Force that causes ions to move to a lower concentration

[PhysicsInterest]

Apologies if this question may come off as simple to some of you and/or its posted in the wrong thread (i'm not sure if its better on the chemistry or physics forum) but i was curious, i'm currently doing neuron work and it involves the diffusion of K+ and Na+ ions one part that made me ask this question was when i read this """During depolarisation of a section of the axon, voltage-gated Na⁺ channels open and large numbers of Na⁺ ions diffuse into the axon. The influx of Na⁺ creates local currents in the cytoplasm of the axon. Na⁺ diffuses through the cytoplasm in the axon, towards regions where their concentration is lower""" upon reading the last bit of Na+ diffusing to lower concentrations it made me wonder what is causing the movement to a lower concentration my chemistry teacher would always say that they don't have consciousness so what is causing them to move from A to B, What force is causing this????

 

[anuttarasammyak]

Fick’s laws of diffusion would give you an explanation. Random molecular motion cause flow of materials from high to low density side.

 

[docnet]

In case of ions, there is also the tendency for the ions to distribute as evenly as possible in solution, to minimize the electric potential energy of the system. I believe this would be the main phenomenon at play in this scenario.

 

[Tom.G]

Longer version:

Anything with a temperature above Absolute Zero has its atoms and/or molecules in constant random motion. The higher the temperature the faster the motion, and that is why most solids and gasses increase in size with temperature, the atoms bump into each other harder driving them further apart. (Note: Ice and water are a completely separate subject, ignore them for this explanation.)

As an experiment, you could inflate a balloon and put it in a freezer, it will likely shrink from the low temperature.

You can get an idea of the random motion by dropping a little ink in a glass of water, it will gradually spread thru the water.

Since the atomic (molecular) movements are random, the end result is they are uniformly distributed in the volume.

That's how the ions get into the cells, there is a higher concentration outside the cell than inside and when the channels in the cell wall open the ions scatter thru openings into the cell.

(extra credit: Ask your instructor how/why the cells can keep a lower ion concentration in them.)

Hope this helps!

Cheers,
Tom

 

[DaveC426913]

Surely there is more than fluidic diffusion involved though. As docnet points out: the ions repel each other, driving them to distribute themselves as widely as possible. Surely that's the primary mechanism.

"Ionic diffusion refers to the diffusion of charged species that interact electrostatically, while molecular diffusion is usually used to describe the migration of neutral species."

 

[BillTre]

In case of ions, there is also the tendency for the ions to distribute as evenly as possible in solution, to minimize the electric potential energy of the system. I believe this would be the main phenomenon at play in this scenario.

All the charged species considered together will effect the overall charge across the membrane.
However, different kinds of ion channels will only permit certain species to cross the membrane. This is determines the kind of ions moving (if only one kind of channel opens).
The concentration driven movement of ions will overcome or or add to the influence of the membrane potential on the ion flow.
The open channels can be thought of as little batteries in the membrane. The potential of each kind of channel battery will be determined by the concentration differences of the ions that go through the channels and across the membrane. The balance of the channel battery potential vs. the membrane potential will influence whether and which way the ions go across the membrane.
The voltage differences can drive ions against their individual concentration gradients.
The state of the channel open vs. closed can be thought of a variable resister.
Alone the concentration gradients will determine ion movement.
The membrane acts as a capacitor.

Patch electrodes allow the electrical characteristics of individual channels to be measured.
Patch voltage clamps and maybe other methods allow a cell's capacitance to be determined.

 

[docnet]

To add, ion-specific 'pumps' A.K.A. cross-membrane proteins build concentrations gradients by moving ions against their concentration gradients. These pumps commonly use energy from other molecules such as ATP, xor by utilizing a pre-existing concentration gradient of a second ion species. - quoted from my undergraduate biology professor.

 

[Borek]

Technically there is no "force" behind diffusion, only statistics and entropy. Molecules (or ions) move in random directions, it happens that their most probable distribution (and one that has the largest entropy) is that with the uniform concentration throughout the system.

In other words diffusion is an emergent property - google for Einstein theory of a Brownian motion.

 

[Lord Jestocost]

Simply spoken, transport processes of mobile components occur in materials which are exposed to thermodynamic potential gradients (i.e., gradients of chemical potentials, electrical potential, temperature, or pressure).

 

[DaveC426913]

Technically there is no "force" behind diffusion,

Ionic diffusion is driven by EM repulsion.

 

[Borek]

Ionic diffusion is driven by EM repulsion.

Source please.

The sciencedirect page that you quoted says about diffusion of charged species that interact electrostatically, but it doesn't say anything repulsion being the driving force. Plus, it uses the same Fick's equation for diffusion to describe the process, I am more than sure if the process were run by the repulsion it will be much faster.

 

[DaveC426913]

OK, this is not my area of expertise, so I defer to more knowledgeable members.

 

[James Demers]

Almost every sodium ion will have an anion not too far away, so electrostatic repulsion won't be much of a force.

Diffusion can be analyzed in several ways, all of them inter-related. As chemist, I'm accustomed to the Second Law explanation: free energy tends to decrease, and entropy tends to increase (ΔG=ΔH-TΔS), and an even, random distribution has maximum entropy.

There's the statistical mathematical explanation: you can prove mathematically that there are more ways particles can be arranged in an even, random distribution than there are ways to arrange them in any other manner. For the hard-core math junkies, there's the Einstein Diffusion Equation, which starts with the individual particles and the forces acting on them, and (with some impressive mathematical jujitsu) arrives at the same result: an even, random distribution is the most favored outcome.