Principles of Transmembrane Transport

So in this video we're gonna be talking about the principles of trans membrane transport. So the first thing we're going to talk about is mainly just membrane gradients and what allow what property of the membrane allow them to develop gradients. So first sales must be able to communicate across their membrane barriers in order to exchange materials with their environment. So the internal part of the cell has to be able to expel or take in things that are found in the extra extra cellular environment, which means that membranes are semi permeable. So they only allow certain certain molecules to cross. So you can't have everything in the extra cellular environment entering into cells. You only want certain amounts of things. And so that makes the membrane semi permeable. So things like small, non polar molecules, oxygen or carbon dioxide, these can rapidly cross the membrane um uncharged polar molecules can cross if they're small, but the large ones are pretty much excluded and charged molecules and ions pretty much cannot pass the membrane really at all. So um the semi permeability allows for the cell to regulate what things are getting in and what things are not. So because there are differences between the internal environment and the exterior environment. Um This is this is a really important concept. Um So the intracellular concentration of things vary from the external concentration. So this results in four terms that I really want to make sure that we understand and it's really important for this chapter. The first our concentration gradients and that's when the concentrations of molecules differ on either side of the membrane. Then you have the electrical potentials. And that is when the charge there's a charge different. So the intracellular environment is more positive than the extra cellular environment. The electrochemical potential defines or describes the combination of the concentration gradient and the electrical potential. Um So that's going to just sort of sum up, you know, the concentration difference and the electrical difference into one um sort of driving force. And that's the electrical chemical or electrochemical potential. And then finally you have membrane potential which is actually the difference. So whereas the electrochemical potential was adding them together, the membrane potential is the difference between the concentration gradient and the electric potential and so generally the overall net charge show the overall difference. So the electrical difference can be different. Um and the concentration difference or concentration gradients can be different. But when you combine all these factors together, the overall net charge really has to be balanced or the cell is just gonna kind of explode. So what this looks like. So here we have a membrane and we have all these differences across the membrane. So you have an electrical difference where there's more positive charges on this side and more negative charges on this side. So it's electrical difference. We have a concentration difference where there's more sodium on this side and more potassium on this side, or potassium on this side. And so the combination of these refer to the electrochemical different potential and the difference between them refers to the membrane potential. So now let's move on.

So now let's talk about the two different types of transport across membranes and that's passive and active. So molecules cross the membrane barrier in these two ways passive transport and active transport. And I know that we've talked about these before in some intro bio courses but let's just review them again. So passive transport moves molecules through a gradient, usually from some type of high concentration to an area of low concentration. Now this can happen in a couple of ways. It can happen through simple diffusion which is means that the molecule needs no assistance and crossing and passing through the membrane or facilitated diffusion, which means that the molecule needs some type of assistance to cross the membrane. But generally this passive way of transporting our passive diffusion requires no energy input. It'll make it across whether it does it by itself or whether it's facilitated by something. And this is very different than active transport, which is a molecule moving against sense gradient from low concentration, too high concentration. And in order to do this, active transport requires energy. And usually this energy is taken from A. T. P. So if we're looking at simple versus facilitated diffusion, what we get is you have your passive diffusion here where the molecules are just passing through the membrane and they don't really need anything else. Um And you have facilitated diffusion where the molecules are getting through the membrane but they're doing it through the help of some type of protein. So that's the difference. But generally this entire process requires no energy because it's passive. Now there are three classes of trans membrane proteins that transport molecules across the membrane. The first are channels and they provide some type of portal for a molecule to pass. But they are specific. So generally they let molecules of a specific size or a specific charge to pass the membrane. So anything that has a certain negative charge can get through this channel, whereas things that don't have that negative charge camp, then you have transporters and these are highly selective in allowing molecules to pass. So usually transporters only allow for a specific molecule that can bind to specific binding site. So it has to only be this molecule um and not others. And then you have a teepee powered pumps and these pumps crossing allow things to cross the membrane through using energy from A T. P. So if we're looking at our three choices here, what we see is we have our channel protein which allows similar molecules and things with a similar charge or shape to get through. We have our transporter which only allows a specific molecule. So in this case it's going to be glucose and sodium. So it only allows those two molecules to get through. And then we have our A T. P. Power pump which uses a teepee in order to transport different things across the membrane. So with those three things let's now move on