To this point we've talked about movement of molecules across the membrane, and we've talked about passive movement. Passive movement means that no external energy is required. Diffusion is passive movement. So anytime we see the word diffusion in any of these descriptors, that automatically means that this is a passive process. Simple diffusion was dealing with molecules that could go straight through the membrane.
And we showed different characteristics or different factors that could influence the rate of simple diffusion. These included concentration gradient of the ligand, the partition coefficient of the ligand, the diffusion coefficient of the ligand, the thickness of the membrane, as well as the surface area over which the ligand was moving. Now, this type of diffusion, simple diffusion, again, we primarily think of lipophilic molecules moving straight through the membrane.
The other type of diffusion is facilitated diffusion, and we had a couple of different types of facilitated diffusion. The first was what we called facilitated diffusion using a channel protein. So this primarily dealt with small molecules such as ions that can pass directly through this transmembrane protein.
For larger molecules, a channel protein is not adequate, so this is when we introduced the concept of a carrier molecule. And with the carrier molecule, that deals with having a binding site, and so from that, we discussed several different characteristics that can affect a carrier molecule or that come into play. That included specificity, affinity, and saturation.
So active transport now is going to be our new topic, and active transport is going to be moving molecules against their concentration gradient. So passive transport always moves things with their concentration gradient or downhill. Active transport moves molecules against their concentration or uphill. Therefore, they require energy in order to power this.
The first type of active transport is primary active transport. And primary active transport uses for its energy, it directly utilizes the hydrolysis of ATP. So if we take a standard cell here, we can draw in our primary active transporter.
And as we mentioned, this is going to be something that's going to be moving molecules against their concentration gradient. So one of the things that we will emphasize in this class is the relative concentrations of sodium and potassium with regards to energy. intracellular concentration and extracellular. So with sodium, sodium is much more concentrated outside than inside the cell. So this particular transporter, this particular active transporter, is going to be moving sodium out of the cell.
Now at the same time, this particular active transporter also actively transports potassium into the cell. And you'll notice that the relative concentration of potassium inside the cell is much greater than outside the cell. Now, Now, what causes this transporter to be able to be powered to move both of these molecules against their gradients? That would be the hydrolysis of ATP to ADP.
So with this hydrolysis, energy is released. That energy is harnessed by this transporter and is able to move these molecules against their gradients. Now a common term that we use to describe a primary active transporter would be to utilize the term called a pump. So as we've demonstrated here, this is our sodium-potassium pump. Another pump that we'll talk about will be the calcium pump.
So this is primary active transport. The other type of active transport is secondary active transport. So secondary active transport uses the sodium gradient that's created by the sodium-potassium pump.
So remember we said that sodium is much more concentrated. outside the cell than inside the cell. The reason that's so is because the sodium-potassium pump helps to maintain that gradient.
So now that we have sodium in greater concentration outside the cell, then it has a natural tendency to... to want to move down its chemical gradient, and as we'll learn in the next lecture or two, its electrochemical gradient, it wants to move downhill into the cell. So what's going to happen is, is we have here again our transporter, but in this case, and we're going to demonstrate this by the green arrow, sodium is going to be moving passively down its electrochemical gradient into the cell. So as we're moving down this electrochemical gradient, gradient, we now are releasing potential energy that can be harnessed. That energy is harnessed by this transporter.
So now this transporter is able to move other molecules against their gradient. So in this case we're showing this molecule X moving into the cell. It's moving with sodium, And in this particular case, we would say that X is being co-transported along with sodium into the cell. There's going to be other substances that we'll learn about that will actually be moving in the opposite direction.
So in this case, we're simply denoting that by the letter Y. So in this case, this... ligand Y is being moved out of the cell.
And notice the red arrow. It's moving against its concentration gradient out of the cell. And it's able to do so because it's being powered by this secondary active transporter. So notice that sodium is once again moving into the cell because that's what's powering this transporter.
Y is moving out of the cell. So in this case, we would say that Y is being counter-transported. So notice that both X and Y in these examples are the substances being actively transported. Sodium is moving passively into the cell.