ICPP 5 - Primary & Secondary AT's (Part 1)

Question 1

What is primary active transport?

Answer 1

When transport is driven directly from the hydrolysis of ATP. ATP hydrolysis catalysed by the transporter as part of the reaction.

Question 2

Give 2 main examples of primary AT’s within the cell?

Answer 2

1) Ca2+ ATPase (PMCA) - drives Ca2+ out of the cell against concentration gradient.
2) F1F0 ATPase (ATP synthetase) - This occurs in reverse mode which utilised the proton gradient created by the ETC to drive the synthesis of ATP from ADP.

Question 3

What is a co-transporter?

Answer 3

A transporter that transports more than 1 type of ion or molecule per reaction cycle.

Question 4

What is meant by uniport, symport and antiport transport?

Answer 4

Uniport = 1 ion carried through a transport channel on its own.
Symport = 2 or more ions transported in the same direction.
Antiport = 2 or more ions transported in opposite directions.

Question 5

What does the Na/K ATPase pump do?

What kind of transporter is it?

Answer 5

• Transports 3 x Na+ ions out of the cell brining in 2 x K+ ions.
• Primary active antiporter.

Question 6

How much of the BMR does the Na/K ATPase use?

Why is it a P-type ATPase?

Answer 6

• 25%

- Because it phosphorylates an aspartate residue to produce a phosphoenzyme intermediate

Question 7

What is the Na/K pump important for?

Answer 7

Generating ion gradients that are used to allow secondary AT and action potentials.

Question 8

How responsible is the Na/K pump for generating the resting membrane potential?
Where does the main responsibility for this lie?

Answer 8

• It only provides a small contribution (via efflux of positive charge) - around -5 to -10mV
• The main responsibility lies with K+ efflux through K+ channels

Question 9

What is secondary active transport?

Answer 9

Secondary AT is driven indirectly by hydrolysis of ATP. Dissipation of graidents formed for another ion provides energy for secondary AT.

Question 10

Give an example of an important secondary AT and explain how it works.

Answer 10

• The Na/Ca exchanger. Efflux of Na+ from the Na/K ATPase allows Na+ to move into the cell through the NCX.
• This (dissipation of Na+ gradient) provides energy for Ca2+ to be removed from the cell.

Question 11

What are the affinities and capacities of the Ca2+ ATPase and Na/Ca exchanger proteins?

Answer 11

Ca2+ ATPase = high affinity, low capacity

NCX = low affinity, high capacity

Question 12

Give another two examples of secondary AT’s.

Answer 12

1) Na/H exchanger - uses Na+ gradient created by Na/K ATPase

2) Na-glucose co-transporter - uses entry of Na+ to provide energy for the entry of glucose against it conc gradient.

Question 13

Explain how mutation in CFTR gene leads to sticky mucous build up in cystic fibrosis.

Answer 13

• Na+ gradient by Na/K ATPase allows Cl- ions to move into cell via Na2ClK transporter (alongside Na+ & K+)
• Cl- transfered across membrane and usually out into lumen using CFTR
• CFTR no longer present/functional, so no transfer of Cl- into lumen
• Thus no movement of water with Cl- ions, leading to loss of mobility of mucus covering the epithelial layer of the alveoli.

Question 14

Explain how vibrio cholera infection leads to diarrhoea.

Answer 14

• PKA activated by bacteria
• PKA phosphorylates CFTR gene and stimulates increased cl- movement
• More Cl- ions into lumen of gut, therefore more water leading to diarrhoea.