M&R S2 - Membrane Permeability and Cell Volume/pH Regulation

Question 1

What properties of solutes determines whether they can pass through a phospholipid bilayer?

Answer 1

Hydrophobic
Small
Uncharged Polar

Question 2

What types of solutes can’t pass through a phospholipid membrane?

Answer 2

Large, charged, or polar molecules cannot make it through a membrane

Ions can also not pass through a membrane

Question 3

What is passive diffusion?

What determines rate of PD?

Answer 3

Diffusion that is dependent on:

Permeability
Concentration gradient

Rate increases linearly with increasing concentration gradient

Question 4

What is a permeability coefficient?

What is the permeability coefficient of water?

Answer 4

A numerical expression of permeability of a solute

Expressed in cm/s

5x10-3 cm/s

Question 5

What could be said about the permeability of hydrophobic vs hydrophilic solutes in terms of permeability coefficient?

Answer 5

Hydrophobic molecules have a higher PC

Hydrophilic molecules smaller (tend to be <10-10 cm/s)

Question 6

What is the key function of a phospholipid bilayer in terms of their interaction with hydrophilic molecules?

Answer 6

Act as a permeability barrier to hydrophilic molecules

Question 7

How is the movement of hydrophilic molecules across a membrane mediated and regulated?

What roles do these processes have?

Answer 7

By specific membrane transport systems

Transport processes are important in:

Maintenance of intracellular pH, ionic composition, metabolic fuels and volume

Extrusion of metabolic waste and toxins

Generation of ion gradients

Question 8

What is facilitated diffusion?

Name two models of facilitated diffusion

Answer 8

Permeability of a membrane to a substance is ‘facilitated’ by specific proteins in the bilayer

Models include Carrier molecules (ping pong) and channel proteins

Question 9

Why is facilitated diffusion a saturable process?

Answer 9

Limited number of proteins in the membrane means a maximum rate can be achieved when all transporters are busy

Question 10

What effect does facilitated diffusion have on equilibrium points of solutes moving across the membrane?

Answer 10

Has no effect, similar to enzymes

Question 11

What is channel protein gating?

Answer 11

Some channel proteins may be gated, meaning that they open or close in response to stimulus such as:

Ligand binding
Voltage change
ATP binding

Question 12

What is active transport?

Answer 12

Active transport allows transport of ions or molecules against unfavourable electrical or chemical gradients requiring energy from the hydrolysis of ATP

Question 13

What determines whether a solute must be transported by active transport?

Answer 13

The free energy change of the transported species

This is in turn determined by:

Concentration gradient

Electrical potential across the membrane bilayer (only when the molecule is charged)

Question 14

What percentage of cellular energy can be expended on active transport?

Answer 14

Some cells spend 30-50% of ATP on AT

Question 15

What are co-transporters?

Hint: Secondary active transport

Answer 15

Membrane proteins that will transport more than one ion or molecule per reaction cycle

In this way the transport of one substance can be linked to the concentration of another (secondary active transport)

Question 16

What are the three types of membrane transport?

Answer 16

Uniport - One molecule/ion transported, one direction

Symport - Two molecules/ions, one direction

Antiport - Two molecule/ions, opposing directions

Question 17

Give an example of Symport SYSTEM found in the small intestine and kidney

Answer 17

Na+/Glucose co-transport system

Entry of Na+ provides energy for the entry of glucose against the concentration gradient

Question 18

What is Na+/K+ - ATPase and how does it work?

Hint: Form AND function

Answer 18

What it is:

A plasma membrane associated pump with 2 subunits, Alpha and Beta

Alpha - Contains binding sites for Oubain, K+ extracellularly and Na+ and ATP intracellualrly

Beta - Gycoprotein, Oligosaccharide chains direct pump to the surface

Called a P-type ATPase
(ATP phosphorylated Asp, producing phosphoenzyme intermediate)

What it does:

Uses ATP to transport 2K+ into the cell and 3Na+ out (Active transport/Antiport)

Question 19

How much of the BMR is used by the Na+/K+ ATPase proteins in a cell?

Answer 19

25% of BMR

Question 20

How is Na+/K+ ATPase controlled?

Answer 20

Oubain binding inhibits transport

Question 21

What processes is Na+/K+ ATPase involved in?

Answer 21

Forms Na+ and K+ gradients necessary for electrical excitability

Drives secondary active transport processes such as:

Control of pH
Regulation of Cell volume
Regulation of Ca2+ concentration
Absorption of Na+ in epithelia
Nutrient uptake (Eg. Glucose in small intestine)

Question 22

What is the relevance of the interaction of K+ channels and Na+/K+ ATPase?

Answer 22

K+ diffusion down the concentration gradient set up by the Na+/K+ ATPase through K+ channels is mainly responsible for membrane potential

Question 23

What is the importance of intracellular Ca2+ concentration control?

Answer 23

High Ca2+ concentration is toxic to cells

Question 24

What is the concentration difference in intra and extra cellular Ca2+ concentration?

Answer 24

The is a `20,000 fold difference in intra and extracellular concentration

Intra - <100nM
Extra - 2mM

Question 25

What is the function of Ca2+ ATPases?

Answer 25

Control of resting Ca2+ concentration intracellularly Uses ATP to pump ions

Question 26

What are the two major forms of Ca2+ ATPases? Where are they found?

Answer 26

SERCA and PMCA a.k.a. Plasma membrane Calcium ATPase Sarco(endo)plasmic reticulum calcium ATPase Names denote where they're found

Question 27

Describe the action of PMCA

Answer 27

Removes residual Ca2+ from the cell in exchange for H+ Uses ATP (Active transport/Antiport) High affinity, Low capacity

Question 28

Describe the action of SERCA

Answer 28

Accumulated Ca2+ in the SR/ER in exchange for H+ Uses ATP (Active transport/Antiport) High affinity, low capacity

Question 29

What is mitochondrial Ca2+ uniport?

Answer 29

A uniporter that operates at high Ca2+ concentration to move Ca2+ into the mitochondria and help prevent cell damage

Question 30

Outline the normal function of the Na+/Ca2+ Exchanger (NCX)

Answer 30

Utilises secondary active transport Expels 1 Ca2+ in exchange for 3Na+ To do this it uses the Na+ concentration gradient set up by the Na+/K+ ATPase Electrogenic - Current flows in direction of Na+ gradient Activity is membrane potential dependent Antiport Low affinity, high capacity Removes MOST Ca2+ - Expels intracellular Ca2+ during cell recovery (after an action potential)

Question 31

Outline how NCX function changes in Ischaemic conditions

Answer 31

ATP depleted, Na+/K+ ATPase inhibited Na+ builds up in the cell, causing depolarisation NCX reverses (Sodium out, Calcium in) in an effort to repolarise the cell

Question 32

When cellular buffering capacity is exceeded, how is pH controlled/returned to the set point?

Answer 32

Membrane transporters Acidification opposed by expelling H+ or inward movement of Bicarbonate (activation of NHE and NBC) Alkalisation opposed by expelling bicarbonate (activation of AE)

Question 33

What membrane transporters are involved in acid extrusion?

Answer 33

Na+/H+ exchanger (NHE) Na+/Cl-/HCO3-/H+ co-transporter (NBC) Both raise intracellular pH

Question 34

Describe the function of the Na+/H+ Exchanger (NHE) Hint: Include inhibitors/activators

Answer 34

Moves 1Na+ in, 1H+ out, therefore electroneutral Uses Na+ Concentration gradient set up by the Na+/K+ ATPase to achieve this Raises pH intracellularly Helps regulate cell volume Activated by growth factors Inhibited by amiloride

Question 35

Outline the function of the Na+/Cl-/HCO3-/H+ co-transporter (NBC)

Answer 35

Na+ and HCO3- in (base in) H+ and Cl- out (acid out) Uses Na+ concentration gradient set up by the Na+/K+ ATPase Process can be looked at as coupled Na+/H+ and anion exchange performed by the same exchanger Raises intracellular pH Involved in cell volume regulation

Question 36

What is the major cellular base extruder? Describe its function

Answer 36

Cl-/HCO3- Exchanger (Anion exchanger/AE) Cl- in HCO3- out (base out) Acidifies cell Involved in cell volume regulation

Question 37

How is cell volume regulated?

Answer 37

Osmotically active ions (E.g. Na+, K+, Cl-) or organic osmolytes (amino acids) are transported in or out of a cell and water follows via osmosis causing cell shrinking or swelling There is no standard regulation method, different cells use different combination of transporters to achieve regulation If cell is shrinking: Influx ions (E.g. Na+ or Ca2+ channels) and osmolytes If cell is swelling: Extrude ions (E.g. K+ or Cl- channels) and osmolytes

Question 38

Outline how bicarbonate is reabsorbed from the kidney's proximal tubule

Answer 38

Na+/K+ ATPase removes Na+ from proximal tubule epithelia NHE can therefore pump Na+ from the lumen into the cell along the concentration gradient in exchange for H+ H+ goes into the lumen and 'picks up' HCO3- to bring it back into the cell

Question 39

What is the main reason for bicarbonate reabsorption from the proximal tubule?

Answer 39

To retain base for pH buffers

Question 40

What is the main goal of renal anti-hypertensive therapy?

Answer 40

Reduce the re-uptake of Na+ and other molecules so less water is absorbed via osmosis With less water absorbed, blood volume, and therefore blood pressure falls

Question 41

What is the function of Aquaporins? Describe their involvement in regulation of water reabsorption form the kidney tubules

Answer 41

Allows water to more easily cross the membrane It's inclusion in the membrane of Kidney epithelia is stimulated by Anti-Diuretic Hormone (ADH) causing increased water re-absorption.

Question 42

What are Loop Diuretics?

Answer 42

Block Na+ re-uptake in the thick ascending Limb of the Loop of Henle

Question 43

What type of drug is amiloride? How does amiloride affect Na+ re-uptake in the kidney?

Answer 43

Is a potassium sparing diuretic Acts on both distal (ENaC - Epithelial Na+ Channel) and proximal (NHE) tubules to prevent Na+ re-uptake

Question 44

What is the result of excess aldosterone and how is this treated?

Answer 44

Aldosterone upregulates ENaC and NHE in the kidney tubules to increase Na+ re-uptake In excess, this can be a contributing factor to hypertension Spironolactone (Glucocorticoid receptor agonist) is used to treat

Question 45

How is glucose moved from the lumen to the blood by intestinal lumen and kidney tubule epithelium?

Answer 45

SGLUT-1 is a sodium and glucose symporter that moves them into the epithelia from the lumen via facilitated diffusion Na+/K+ ATPase produces the Na+ gradient that allows co-transport of glucose as Na+ moves down its concentration gradient in SGLUT-1 GLUT transporters move glucose from the cell into the bloodstream

Question 46

What are GLUT transporters?

Answer 46

Transporters that move glucose into the cell

Question 47

What are the different types of GLUT transporters and where are they found? By what method do these transporters move glucose into cells?

Answer 47

GLUT 1+3: - Found throughout the body - Facilitated diffusion (uniport) GLUT 2: - Hepatocytes and pancreatic beta cells - Facilitated diffusion (uniport) GLUT 4: - Striated muscle and adipose - Facilitated diffusion (uniport), Insulin stimulates SGLUT-1 - Intestine and kidney - Secondary active transport (symport)

Question 48

What is the main function of GLUT 1 and 3?

Answer 48

Maintain a basal glucose level in cells

Question 49

How does insulin promote glucose uptake into cells?

Answer 49

Up-regulates expression of GLUT4, resulting in more GLUT4 receptors on the cell surface (from storage vesicles)

Question 50

How does the cell prevent glucose efflux when blood concentration of glucose falls to resting levels following a meal?

Answer 50

Glucose is rapidly metabolised to G-6-P upon entering the cell, so intracellular glucose concentration never rises higher than blood glucose concentration

Question 51

What other metabolites use the sodium gradient for their uptake into cells?

Answer 51

Amino acids