Permeability Barriers (2)

Question 0

What type of molecules can’t cross lipid bilayers? Give 2 examples of each.

Answer 0

• Large uncharged polar: Glucose/Sucrose

- Ions: H+/Na+

Question 1

What types of molecules can cross lipid bilayers. Give 2 examples of each.

Answer 1

• Hydrophobic: O2/CO2

- Small uncharged polar: H20/urea

Question 2

What 2 things is passive transport dependent on?

Answer 2

• Permeability

- Concentration gradient.

Question 3

Passive transport increases linearly with what?

Answer 3

• Concentration gradient.

Question 4

What are some of the roles of transport in cells?

Answer 4

• Maintenance of ionic composition
• Maintenance of Intracellular pH
• Regulation of cell volume
• Concentration of fuels and building blocks
• Extrusion of waste products
• Generation of ion gradients necessary for the electrical excitability of nerve and muscle.

Question 5

What is ‘ping pong’ transport?

Answer 5

• Substrate enters carrier protein

- Carrier protein changes shape to release substrate on other side of membrane.

Question 6

What is facilitated diffusion?

Answer 6

• Channel proteins are open or closed.
• When open, allows large insoluble molecules to cross membrane.
• Opens in response to a ligand binding.

Question 7

Give examples of ligand gated ion channels.

Answer 7

• Nicotinic acetylcholine receptor (Ach for Na+)

- ATP sensitive K+ channel (ATP for K+)

Question 8

What are voltage gated ion channels?

Answer 8

• Membrane depolarisation allows channels to open

Question 9

What are the differences between passive and active transport?

Answer 9

• Passive: down conc gradient, -delta G (release of energy)

- Active: against conc gradient, +delta G (requires energy)

Question 10

What does active transport allow?

Answer 10

• Transport of molecules against an unfavourable conc and or electrical gradient.
• Energy is supplied directly or indirectly from ATP.

Question 11

What are the Extracellular amounts of Na/Cl/Ca/K?

Answer 11

• Na: 145mM
• Cl: 123mM
• Ca: 1.5mM
• K: 4mM

Question 12

What are the Intracellular values for Na/Cl/Ca/K?

Answer 12

• Na: 12mM
• Cl: 4.2mM
• Ca: 1x10^-7M
• K: 155mM

Question 13

What does primary active transport mean?

Answer 13

• Gets its energy from ATP directly.

Question 14

What is co-transport?

Answer 14

• More than one type of ion or molecule may be transported on a membrane transporter per reaction cycle.

Question 15

What are the different types of transport through a membrane?

Answer 15

• Uniport: Only one molecule transported in one direction
• Symport: Two molecules transported in the same direction
• Antiport: Two molecules transported, one in each direction.

Question 16

What is the Na/K pump?

Answer 16

• Antiport, primary active transport.
• Plasma membrane associated pump
• P type ATPase: ATP phosphorylates aspartate, producing a phosphoenzyme intermediate.

Question 17

• The Na/K pump is made up of two subunits, what are these and what are their roles?

Answer 17

• Alpha: K/Na/ATP ouabin binding site

- Beta: Glycoprotein directs pump to surface.

Question 18

What proportion of BMR is used for the Na/K pump?

Answer 18

• 25%

Question 19

What is the role of K being pumped through the membrane?

Answer 19

• Responsible for membrane potential

Question 20

What are the two types of Ca transport?

Answer 20

• Uniport: Ca-Mg ATPase.
• High affinity, low capacity
• Antiport: Na-Ca exchanger.
• Low affinity, high capacity.

Question 21

What are the main two sodium transport systems?

Answer 21

• Na-H+ exchange: inward flow of Na down conc gradient, causing cell alkalisation by removing H+ (antiport)
• Na-glucose co-transport: entry of Na provides energy for entry of glucose against conc gradient (symport)

Question 22

What is the role of transport proteins in diarrhoea?

Answer 22

• CFTR is overstimulated by protein kinase A

- Cl outflow increased so does H2O as it follows.

Question 23

What are the intracellular and extracellular Ca concentrations?

Answer 23

• Intracellular: 50-100 nM

- Extracellular: 2mM

Question 24

What are the 6 different ways of controlling resting Ca?

Answer 24

- Na/K pump using ATP - Ca ion channel - NCX (Na/Ca exchange) - PMCA (Ca/H exchange) - SERCA (Ca/H exchange in sarcoplamic reticulum) - Ca facilitated transport (mitochondria)

Question 25

How can NCX work in both directions?

Answer 25

- As Na enters this causes depolarisation, membrane potential reverses mode of operation.

Question 26

How is the NCX pump affected in ischaemia?

Answer 26

- Decrease in ATP - Na pump inhibited - Na accumulates, cell depolarises - NCX reverses, Na is exchanged for Ca - High Ca levels is toxic causing cell damage.

Question 27

What are the ways in which cell pH is controlled?

Answer 27

- Acid extrusion: Na/H exchange -NHE Na dependent Cl/HCO3 exchanger -NBC - Base extrusion: Cl/HCO3 exchange (Cl in) -AE

Question 28

How is cell volume regulated?

Answer 28

- Transport of osmotically active ions (Na/K/Cl/organic osmolytes) - Water follows - Cell swelling (extrude ions) - Cell shrinkage (influx ions) - Conductive and co-transport systems to maintain electroneutrality

Question 29

Outline how bicarbonate reabsorption occurs in the proximal tubule.

Answer 29

- NaHCO3 splits into Na and HCO3- - Na is transported into the tubule from the lumen by NHE (Na/H) - Na pump, pumps Na into the capillary. (K opposite direction, but diffuses back into capillary) - In the proximal tubule H20 and CO2 from metabolism are combined to form H2CO3 - This splits into HCO3- and H+ - HCO3- is transported into the capillary by an AE pump (Cl the other way) - H+ is used in the NHE pump to be transported to the lumen. - H+ and HCO3- combine to form H2CO3 - This can be changed into CO2 and H2O by carbonic anhydrase - These can be transported into the tubule.

Question 30

What are the mechanisms to resist cell swelling?

Answer 30

- Conductive: equal ion transport (K+ and Cl- efflux) - Co-transport: K+ and Cl- efflux (co-transport) also amino acid and H2O out. - Co-transport: Cl- and HCO3- exchange and K+/H+ exchange. H+ and HCO3- combine to form H2CO3 -> H20 and CO2 which both leave the cell.

Question 31

What are the mechanisms to resist cell shrinkage?

Answer 31

- Co-transport: Na/K and 2Cl influx. Na and organic osmolytes influx and Na/Cl influx. -> H2O influx - Co-transport: CO2 and H2O influx combine to form H2CO3. H2CO3 splits into H+ and HCO3-. Cl/HCO3 exchange (Cl influx, HCO3- out) H+/Na+ exchange (H+ efflux) - Na/Ca transported in. H2O in.