 As I mentioned, one of the beauties of membrane is how seemingly simple they are and yet they are so physical, and that is more true than ever when it comes to transport. In fact, it turns out there are at least four different ways of doing transport through a membrane, possibly more ways of classifying it. Some of them need membrane proteins, but not all. I'll show you an image here just to illustrate this. I already mentioned to you that in the simplest case, if we forget about the membrane proteins for a second, a molecule like water will typically not be able to get through the membrane because that single water would reduce all its hydrogen bonds, even if you try to form a water wire or something through a membrane, it's going to be rare. While other molecules, such as a gas, oxygen will be able to get through a membrane. That's the first simple type of transport, and it's so-called passive transport. Passive transport happens purely to the loss of diffusion of physics. That is, we have a molecule out here, and under some conditions it will just end up on the other side. If we forget about the proteins here for a second, the obvious case here is oxygen, and that means that oxygen will on average go from a high concentration on the outside to a low concentration on the inside. It can only go, again, on average it will only go in the way of even smearing out the concentration, because otherwise it would be too costly in terms of entropy, right? You will never go spontaneously from low concentration to high concentration. It's also a non-mediated transport. It happens spontaneously. You have no health, you have no protein, literally. The oxygen molecule will go through the membrane on its own. Passive transport will not require energy for this reason. In fact, it will not even require a protein when it's oxygen. So that was simple, but it's also a little bit boring. The other simple case you can imagine, you can imagine something like an ion. An ion might, under some conditions, be able to go through a membrane. I will look at that in a few cases, but the ion itself, well, will it go through a membrane or not? You have the tools to determine that. You should draw the free energy on one side and the free energy on the other side, right? If the ion concentration is high on the upper side of the membrane but is low on the other side of the membrane, will that be an advantages process or not? It will be advantages and will always be advantages, whether you have an ion channel or not. The only problem is what happens in between. There might be a gigantic free energy barrier, and in fact, putting something on the charge you will see in a minute, putting something charged on the inside is so expensive that it eats my left shoe territory. So while it would technically be favorable comparing the initial to the end state, the barrier between them is so high that it would never happen spontaneously without help. That help exists in the form of a channel, and we're going to be looking at that channel in a second so that there is some sort of environment that makes it easier for the ion to go through the membrane. But it's still just passive diffusion. It's a whole, think of it as a window and a door, and if the door is open occasionally, something might sneak through, but it can only even out concentrations. In some cases, we might have something that is mediated, and the key thing is between mediated versus non-mediated. Leave some space here. So these are non-mediated, but you can also think of mediated transport. So mediated transport is somebody who needs help, and this help could be like, you might have a molecule that sometimes looks like this and sometimes looks like this. So something will actually have to change. Somebody might have to think of this like a rotating door or something. That would, for instance, be transporters, and again, we will come back and talk to them later, but mediated transport per se can still be passive. It does not necessarily require energy. It's just I might have some really complicated process here that doesn't just open the door, but makes sure just one person at the time can go through the door, and the door itself might be a revolving door. That will also require a protein, and we will look at some examples like that shortly. But it's still passive transport that it will only go from high concentration to low concentration on average. One molecule now and then might go the other direction, but it does not need energy. There is one more type of transport though. We can have active transport. In fact, we need active transport. If all we had was passive transport, life would cease because these processes would only happen until we've reached equilibrium, and at equilibrium we're dead. Active transport uses energy, primarily in the form of ATP, to drive something against a concentration gradient. And in this case, you see that something go from low D to uppercase D. So you can take something from low concentration and move it to high concentration. That is thermodynamically unfavorable. Now we're going uphill from low to high free energy. The only reason we can do that is because we're paying with energy in the form of chemical energy stored in ATP. So those are the four different types of transports you should know. You have passive transport or active transport, where active requires energy. You have non-mediated transport that is just something statically not moving, either directly through the membrane or a channel. But we can also have mediated transport where protein actually has to go through and change this conformation. You need a bit of help. But just because it's mediated doesn't necessarily mean that it requires energy in form of ATP.