who knows

[michel_mr]

hello my freinds ,this is my first visit and i am really need your help. in neurophysiology we know that we have Na ions and K inos gates,but why the Na ions cannot pass through the K gates although the are smaller in size with respect to these gate?

[Phi for All]

Is size the only factor in determining what passes through an ion gate?

[iNow]

hello my freinds ,this is my first visit and i am really need your help. in neurophysiology we know that we have Na ions and K inos gates,but why the Na ions cannot pass through the K gates although the are smaller in size with respect to these gate?

 

You might check here:

 

http://people.eku.edu/ritchisong/301notes2.htm

 

 

 

 

Basically, how sure are you about this part?

 

in neurophysiology we know that we have Na ions and K inos gates,but why the Na ions cannot pass through the K gates

[Paralith]

I agree with INow. As far as I know, potassium and sodium use the same gate molecule in neurons.

[CharonY]

Psst michael_mr, they are antiporters

 

Also, if that was what confused you, the gradient is built at the expense of energy (ATP).

[thedarkshade]

I think what we have to deal with here is called "active transport" (at least we call it that way!)

 

When our muscles are in rest, the level of Na is 10 times bigger outside than inside, and the level of K is 10 times bigger inside than outside. So according to physical laws, Na must move in inside until the balance is reached, and also K must move outside until the balanced is reached too. But here is a specific case that act some stuff called "Na and K pomps". Na pomp prevents Na to enter inside, and K pomp prevents K to go outside. So it acts from the place with less concentration to a place with higher concentration, and of course this does not occur spontaneously so energy is needed. And this energy is ATP (adenozim tria-phosphate), and it consumes quiet a lot energy. 20% of our entire body energy is consumed only by this "active transport"

 

It is also called active process because despite diffusion and osmosis (which need no energy), this needs energy to happen!

[iNow]

I think what we have to deal with here is called "active transport" (at least we call it that way!)

Yes. That is correct. It requires energy, which comes from ATP, to perform this transport process since it is not occurring along natural gradients. In essence, the charge is being pushed "uphill" and it requires energy to do this.

 

 

When our muscles are in rest, the level of Na is 10 times bigger outside than inside, and the level of K is 10 times bigger inside than outside.

This is close, but remember we are speaking of nerve cells and not muscles. In nerve cells, the actual numbers are about ten times more sodium (Na+) on the outside and twenty times more potassium (K+) on the inside of the cell, and this is called the concentration gradient. This results in a resting potential of the cell of about -70Mv (millivolts, and it's negative because the inside of the cell is negatively charged relative to the space outside of and around it).

 

 

So according to physical laws, Na must move in inside until the balance is reached, and also K must move outside until the balanced is reached too. But here is a specific case that act some stuff called "Na and K pomps". Na pomp prevents Na to enter inside, and K pomp prevents K to go outside. So it acts from the place with less concentration to a place with higher concentration, and of course this does not occur spontaneously so energy is needed.

Again, very close. Let me clarify a little below.

 

To keep this resting potential, the Na/K pump moves three sodium (Na+) ions out of the cell for every two potassium (K+) ions that it brings in, and because of the outward leakage of potassium and the presence of negatively charged ions within, the cell maintains a separation of charges; the inside of the neuron is made negative relative to the outside, and is thus "polarized" (at -70Mv).

 

More is available on this here:

 

http://www.miracosta.edu/home/sfoster/neurons/rest.htm

 

 

...and also here:

 

http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/sppump.html

 

 

And this energy is ATP (adenozim tria-phosphate), and it consumes quiet a lot energy. 20% of our entire body energy is consumed only by this "active transport"

Except for your spelling, your are right. It is adenosine triphosphate, which is a nucleotide derived from adenosine that occurs in muscle tissue, and is also the major source of energy for cellular reactions. However, the percentage of the cells energy (where it uses it's ATP) is closer to 50-70% to power the sodium/potassium pump. Again, nearly 70% of all of the nerve cells energy, which it derives from ATP, goes to powering the Na/K pump. Notice also that I said the "cell's" energy, not the "entire body's" energy. This is an important distinction.

 

 

It is also called active process because despite diffusion and osmosis (which need no energy), this needs energy to happen!

Absolutely. Very well said.

[thedarkshade]

Thnx iNow, that really helped!

[michel_mr]

thx my freind but really this is not my question let me ask the question again.1- i am not speaking about na /k pump,there is voltage dependent Na and K gates,and these gates open or close during action potential.

2- my question is why when K GATES open the Na ions wont cross it although they are smaller in size

[Paralith]

oh, you mean this:

 

http://www.blackwellpublishing.com/matthews/channel.html

 

(a pretty nice animation)

 

transport proteins are specific. they are designed to only allow certain kinds of molecules pass through them. because of this, it doesn't matter which ions are smaller. the transport proteins will only recognize the specific molecule they are meant to transport.

[iNow]

my question is why when K GATES open the Na ions wont cross it although they are smaller in size

They pass through the same gate. When K goes through, Na cannot. When Na goes through, K cannot. The basic answer is that when the ion binds to the gate, it "occludes" further binding until released on the other side.

 

It's generally referred to as the "alternating-access" model, and suggests that ion pumps allow the entrance and exit of ions at only one side of the membrane at a time, much like a revolving door.

 

 

Below I've shared a study that is probably more information than you want, but it addresses the question you have asked. It is a full article, and is free. It does not require subscription or payment. Good luck.

 

 

http://www.pnas.org/cgi/content/full/100/2/501

Typically, in an ion-motive pump, ions at one membrane surface bind to sites within the protein core and become temporarily occluded there before being deoccluded and released at the opposite surface (2, 3). Thus, an ion pump behaves, at least qualitatively, like an ion channel with two gates, one on each side of the ion-binding cavity, that are constrained to open and close alternately (4-7). The fact that the ion flux through an open ion channel can be as much as 106-fold larger than that mediated by an active ion pump dictates that the probability of the pump's hypothetical two gates being open simultaneously must be negligibly small, i.e., <106, lest the pump fail to effect useful ion transport. Presumably, just as evolution optimized ion-channel structure to maximize conductance while retaining selectivity (8), it optimized the coupling between the gates of ion pumps to preclude open-channel conformations while facilitating appropriate ligand-mediated gating reactions.

 

 

Fig. 1. Alternating-gate model of the Post-Albers (9, 10) transport cycle of the Na+/K+ pump represented in cartoon form as a channel with two gates never simultaneously open. Extra- (OUT) and intracellular (IN) surfaces of the membrane (yellow) are indicated, as are E2 states (Upper; external gate may open) and E1 states (Lower; internal gate may open). Occluded states with both gates shut (Upper Right, Lower Left) follow binding of 2 external K+, and of 3 internal Na+ and subsequent phosphorylation, respectively. ATP acts with low affinity to speed opening of the internal gate and concomitant K+ deocclusion, and acts with high affinity to phosphorylate the pump.

 

 

The Na+/K+ pump is a P-type ATPase crucial to the life of practically all animal cells. Its transport cycle, in which three Na+ ions are extruded from the cell and two K+ ions are recovered, at the expense of one molecule of ATP, is cartooned in Fig. 1 as a sequence of steps that alternates access to ion binding sites within an ion channel (4-7). The pump adopts two principal conformations, E1 with the binding sites accessible from the cytoplasm (Fig. 1 Lower), and E2 with access from the extracellular space (Fig. 1 Upper). Binding of the third cytoplasmic Na+ to E1 triggers phosphorylation (denoted "P", Fig. 1 Lower Left) of the pump from ATP, acting with high apparent affinity (1 µM), and concomitant occlusion of the 3 Na+, so trapping them within the pump (11). Spontaneous relaxation to the E2 conformation opens the external gate, allowing release of the Na+ to the exterior followed by binding of 2 extracellular K+ (Fig. 1 Upper Left); the latter promotes occlusion of the K+ ions (2, 3) and dephosphorylation of the pump (Fig. 1 Upper Right). Subsequent binding of ATP, with relatively low apparent affinity (100 µM), speeds the conformational change of the dephosphorylated pump, E2E1, which opens the internal gate and releases the K+ into the cytoplasm (Fig. 1 Lower Right). In contrast to the high-affinity, phosphorylating action of ATP, its low-affinity binding effect is mimicked by ADP (12, 13), and by poorly hydrolyzable ATP analogs (14).