Lecture 2 - Membrane Permeability, Cell Volume and pH Regulation; ATP Dependent Ion Pumps and Ion Exchangers

Question 1

What molecules can pass through membranes?

Answer 1

Hydrophobic molecules

Small, uncharged, polar molecules

Question 2

What molecules can’t pass through membranes?

Answer 2

Large, uncharged, polar molecules

Ions

Question 3

What is passive diffusion?

Answer 3

Dependent on permeability and concentration gradient

Rate increases linearly with increasing concentration gradient

Question 4

What is facilitated diffusion?

Answer 4

Permeability for substance is increased by incorporation of a specific protein in the bilayer.

Question 5

What is active transport?

Answer 5

Transport of ions or molecules against the favourable concentration gradient
Energy may be required depending on concentration gradient and electrochemical gradient

Question 6

What is a cotransporter?

Answer 6

Membrane transporter that transports more than one molecule.

Question 7

What is uniport?

Answer 7

One molecule is transported in one direction.

Question 8

What is symport?

Answer 8

Two molecules are transported in the same direction.

Question 9

What is antiport?

Answer 9

Two molecules are transported in opposing directions.

Question 10

What is Na K ATPase?

Answer 10

Plasma membrane associated pump
Uses ATP to pump ions
P type ATPase

Question 11

What is the structure of Na K ATPase?

Answer 11

alpha subunit - binding site for K, Na, ATP and ouabain

beta subunit - glycoprotein directs pump to surface

Question 12

What does ouabain do?

Answer 12

Binds to alpha subunit - inhibits Na K ATPase.

Question 13

What does Na K ATPase do?

Answer 13

Uses energy from ATP hydrolysis
moves 2K into the cell
moves 3Na out of the cell
forms Na and K gradients

Question 14

what does Na K ATPase driving secondary active transport do?

Answer 14

control of pH
regulation of cell volume
regulation of Ca concentration
absorption of Na in epithelia
nutrient uptake

Question 15

what is the function of K channels?

Answer 15

allows K diffusion down the concentration gradient set up by Na K ATPase
mainly responsible for membrane potential

Question 16

what is the resting membrane potential?

Answer 16

-70mV

Question 17

what is the role of Ca ATPases?

Answer 17

control of resting Ca concentration

uses ATP to pump ions by active transport

Question 18

what is the role of pmca?

Answer 18

Expels Ca from the cell, high affinity, low capacity

Question 19

what is the role of serca?

Answer 19

Accumulates Ca in the SR/ER, high affinity, low capacity

Question 20

what is the role of ncx?

Answer 20

Inward flow of Na down its concentration gradient drives outward flow of Ca up its concentration gradient, low affinity, high capacity

Question 21

what happens to ncx in ischaemia?

Answer 21

atp is depleted in ischaemia
Na K ATPase is inhibited
Na accumulates in cell leading to depolarisation
ncx reverses
Na moves out, Ca moves in 
high Ca is toxic

Question 22

what is the role of nhe?

Answer 22

Inward flow of Na down its concentration gradient leads to cell alkalisation by removing H

Question 23

what is the role of nbc (sodium bicarbonate cotransporter) and Na dependent Cl HCO3 exchanger?

Answer 23

acid out
base in
uses Na gradient set up by Na K ATPase
raises intracellular pH
regulates cell volume

Question 24

what is the role of ae (anion exchanger) and Cl HCO3 exchanger?

Answer 24

base out
acid in
cell volume regulation

Question 25

how is cell swelling reversed?

Answer 25

extrude ions

Question 26

how is cell shrinking reversed?

Answer 26

influx ions

Question 27

what are osmotically active ions?

Answer 27

Na K Cl organic osmolytes (amino acids)

Question 28

how is bicarbonate reabsorbed in the proximal tubule?

Answer 28

Na K ATPase keeps intracellular Na low so nhe can pump H into the proximal tubule lumen H picks up bicarbonate in the lumen and brings it back into the cell

Question 29

what do loop diuretics do?

Answer 29

block Na reuptake in the thick ascending upper limb

Question 30

what does amiloride do?

Answer 30

acts in distal convoluted tubule (ENaC) and proximal convoluted tubule (Na/H) to prevent Na reuptake

Question 31

what does aldosterone do?

Answer 31

upregulates these transporters

Question 32

what does sprionolactone do?

Answer 32

glucocorticoid receptor antagonist - used to treat if aldosterone is high

Question 33

why does cystic fibrosis cause thick mucous?

Answer 33

normally transport of Na out of the cell by Na K ATPase allows for symport of Cl into cell with Na and K faulty ctfr protein leads to accumulation of Cl in cell water moves into the cell via osmosis leaving thick viscous mucous in the lumen

Question 34

what happens in diarrhoea?

Answer 34

ctfr is overly active once phosphorylated by pka Cl is excessively transported into lumen water follows leading to symptoms of diarrhoea

Question 35

How does water move across membranes?

Answer 35

Osmosis

Question 36

What increases a membrane's permeability?

Answer 36

Specific channels

Question 37

What is the difference in Cl permeability in a phosphatidylserine bilayer compared with an erythrocyte membrane?

Answer 37

10^7 fold.

Question 38

What protein is responsible for Cl transport?

Answer 38

Band 3

Question 39

What does the Band 3 protein do?

Answer 39

Carries out the specific exchange of Cl for HCO3.

Question 40

What does a ligand gated ion channel do?

Answer 40

Open or close in response to ligand binding to a receptor site.

Question 41

What does a voltage gated channel do?

Answer 41

Open or close in repsonse to potential difference across the membrane.

Question 42

What does a gap junction do?

Answer 42

Close when cellular calcium concentration rises above 10 micromoles or the cell becomes acid.

Question 43

What determines whether a molecule moves by passive or active transport?

Answer 43

The free energy change of the transported species.

Question 44

How is the free energy change of the species determined?

Answer 44

The concentration gradient for the transported species and the electric potential across the membrane when the species is charged.

Question 45

Where does the free energy driving active transport come from?

Answer 45

Directly or indirectly from the hydrolysis of ATP

Question 46

What drives ATP synthesis in mitochondria?

Answer 46

A gradient of H+ ions via an ATP dependent proton transporter.

Question 47

What is secondary active transport?

Answer 47

The transport of one substance is linked to the concentration gradient for another via a cotransporter, so hydrolysis of ATP is used indirectly

Question 48

What is the role of the Na glucose co transport system of the small intestine and kidney?

Answer 48

Entry of sodium provides the energy for entry of glucose

Question 49

What is the role of mitochondrial Ca uniports?

Answer 49

Operate at high Ca concentration to buffer potentially damaging concentrations

Question 50

When cellular buffering capacity is exceeded, how is cellular pH controlled?

Answer 50

Plasma membrane transporters

Question 51

How is acidification opposed?

Answer 51

Expelling H+ or inward movement of bicarbonate ions

Question 52

How is alkalinisation opposed?

Answer 52

Expelling bicarbonate via the anion exchanger

Question 53

What does electroneutral transport of ions allow?

Answer 53

The osmotic strength of the cytoplasm to be varied without affecting the membrane potential