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Movement of Ions Across a Membrane
N/A
8
Physiology
Undergraduate 2
05/05/2012

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Cards

Term
chemical gradient
Definition
concentration gradient

describes the difference from one side of a membrane to another as it relates solely to the concentration of a molecule

electrical charge is not relevant with discussing chemical gradient
Term
electrical gradient
Definition
describes the electrical forces acting across a membrane

significant when the molecules involved are charged

(anions are negative, cations are positive)

(like charges repel, opposite charges attract)
Term
electrochemical gradient
Definition
combined net effect of the chemical and electrical gradients
Term
equilibrium potential
Definition
magnitude of the electrical gradient that exactly opposes the diffusion of an ion across a membrane, down its chemical gradient

no net flux
Term
relationship of Na+, K+, and proteins in intracellular and extracellular fluid
Definition
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Term
resting membrane potential
Definition
reflects the overall effect of multiple ions on the cell's resting electrical charge (taking into account multiple different ions, varying permeability to specific ions, etc.)

-70 mV for a typical neuron

membrane is most permeable to K+ at rest

if a membrane is made more permeable to a specific ion, the membrane potential will move toward that specific ions equilibrium potential
Term
establisment of resting membrane potential
Definition
dependent on
--relative permeability of the cell membrane to Na+ and K+ (relative ease with which these ions can move across the cell membrane. K+ is able to move across the cell membrane much more readily than Na+
***It is the relative permeability of the cell membrane via leak channels that creates the resting membrane potential. There are many more K+ leak channels than any other, thus cell membranes are ~50-100x more permeable to K+ than Na+***
--Concentration gradients of Na+ and K+ (see "Concentration Gradients of Na+ and K+" notecard)
Term
Concentration gradients of Na+ and K+
Definition

Na+/K+ pumps maintain a concentration gradient of Na+ and K+ the final membrane potential is the weighted sum of equilibrium potential of Na+ and K+ (in addition to other ions). Because the membrane is much more permeable to K+ ions, the resulting resting membrane potential of -70 mV is closer to the K+ equilibrium potential (-90 mV) than that of Na+ (+60 mV) The fact that the resting membrane potential of -70 mV is not equal to either the Na+ or the K+ equilibrium potentials, both ions have forces acting on them. At resting membrane potential, the electrochemical force on K+ is directed out of the cell. The electrochemical force for Na+ is directed into the cell. In light of K+ "leak" channels, K+ continuously moves out of the cell, down its electrochemical gradient. Na+ may also move down its electrochemical gradient into the cell. The Na+/K+ pump is incessantly working. The activity of the pump maintains the Na+ and K+ concentrations at a relatively stable concentration.

 

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