Membrane transporters Flashcards
Primary active transporters derive their energy from:
directly from the splitting of ATP
Other than the _____, there are no other ubiquitous primary active transporters in the plasma
membrane of cells
Na/K pump
Inside mitochondria is a very special proton pump that, when running backwards, lets
(the F1-ATPase)
protons leak across a membrane and synthesizes, rather than hydrolyzes ATP
Secondary active transport Energy to do the work of pumping comes from
not from metabolism (ATP), but from a secondary source
• Usually this energy source is the ‘downhill leak’ of Na+ into the cell.
Secondary active transport is
Mechanism by which most substances are pumped.
There are two basic types of secondary active transporters:
Cotransport:
Antiport or Exchange:
Cotransport:
those that move different solute species in the same direction
Antiport or Exchange:
those that move solute in opposite directions
Secondary active transport For example,cells can accumulate amino acids
This active uptake is dependent on ______
against their energy gradients
external Na+; if external Na+ is removed, amino acid
uptake is abolished
Conversely, removing the amino acid reduces the entry of Na+
Secondary active transport The carrier ingeniously captures the energy released by
the inward leak of Na+ and instead
of letting it escape as heat, uses it to pump the amino acid into the cell.
Secondary active transporters do not necessarily.
always run in the same direction
All secondary transport mechanisms depend ultimately on the ______
Na+/K+ pump (and therefore on ATP).
Some secondary active transporters are
electrogenic in that one cycle produces a net charge transfer across the membrane.
o
Other secondary active transporters are
non-electrogenic.
The main feature of electrogenic secondary active transporters is that their activity is
governed by the membrane potential.
Secondary active transporters The will always tap the ____, because of this, they can _____.
bigger leak to drive the smaller pump.
reverse direction sometimes
example of secondary active transporters moving in reverse
the sodium-calcium exchanger, which reverses direction in heart muscle cells every time the heart beats.
Electrically silent transporters could not care less about
membrane potential
if the Na+/K+ pump is blocked, cells fill up with Na+, and thus the ______ is reduced. Because this is the energy source for ______
Na+ electrochemical gradient
secondary active transport, all of these transport mechanisms suffer.
For example, Na+/ amino acid transporters are electrogenic, because
one cycle transfers a net positive charge (Na+) into the cell.
An example of non-electrogenic. is the
Na/K/2Cl cotransporter, which a ch cycle moves one sodium ion,one
potassium ion, and two chloride ions into the cell.
Cotransport:
secondary transporters that move different solute species in the same direction
Exchange:
secondary transporters that move solute in opposite directions
A non-electrogenic secondary active transporter is one in which
no net charge is moved across the membrane.
Driving Force =
Vm-E
as Vm becomes more positive, the net driving force will
continue to decrease.
reversal potential (Vrev);
it’s the value of Vm at which there will be no net movement via this exchanger.
Vrev = Vm at which
there is no net movement
At values of Vm more positive than Vrev, the exchanger will run
in the other direction: moving H+ into the cell and pumping Na+ out of the cell.
infusing K+ causes _____, and infusing acid causes _____
acidemia (the K+ is taken up by cells ‘in exchange’ for H+)
hyperkalemia
hyperkalemia will cause _____. Hyperkalemia also will ____ cells (by shifting EK in a positive direction), and the change in membrane potential can affect ____
extra K+ uptake via the Na/K pump.
depolarize
the rate of activity of electrogenic transporters.
facilitated diffusion.
Some transporters act like ion channels, shuttling a single solute species in either direction.
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.
How to treat Hyperkalemia
C BIG K C: calcium B: Bicarbonate I: insulin G: glucose K: kayexalate
Hyperkalemia is very dangerous because it can lead to
heart arrhythmias.
Increased extracellular potassium levels result in ______ of the membrane potentials of cells. Which results in _____
depolarization
This depolarization opens some voltage-gated sodium channels, but not enough to generate an action potential.
the open sodium channels inactivate and become refractory, increasing the threshold needed to generate an action potential.
This leads to the impairment of neuromuscular, cardiac, and gastrointestinal organ systems.
Biggest concern in hyperkalemia
the impairment of cardiac conduction which can result in ventricular fibrillation.
Hyperkalemia treatment with calcium
Ca+2 ions bind to the outside surface of cell membranes.
they trick the Na+ channels into thinking the membrane has been hyperpolarized. This increases threshold potential and restoring normal gradient between threshold for an action potential,
quieting down the aberrant, spontaneous depolarization of individual cardiac cells.
VERY LOCALIZED EFFECT!
Hyperkalemia treatment with bicarbonate
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.
hyperkalemia treatment with insulin and glucose
stimulates the Na+/K+ pump, drawing K+ into the cell from the blood
Hyperkalemia treatment with Kayexalate
Ionic species in which Na+ is bound to a large, negatively charged kayexalate ion
Kayesalate prefers to bind to K+, so when introduced into K+ saturated blood, Kayexalate will exchange Na+ for K+ and sequester free K+ ions.
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).
