S5) Regulating Flow — Ion Channels, Transporters and Electrochemical Gradients

Question 1

What are gated channels?

Answer 1

Gated channels are channels which open/close in response to different stimuli and control the movement of molecules both into and out of the cell

Question 2

What are ligand gated ion channels (LGIC)?

Answer 2

Ligand-gated ion channels are channels which open or close in response to ligand binding to a receptor site

Question 3

What are voltage gated ion channels (VGIC) ?

Answer 3

Voltage-gated ion channels are channels which open and close in response to the voltage potential difference across the membrane

Question 4

What is the sodium potassium pump and what does it do?

Answer 4

• The Na⁺-K⁺-ATPase protein in the membranes of all excitable cells ‘pumps’ 3 Na⁺ out of the cell and 2 K⁺ into the cell
• This is against the respective concentration gradients, at the expense of one ATP molecule hydrolysed

Question 5

Which secondary active transport processes are driven by the Na pump?

Answer 5

• Ion homeostasis
• [Ca²⁺]_i
• Intracellular pH
• Cell volume
• Action potential

Question 6

In terms of primary active transport, outline the control of resting [Ca²⁺]_i

Answer 6

• PMCA expels Ca²⁺ out of the cell
• SERCA accumulates Ca²⁺ into the SR/ER

Question 7

Outline the role of the sodium-calcium exchanger (NCX)

Answer 7

The NCX exchanger exchanges 3 Na⁺ for 1 Ca²⁺ and so creates a net change in the electric field across the plasma membrane of 2+

• In polarised cells, Ca²⁺ move out of the cell, whilst Na⁺ move in
• In depolarised cells, NCX is inhibited to bring Ca²⁺ into the cell (influx)

Question 8

When cellular buffering capacity is exceeded, cellular pH is controlled by the activity of a variety of plasma membrane transporters.

Explain the role of ion transporters in intracellular pH regulation

Answer 8

• Acidification can be opposed by expelling H⁺ / the inward movement of bicarbonate ions.
• Alkalinisation is opposed by expelling HCO₃^- via the anion exchanger

Question 9

Different cell types use particular combinations of transporters to achieve cell volume regulation.

Explain the principles of electroneutral transport to influence water movement

Answer 9

• Cells extrude ions in response to cell swelling → water follows
• Cells influx ions in response to cell shrinking → water follows

Question 10

Outline the use and action of diuretic drugs to control water balance and fluid loss

Answer 10

Diuretic drugs are used where fluid loss is required to treat oedema or hypertension:

• Na⁺ reabsorption mechanism(s) are blocked
• Increased Na⁺ excretion to produce a hyperosmotic urine