Direct Active Transport: the Na+/K+ Pump Maintains Electrochemical Ion Gradients

Question 1

what are the relative concentrations of the K+ on the inside and outside of the cell? What about Na+?

Answer 1

[K+]inside/[K+]outside is about 35:1

[Na+]inside/[Na+]outside is about 0.08:1

Question 2

What does the pumping of both Na+ and K+ ions against their gradients require?

Answer 2

an input of energy

Question 3

where does the Na+/K+ pump get its energy to function?

Answer 3

pump is Na+/K+ ATPase, uses exergonic hydrolysis of ATP to drive the transport of both ions, which is responsible for the asymmetry distribution of ions across the plasma membrane of animal cells

Question 4

structure of Na+/K+ pump?

Answer 4

the pump is a tetrameric transmembrane protein with two a (alpha) and two B (beta) subunits

Question 5

What do the a (alpha) subunits contain?

Answer 5

contain binding sites for Na+ and ATP on the cytoplasmic side and K+ and ATP on the external side

Question 6

how many Na+ ions move out and how many K+ move in?

Answer 6

3 Na+ are moved out and 2 K+ moved in per molecule of ATP hydrolyzed

Question 7

Na+/K+ pump and conformational states …

Answer 7

the pump has 2 alternative conformational states , E1 and E2.

• E1 conformation is open to the inside of the cell and has a high affinity for Na+ ions
• E2 conformation is open the outside of the cell and has high affinity for K+ ions

Question 8

Mechanism of the Na+/K+ pump (6)

Answer 8

1. 3 Na+ ions bind to the E1 conformation
2. this triggers the phosphorylation of the alpha subunit by ATP
3. the pump undergoes a shift to the E2 conformation, causing release of the Na+ ions to the outside of the cell
4. 2 K+ ions from the outside bind to the E2 conformation on the outside of the cell
5. this triggers dephosphorylation of alpha subunit by ATP and a shift back to the original conformation
6. K+ ions are moved to the inner surface, where they dissociate and the process repeats