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Concentration gradients across membranes with different ions

Concentration gradients across membranes with different ions


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I'm trying to gain an intuition for the dynamics of across neuronal membranes. The overarching idea here is they are controlled by ion concentration gradients across the membrane (which we can describe mathematically).

I'm looking for someone to delineate every present concentration gradient across neuronal membranes that are relevant here(involving $Na^+$, $Ca^{2+}$, $K^-$, and $Cl^-$).

It's unclear to me what creates a concentration gradient:

  • Are the concentrations of each type of ion independent from eachother, i.e. in a world without charge, diffusion across the membrane is individual to the concentration of each ion, or, is mediated by the net concentrations of mass (which makes less sense)?
  • Is the flow of ions across the membrane driven by net charge on each side, or the charge concentrations of each individual ion?

I try to answer your two questions briefly:

  • Ion gradients are dependent on charge, but there exist independent transport mechanisms, extending diffusion. Trans-membrane transport proteins can specifically move only one sort of ions. Also symport or antiport exists, that can depend on gradients of other (e.g. non-charged molecules). In addition, the mentioned inorganic ions can form chemical bonding to organic ions for example, which neutralizes the electrical charge.

  • Beside an electrical potential, also chemical (concentration) potential can act as force on ios. In addition active transport, inactivation as mentioned above and other factors have an influence on the molecule flow.


Electrochemical Gradient

We have discussed simple concentration gradients—differential concentrations of a substance across a space or a membrane—but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium (K + ) and lower concentrations of sodium (Na + ) than does the extracellular fluid. So in a living cell, the concentration gradient of Na + tends to drive it into the cell, and the electrical gradient of Na + (a positive ion) also tends to drive it inward to the negatively charged interior. The situation is more complex, however, for other elements such as potassium. The electrical gradient of K + , a positive ion, also tends to drive it into the cell, but the concentration gradient of K + tends to drive K + out of the cell (Figure). The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient .


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