Membrane Transport

Question 1

What is the transport of inorganic ions and small molecules across the membrane required for? (3)

Answer 1

• Regulation of intracellular ion concentrations
• Uptake of nutrients
• Excretion of metabolic waste products

Question 2

Does passive transport require energy?

Answer 2

no (driven by electrochemical gradients/conc)

Question 3

What are the types of passive transport?

Answer 3

Simple diffusion

Facilitated diffusion

Question 4

Whats the difference between simple and fac?

Answer 4

In fac membrane proteins are involved

Question 5

What is the electrochemical gradient?

Answer 5

The combined force of conc gradient and membrane potential (membrane potential only applies to charged molecules)

Question 6

What is the membrane potential?

Answer 6

The difference in charge between the inside and the outside of a cell

Question 7

What affects ability of a solute to cross the membrane by simple diffusion? (4)

Answer 7

• conc gradient
• electrochem gradient (ions don’t cross)
• hydrophobicity (polar molecules don’t tend to cross bilayer)
• size

Question 8

What types of membrane protein are involved in fac diffusion?

Answer 8

• channels

- transporters

Question 9

What are the channels and transporters required for in fac?

Answer 9

to transport inorganic ions and small molecules across the membrane (passively along conc/electrochem grad)

Question 10

What are channels required for?

Answer 10

To transport ions (membranes impermeable to them)

Question 11

What are 4 features of ion channels?

Answer 11

1. exhibit ion selectivity
2. driven by conc/electrochem grad
3. V fast (10^7 molecules per sec)
4. Regulated (open + close in response to stimulus)

Question 12

What is one of the most common ion channel?

Answer 12

K+ channel

Question 13

What are K+ leak channels

Answer 13

K+ ion channels that are constantly open (not gated)

Question 14

How many times more effective are K+ channels than Na+ channels?

Answer 14

10,000 fold more effective

Question 15

Describe how a K+ ion moves through a K+ channel

Answer 15

1. Due to being in a high conc a hydrated K+ ion enters the channel where there are negatively charged amino acids at the pore entrance
2. As it passes through it gets dehydrated
3. The K+ ion passes through the selectivity filter which is lined with carbonyl oxygens of amino acids
4. As the K+ leaves the channel it gets rehydrated and moves out into the are of lower electrochem gradient

Question 16

How is the K+ ion channel selective?

Answer 16

1. Anions prevented by entering due to negative amino acids at entrance
2. Hydrated cations including K+ are too big to pass through selectivity filter + must be dehydrated (requires energy). In K+ the energy lost by dehydration is regained by dehydrated K+ ions interacting with carbonyl oxygens of amino acids lining selectivity filters (act as surrogate water molecules) (Na+ would only be able to interact with 2 O molecules in contrast to K+ 4 (energetically unfavourable))

Question 17

What are 5 features of glucose transporters (Gluts)

Answer 17

• uniporters (only transport glucose)
• expressed by most cells
• many isoforms
• 12 pass membrane spanning proteins
• alternate between 2 conformations

Question 18

What happens when someone has Glut1 deficiency syndrome?

Answer 18

• seizures
• microcephaly
• retarded development

Question 19

What 2 transporters are involves in active transport?

Answer 19

• ATP-driven pumps

- coupled receptors

Question 20

What is the role of ATP driven pumps?

Answer 20

To move solutes against their conc/electrochem gradient by expending energy (primary active transport)

Question 21

What are 7 features of the (Na+K+ pump)

Answer 21

• expressed by all cells
• up to 30 million of them on some cells
• Hydrolyses ATP (primary active transport
• Utilises 30% of energy available in most cells (more in nerve cells)
• Continually expels Na+ from the cell in exchange for K+)
• Maintains k+ and Na+ electrochem gradients
• Stabilises osmotic balance so that secondary active transport can’t occur

Question 22

How does the Na+K+ pump work?

Answer 22

1. 3Na+ ions bind, the pump hydrolyses ATP and is phosphorylated
2. Na+ dependent phosphorylation causes the pump to change conformationally and Na+ is transferred across the membrane and released
3. 2K+ ions bind and the pump is dephosphorylated
4. K+ dependent dephospho rylation causes the pump to return to normal conformation and K+ is transferred across the membrane and released

Question 23

What is the role of coupled receptors?

Answer 23

To move solutes against their conc/electrochem gradient by coupling their transport to the Na+ gradient created by Na+K+ATPase (does not depend directly on the hydrolysis of ATP)

Question 24

What are 2 features of the Na+/glucose symporter?

Answer 24

• oscillates between two conformations

- binding is cooperative (Na+ binding increases affinity for glucose)

Question 25

Where is glucose most likely to bind to and why?

Answer 25

extracellular side where Na+ conc is higher (glucose more likely to enter cells then leave them)

Question 26

Why are glucose symporters required for absorption of glucose in the GI tract?

Answer 26

There are tight junctions so glucose can’t pass through paracellular pathway

Question 27

What do tight junctions prevent?

Answer 27

Paracellular movement

Question 28

Name the 2 functions of tight junctions and explain.

Answer 28

Barrier-prevents things from moving through paracellular pathway
Fence-Prevents proteins diffusing through the basal domain to the apical domain and vice versa

Question 29

How is glucose absorbed

Answer 29

1. Na+ grad created by Na+/K+ATPase
2. Na+/ glucose symporter transports glucose into the epithelial cells (against conc grad) using Na+ electrochem gradient
3. Glut2 transports glucose out of cells (along conc grad) and into blood supply (fac diffusion)

Question 30

What does glucose absorption depend on?

Answer 30

The asymmetric distribution of transporters in epithelial cells
Na+/glucose symporter must be in apical membrane
Glut2 must be in basolateral membrane

Question 31

Where is the Na+/Ca+ anti porter expressed by?

Answer 31

Cardiomyocytes in heart muscles

Question 32

Is the conc Ca+ conc greater in the interstitial fluid or in the cardiomyocytes?

Answer 32

Interstitial fluid

Question 33

What is the role of the antiporter in cardiomyoctes?

Answer 33

To use Na+ electrochem gradient to transport Ca2+ out of the cell against its gradient (causes muscle relaxation)

Question 34

How is muscle contraction caused?

Answer 34

By a rise in intracellular Ca2+

Question 35

How would you treat heart failure?

Answer 35

By preventing Ca2+ being transported out of cell (increase strength of heartbeat)

• ouabain and digoxin inhibit Na+K+ATPase
• This inhibition increases intracellular Na+ and a reduction in Na+ gradient
• So effectiveness of anti porter reduced so fewer Ca2+ transported out of the cell

Question 36

What is the structure of an aquaporin?

Answer 36

• tetrameric complex
• made up of four monomers (each acting as a water channel)
• Each monomer has a six pass membrane spanning protein

Question 37

What is the role of the aquaporin?

Answer 37

to allow passage of water whist preventing the passage of ions