Membrane transport processes

Question 1

Membrane transport mechanisms

Answer 1

• Pores
• Ion channels
• Solute carriers
• Pumps (ATP dependent)
• Vesicular transport

Question 2

Types of facilitated diffusion

Answer 2

Two types; channel mediated (pores), carrier-mediated (binding)

Question 3

Differences between facilitated diffusion and simple diffusion

Answer 3

• facilitated diffusion can be saturated as limited no. of channel proteins, simple diffusion depends linearly on solute conc.
• facilitated diffusion more temp dependent as temp changes protein composition
• facilitated diffusion far more efficient

Question 4

Water permeability of membranes

Answer 4

Low H2O permeability: ascending loop of Henle
High H2O permeability: red blood cells
Depends on:
-Lipid composition (unsaturated phospholipids increase membrane fluidity -> more permeable to water) (sterol content -> decrease fluidity & water permeability)
-water pores: aquaporins (more important factor)

Question 5

Aquaporins

Answer 5

• Consist of 4 subunits (tetrameric)
• Fast transport rate: 10^9 molecules/sec
• 6 a-helical domains form pore -> 4 pores form aquaporin
• Some pores also permeable to small molecues (e.g. glycerol) -> aquaglyceroporins
• Almost always completely open

Question 6

Main method of control for aquaporin permeability

Answer 6

• Ion channels -> change osmolality inside or outside of cell
• Also effected by pH

Question 7

What are ion channels gated by?

Answer 7

• Membran voltage
• Extracellular messengers
• Intracellular messengers
• Mechanical stress (i.e. stretch mediated Na+ channels)

Question 8

Solute carriers operating principle

Answer 8

1. Binds solute on one side of membrane
2. Protein undergoes conformational change
3. Release solute on other side of membrane
   - Only allows 1 molecule at a time
   - 2 conformational changes

Question 9

Difference between channels and carriers

Answer 9

Both allow facilitated diffusion (passive), but:

• Ion channel has central pore
• Solute carrier undergoes conformational change (slower and less efficient)

Question 10

Primary active transport

Answer 10

Hydrolysis of ATP to generate energy for transport

Question 11

Types of ATPase ion transporters

Answer 11

• P-type
• V-type
• F-type

Question 12

P-type ATPase ion transporter

Answer 12

• e.g. Na+/K+ATPase

- ATP hydrolysis leads to phosphorylation causing conformational change

Question 13

V-type ATPase ion transporter

Answer 13

• Vacuolar-type H+ATPase

- Contributes to set up of pH gradients

Question 14

F-type ATPase ion transporter

Answer 14

• F-ATPase or ATP synthase (mitochondria)

- Uses proton gradient for ATP synthesis

Question 15

Na+/K+-ATPase (Na+/K+ pump) structure

Answer 15

4 main domains:

• N nucleotide (ATP) binding domain
• P phosphorylation domain
• A actuator domain
• M transmembrane domain

Question 16

Na+:K+ transmembrane protein transport ratio

Answer 16

3 Na+ out of cell : 1 K+ into cell

Question 17

ABC transporters

Answer 17

Different types for transport of wide range of complex molecules i.e. Cl- and organic anions
-> Good at low concentrations

Question 18

ABC transporter common features

Answer 18

• ATP-binding cassette (ABC)
• Usually homodimer (2 identical subunits)
• Each subunit consists of; transmembrane domain, nucleotide binding domain

Question 19

ABC transporter mode of action

Answer 19

1. Open dimer has high ligand affinity
2. Ligand binding increases ATP affinity -> ATP binding
3. Conformational change -> reduces ligand affinity, ligand releases
4. ATP hydrolysis and release -> return to open configuration

Question 20

Types of co-transport

Answer 20

• Symporters- both solutes are transported in same direction

- Antiporters- solutes are transported in opposite directions

Question 21

Co-transport

Answer 21

• Movement of solute A down its electrochemical gradient can be used to drive co-transport of solute B against electrochemical gradient -> secondary active transport
• Not directly coupled to ATP hydrolysis