Nutrients like ions can easily pass through the cell wall, but the plasma membrane acts like a filter. Remember that term we talked about, selective permeability? The plasma membrane gets to decide what gets in and what doesn't. Plants use proton pumps to create electrochemical gradients, and these gradients allow ions to enter through transporters. These electrical, electrochemical gradients are actually strong enough in some cases to overpower counteracting forces. So through channels. Remember, that's a type of facilitated diffusion. Basically, you just need the channel there, and those ions will move through of their own accord. However, anions, like NO3 here, have to use cotransporters. And those will often use a proton gradient, and they'll bring a proton into the cell as they bring in the desired anion. And remember that this is a form of secondary active transport. Now, ion exclusion is the idea that plants are able to filter harmful ions and poisonous metals, and prevent them from getting into the cells.
They can do this in two ways, what's called passive exclusion, and active exclusion. Passive exclusion basically doesn't require any sort of extra energy input. Basically, if the membrane lacks the necessary transporter to allow the ion to pass, it's not getting in. And you might also recall that the Casparian strip is going to force ions into those endodermal cells, because it's going to prevent them from moving all the way through the apoplast to the xylem. So it's going to force ions to cross a membrane, which gets to act as a filter. So basically here, these transporters, they're kind of like bouncers. Right? They're bouncers, and they get to decide who gets into club cell and who doesn't.
Now, active exclusion comes in the form of antiporters. In fact, usually we see these antiporters at the tonoplast, which is the membrane of the vacuole. In our diagram here, you can see we have a vacuole, this large purple structure inside the plant cell. And the tonoplast is going to be that membrane of the vacuole there. Now a great example of active exclusion is the sodium proton antiporters, or sodium hydrogen ion antiporters, whatever you want to call them. And these are going to help prevent sodium from poisoning plant cells. Plant cells are actually very sensitive to sodium, they have to carefully monitor their sodium concentrations. And if the sodium gets too high, they'll actually pull it into the vacuole to get it out of the way, to prevent it from poisoning the plant. The way they do this is they actually use proton pumps to create a proton gradient, so that the concentration of protons inside the vacuole is higher than the concentration of protons just in the cell, or in the cytoplasm. This gradient is going to be taken advantage of by these antiporters. These antiporters will move a sodium in, as they get rid of one of those protons. So they're going to take advantage of the proton gradient established by the pumps in order to get sodium into the vacuole. So this is a type of secondary active transport, and it involves antiporters at the tonoplast.
Plants also can help prevent poisoning through what are known as metallothioneins. These are cysteine-rich proteins that will actually bind to metals and prevent them from poisoning the organism. And these are not unique to plants either. You'll see metallothioneins in bacteria and fungi as well. And that's actually all I have for this lesson, so I'll see you guys next time.
