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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.

facilitated diffusion


The glucose transporter will

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


How, then, do cells concentrate glucose?

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


in the absence of glucose,

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


_______ 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.



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

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


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

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


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

Give Insulin and Glucose
o Similarly stimulates the Na+/K+ pump, drawing K+ into the cell from the blood


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

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).