The permeability of membranes to solutes is proportional to the oil:water partition coefficient for that molecule’

Question 8

What dictates the permeability of a membrane to gases? - what about to Co2 and o2?

Answer 8

The amount of lipid bilayer - gases can freely diffuse across bilayer at the membrane (so where transport proteins are present perm to gases reduced)
Gases such as oxygen and carbon dioxide have a high solubility in oil, so a natural extension of this law was that all biological membranes were freely permeable to gases.

Question 9

What happens when NH3 moves into a cell?

Answer 9

Combines with H+ to create NH4+, causing alkylisation of the cell

Question 10

What happens when NH4+ moves into a cell?

Answer 10

Dissociates into H+, NH3 and acidifies the cell

Question 11

Conditions wrt NH4+/NH3 at lower pH?

Answer 11

Low pH = more NH4 moving in which is a weak base

Question 12

Isotonic drinks - how do they work?

Answer 12

High glucose environment created in the small intestine which creates hypertonic environment, stimulating water to be reabsorbed, giving the hydrating effect

Question 13

Effect of Co2 diffusing into cell?

Answer 13

Co2 moves in and dissoc into HCO3- and H+ (acidification)

Question 14

Effect of HCo3- diffusing into cell?

Answer 14

Combined with H+ to produce Co2 and H20 giving alklysating effect (process slower than acification)

Question 15

Thick ascending limb - impact of adding NH4Cl on the basolateral side

Answer 15

Causes alkalysation of cells and this membrane has high permeability to NH4 and NH3

Question 16

Thick ascending limb - impact of adding NH4Cl on the apical side

Answer 16

Causes acidification and has a high perm to NH4 but extremely low to NH3

Question 17

The gastric gland - perm to Co2 on the basolateral membrane - how is this shown (experiment)

Answer 17

NORMALLY permeable to Co2 on the basolateral membrane - seen by adding to blood on baso side Co2 causing a decrease in pH of the cell (this is done using perfusion pipettes of isolated gastric gland)

Question 18

The gastric gland - perm to Co2 by lumenal changes - how is this shown (experiment)

Answer 18

By changing the conc of Co2 of the lumen to the PK for co2 (gassed with 100% co2) attempting to create pH 6.1 –> NO CHANGE IN PH SEEN!!!! Apical side not permeable!

Question 19

Why would the apical membrane of the epithlia lining the stomach be impermeable to gases?

Answer 19

Allows the maintenance of a tough, tight membrane so that the acidic conditions of the stomach can be maintained –> stops stomach from allowing enzymes to function so that the stomach doesnt digest itself!

Question 20

Colonic crypt cells - composition of membrane and why important?

Answer 20

77% cholesterol so tight membrane –> acts as a barrier for gases by having low gas permeability. High level of transport proteins found to decrease proportion of lipid bilayer which limits gas movement

Question 21

MDCK cells - what was impact of stripping cholesterol from the membranes?

Answer 21

Increase co2 permeability

Question 22

Relationship between cholesterol in membrane and perm to gases?
What challenges this?

Answer 22

Low perm membrane = high cholesterol content (decreased membrane fluidity)
RBCs challenge this –> 45% cholesterol content of membrane however impermeable to gases

Question 23

Cancer cell line – evidence for cholesterol/permeability relationship?

Answer 23

Low membrane cholesterol in cancerous cells and cells are metabolically active –> high co2 perm!!

Question 24

Oocytes exposed to co2 and hco3- then water. Measured how long lysis took and corellated this with Co2 permeability. What did the results show?

Answer 24

Acidification seen.
Acidification proportional to the co2 perm of the membrane
In this case, co2 perm was negatively proportional to the time taken for lysis to occur –> less acidification = more time taken for lysis to occur –> MORE AQPS = FASTER ACIDIFICATION

Question 25

what is the action of PCMB and what was seen in regards to AQP1 / cs189 residue?

Answer 25

PCMB is a mercurial agent which binds to the C189 residue of AQP1 -> adding causes decrease in co2 perm.
Therefore, pCMBS inhibits the AQP1 dependent increase in CO2 permeability
This was shown because n c189S mutant, increase in CO2 perm back to WT level when PCMB was added

Question 26

COLTON NULL INDIVIDUALS- whats wrong with them and what was shown wrt co2 perm

Answer 26

Colton null individs have no AQPs in their RBCs.

• Causes a 50% decrease in permability to CO2
• PCMB doesn’t impact permability in colton null individuals

Question 27

Impact of DIDS in WT and colton null individuals

Answer 27

CO2 permeability in Colton null and WT is inhibited by DIDS.

Question 28

AQP1 - structure

Answer 28

Made up of hydrophobic and philic residues
Central pore (mainly hydrophobic, gated by phobic residues, prevents H20 access)

Question 29

AQP molecular dynamics - h2o and co2 movement?

Answer 29

Co2 - 6 molecules through 4 individ monomer water pores, 4 through central pore
O2 - 1 mol through 4 water pores and 6 through central pore

Question 30

Other AQPs - what is different about them? Couple of examples of where they are found?

Answer 30

AQP4 - 2 isoforms –> M1 (kidney) and M23 (astrocytes in the BBB) - high permability to gases!
Differing H20/Co2 and NH3 permeabilities in different AQPs

Question 31

Impact of cGMP on AQP1?

Answer 31

Binds and switches it to a cation (Na+) channel

Question 32

Oocytes expressing AQP1 and added cGMP - impact seen?

Answer 32

Loss of perm to Co2 - due to change in conformation/shutting down of CO2 pore/allowing through cations?

Question 33

Oocytes expressing AQP5 added cGMP - impact seen?

Answer 33

inhibition of Co2 perm but had no impact on H20 transport –> useful for transport of gas but without impacting water secretion (AQP5 found in the salivary glands)