Membrane Transporters II

Question 1

Some transporters act like ion channels, shuttling a single solute species in either direction. This is called _______. The best known example of this is the glucose transporter.

Answer 1

facilitated diffusion

Question 2

The glucose transporter will

Answer 2

transport glucose in either direction, and burns no energy in the process. Thus, it is not a pump.

Question 3

How, then, do cells concentrate glucose?

Answer 3

The answer is that as soon as a glucose molecule gets into the cell, it is phosphorylated to Glucose-6-Phosphate. G-6-P doesn’t fit on the transporter, and so is “trapped” inside. The empty ‘carrier’ can then ‘return’ to the outside of the membrane

Question 4

in the absence of glucose,

Answer 4

the transporter is not even present in the plasma membrane; it is sequestered inside the cell

Question 5

_______ triggers a biochemical cascade that causes the vesicle membranes to fuse with the surface membrane, exposing the _______ transporter to the ECF. The transporter then gets busy and ‘carries’ glucose inside. When insulin subsides, the transporter molecules are reinternalized.

Answer 5

Insulin

glucose

Question 6

Way cells are encouraged to take up K from the ECF: C

Answer 6

Give Calcium
o Ca+2 ions bind to the outside surface of cell membranes. Through an electrostatic action, they trick the Na+ channels into thinking the membrane has been hyperpolarized. This increases threshold potential through a mechanism that is still unclear, thus restoring normal gradient between threshold for an action potential, quieting down the aberrant, spontaneous depolarization of individual cardiac cells.
o Screen the naked fixed negative charges in cell membranes
o Hyperpolarizes the local Vm
o Makes it more difficult for the cell to fire an action potential
o VERY LOCALIZED EFFECT!

Question 7

Way cells are encouraged to take up K from the ECF: B

Answer 7

Give Bicarbonate
o The presence of excess bicarbonate ions will stimulate an exchange of cellular H+ for Na+, thus leading to stimulation of the sodium-potassium ATPase which will stimulate uptake of K+ into the cell

Question 8

Way cells are encouraged to take up K from the ECF: I/G

Answer 8

Give Insulin and Glucose

o Similarly stimulates the Na+/K+ pump, drawing K+ into the cell from the blood

Question 9

Way cells are encouraged to take up K from the ECF: K

Answer 9

Give Kayexalate
o Ionic species in which Na+ is bound to a large, negatively charged kayexalate ion
o Kayesalate prefers to bind to K+
o Thus, when introduced into K+ saturated blood, Kayexalate will exchange Na+ for K+ and sequester free K+ ions.
o This is usually a longer-term treatment, so must be combined with the other short term ones to actually work (patient could die before Kayexalate would have an effect).