2.1 - Role of Membranes as Permeability Barriers

Question 1

What types of molecules can diffuse across the membrane?

Answer 1

Hydrophobic molecules, e.g. O2, CO2

Small Uncharged Polar Molecules e.g. water, glycerol

Question 2

What types of molecules can’t diffuse across the membrane?

Answer 2

Large, uncharged polar molecules e.g. glucose

Ions

Question 3

How is the human erythrocyte membrane different?

Answer 3

It is highly permeable to Cl-, Glucose and water

Question 4

How the squid axon resting membrane different?

Answer 4

Increased permeability of K+, Na+ and Cl-

When excited the permeability of K+ and Na+ increases further to allow propagation of an action potential

Question 5

What are the roles of transport proteins?

Answer 5

Maintenance of ionic composition
Maintenance of intracellular pH
Regulation of cell volume
Concentration of metabolic fuels and building blocks
Extrusion of waster products of metabolism and toxic substances
Generation of ion gradients necessary for the electrical excitability of nerve and muscle

Question 6

What is the main model of membrane transport proteins?

Answer 6

‘Ping-Pong’

Question 7

Describe facilitated diffusion via ion channels

Answer 7

Channel sits in membrane
Can close/open when stimulated
Ions diffuse through a pore specific to that ion

Question 8

Describe ligand-gated ion channels

Answer 8

Ion channels that changes conformation when bound to by a ligand
e.g. nicotinic acetylcholine receptor + ATP sensitive K+ channel

Question 9

Describe Voltage-gated ion channels

Answer 9

Channels that are sensitive to changes in the potential difference across the membrane
Conformational change occurs that results in the channel being opened
e.g. Na+ channels

Question 10

Describe Passive Transport

Answer 10

Movement down a concentration gradient

Energy in the concentration gradient is dissipated as the molecules move, i.e. ions move to discharge the gradient

Question 11

Describe Active Transport

Answer 11

Movement of molecules/ions against a concentration gradient/membrane potential
Energy comes directly/indirectly from ATP hydrolysis
Cells use a lot of their energy in active transport so it is clearly important in cell life

Question 12

What is the intracellular and extracellular concentrations of Na+?

Answer 12

Intracellular = 12mM
Extracellular = 145mM

Question 13

What is the intracellular and extracellular concentrations of K+?

Answer 13

Intracellular = 155mM
Extracellular = 4mM

Question 14

What is the intracellular and extracellular concentrations of Ca2+?

Answer 14

Intracellular = 10^-7 mM
Extracellular = 1.5mM

Question 15

What is the intracellular and extracellular concentrations of Cl-?

Answer 15

Intracellular = 4.2mM
Extracellular = 123mM

Question 16

What does the PMCA transporter do?

Answer 16

Plasma Membrane Ca2+ ATPase

Transports Ca2+ which requires a mole of ATP

Question 17

Describe the action of ATP synthetase

Answer 17

Brings H+ into the cell while converting ADP to ATP

Question 18

Define Uniport

Answer 18

The transport of a single ion through a channel

Question 19

Define Co-transport

Answer 19

The transport of more than one type of ion or molecule through a single membrane transporter

Question 20

Define Symport

Answer 20

Co-transport in the same direction

Question 21

Define Antiport

Answer 21

Co-transport in opposite directions

Question 22

Describe the Sodium-Potassium Pump

Answer 22

• antiport co-transporter
• important in maintain concentration gradients of Na+ and K+
• primary active transporter
• alpha and beta subunit

Question 23

The Sodium-Potassium Pump can be describes as a P-type ATPase, what does this mean?

Answer 23

ATP phosphorylates aspartate on the pump, causing a conformational change that allows Na+ and K+ to pass through

Question 24

What are the two Ca2+ transporters?

Answer 24

Ca2+ Mg2+ ATPase

Na+-Ca2+ exchanger

Question 25

Describe the primary active transporter of Ca2+

Answer 25

Ca2+ Mg+ ATPase
High affinity
Low capacity
Requires Mg2+ coupled to ATP

Question 26

Describe the secondary active transporter of Ca2+

Answer 26

Na2+ Ca2+ exchanger
Low affinity
High capacity
Antiport = brings 3Na+ in and takes 2Ca2+ out

Question 27

Describe the Na+ H+ exchanger

Answer 27

Antiport = Na+ moves in and H+ moves out

Secondary active transporter

Question 28

Describe the Na+ Glucose co-transporter

Answer 28

Symport = both Na+ and glucose are brought into the cell
Secondary active transporter
Occurs in the small intestine

Question 29

What happens in cystic fibrosis?

Answer 29

Defective CFTR protein so Cl- not transported out of the cell
Na+ and water therefore remain in the cell
Results in thick, viscous mucous

Question 30

What happens in diarrhoea?

Answer 30

Increased activity of protein kinase A so increased activity of the CFTR protein
More Cl- leaves the cell so more water follows it leading to increase water in the lumen