How can glucose "know" how much glucose is on the other side of a membrane and then diffuse accordingly? In other words, how could it distinguish between fructose and galactose concentration gradients? Maybe it doesn't and this is just simplified for teaching purposes...
I read that glucose is converted to G6P by glycolysis in order to maintain the favorability of glucose moving into the cell because as G6P its identity is changed so glucose will still want to move IN (giving it a charge also traps it). This is apparently also why blood glucose has to stay hyperosmolar as well.
But I have never read anything that explains how one molecule can "sense" a molecule somewhere else. Electrical potential works based on net charge which is straightforward but what's the deal with concentration gradients?
Thanks in advance
That is an excellent question!
The glucose molecule does not know! lets look at a simplified example.
You have a box that is divided in the middle by a wall, this wall has a
hole in it.
Now on left side of the box you have around 20 equal sized small balls that are moving and bumping between each other and against the wall (if you ever seen how they do lottery winning numbers, that's what I mean). On the right of the box there is not a single ball.
Lets assume those balls actually can fit in the hole, what do you expect to happen if you leave it for a long time?
you would see that some of the balls from the right side go to the left side, and the balls keep going back and forth and exchanged through that hole. After a long time, you will see both sides of the box have about the same number of balls, but they keep exchanging.
This is due simply to 1) probability that the ball will hit the wall where the hole is and actually pass through. 2) the concentration of the balls 3) the nature of how the move (how fast they move). So the balls DON'T know that on the other side of the box there are less balls, they just move around. Since number 1 and number 3 are equal for the balls whether they are on the left side or the right side of the box. ONLY the concentration will matter.
In glucose the same thing happen again. The level of complexity increases a bit, because the "holes"(channels) are sometimes different for each molecule (and they not only depend on size but charge as well). Glucose is TAMED down by a phosphate, because the negative charge on it due to the phosphate no longer makes it a molecule that will probably not hit the hole (due to the new charge). WHY IS THAT EXACTLY? we will need to take a look at the channel structure to know that, and honestly I am not curious enough to look that up.
This also applies to molecule that diffuse from the member without the need of channels, the difference now is going to be that they probably have a big probability to pass the wall once they hit it.
It is also very similar to the concept of gas law , PV= nRT, where R and T give you a rough idea on how the molecules are moving (how fast and such). n/V shows the concentration, and P give you an idea on how they interact with the wall. Now of course you need some tweaking to this formula to make it fit into a system in a water medium, and the fact that you need a factor that is dependent on the kind of "hole" you have.
Hmm this actually might not be very good explanation. Let me know if it makes sense or not.
