I think it is a really good question, I don't know the answer, but I can say that they use this principle sometimes in plantecology. I know it has nothing to do with gradiënt, but in plants it is known that when an ion is abundant, it can causee shortage of another ion even though there is enough of the other ion.

for example (not a real example): when a plant has to much K+ then it can cause problems with intake of Ca2+ despite the fact that there is enough Ca2+ around. So I can confirm that there is an influence or interaction but this is maybe a bit far fetched from your subject.

We talk about concentration gradient for individual ion because we need to calculate equilibrium potential of each ion using Nernst equation when an ion channel selectively pass that ion. However, if an ion channel is less selective with multiple different ions passing through, then we need to add them up. In this case, we use Goldman-Hodgkin-Katz equation.

Hi Khashayar,

I am not sure that I will answer your question, but here it is my take on it.

1. Formally concentration gradient of an ion is a first derivative of the concentration of this ion in a certain direction. let us assume, the direction of a vector "x": Delta(concentration)/Delta(x).

2. In physiology transmembrane gradient is usually considered as a difference between the two concentrations. When "Delta(x)" = thickness of the membrane = constant. Therefore: Gradient = (concentration inside the cell) minus (concentration outside).

3. You "believe, concentration gradient is an emergent property of particles pushing each other, independent of the type of ion. Since ions are constantly colliding and influencing each other -universally-, shouldn't the concentration gradient of one ion (e.g., Na⁺) also depend on the concentrations of other ions (e.g., K⁺, Ca²⁺) present in the environment, as they all contribute to the same dynamic system?"

First, concentration of any ion in physiology in most cases is less than 200 mM = 0.2M. However, concentration of water molecules in physiological solutions is about 55M, which is about 300 times higher than maximal concentration of any ion. Therefore, most of the time the ions are colliding not with each other, but with molecules of water.

4. On the other hand, any ion carries a charge (+1 in the case of Na⁺), which will have strong effects on neighboring charges, attracting negatively charged ions and pushing out positive ones. As a result, redistribution of ions and orientation of dipoles of water molecules will lead to so called "bulk electroneutrality" of physiological solutions in volume relatively far from cell membrane.

5. Importantly, that probability of an ion to be transported through the membrane by an ion channel depends on ion concentration, temperature, selectivity of the channel, and concentration of other ions that may compete with this particular ion to enter the channel. In this case we may see the ion interaction. However, for highly selective channels this interaction is negligible and can be omitted.

6. Therefore, the concentration gradients of different ions are treated as they are specific to individual ions and not influenced by the presence of other ions. However, you are right in a sense that when some ions are coming through the membrane, they carry charge, which affects all other ions via electric filed interactions. For example, when during an action potential Na⁺ enters a cell, it shifts intracellular electric potential in positive direction, which pushes all intracellular cations out and attracts anions from outside of the cell. If the cells would not have ions pumps the transmembrane gradients would gradually dissipate in about an hour. However, the pumps and transporters compensate for the small loss of the ion gradients during the action potential (which is usually less than a second), which allow us to assume that in most case we can consider the ion concentrations and their gradient to be constant. At the same time, in pathological conditions the ion gradients might differ from their normal values. Yet, usually they are still considered to be constant, but at a different level.

I believe your perspective is well taken. Consider the concentration gradient of a single solute A in pure water. While moving along the concentration gradient, it will collide with water molecules, resulting in a zig-zag trajectory. But eventually, that solute will migrate in a definite direction.

Let us now consider the addition of a second solute B to solute A. Let us also assume that there is no concentration gradient for solute B. The logic does not change. Solute A will zig-zag across the water and solute B to follow his own gradient.

Let us now imagine that solute B has his own gradient. Because diffusion is a probabilistic process, both solutes A and B will travel according to their respective gradients. Of course, they will collide, and this will just slow their progress in the direction determined by their individual gradient. However, they will move "independently" along their own gradient.