>>Dr. Ketchum: Okay, then we have bicarbonate
reabsorption. Okay, so same thing is going to hold true here. We’re still in the proximal
convoluted tubule, so that means you still have a sodium-potassium pump on the basolateral
membrane. So we know that we have low sodium in the cell and we have high potassium in
the cytoplasm of the cell. Keep in mind that the plasma is over here, correct? And we want
to reabsorb bicarbonate. We want to get bicarbonate into the plasma, but we also want to secrete
hydrogen ions. Let’s continue with sodium, since we already know sodium’s at a low concentration
inside of the cell. If we look at the concentration of sodium in the lumen of the proximal convoluted
tubule, we know we have a high concentration of sodium moving toward its low concentration.
And then we know that hydrogen ions, right, based on the previous slide, they’re at
a low concentration and they’re moving toward a high concentration. So number two here,
then, is going to be sodium hydrogen counter-transport across the apical membrane. We also know—okay, let’s continue with hydrogen ions.
We just said that we have a low hydrogen ion concentration inside of the cell. So if hydrogen’s going
from low toward a higher concentration in the lumen and this is using some sort of carrier
protein, we know that can’t be facilitated diffusion, correct, because that gradient
is going low to high. So the only type of transport that number three here could be
then is a type of primary active transport. If you go clear back to your chapter four
notes, there is a table or a slide that gives you examples of different types of primary
active transport, and this is listed as one of those. We’ve now secreted hydrogen ions. So once those have been secreted, okay, then
they can combine with bicarbonate. So we filtered bicarbonate. That will allow us to
form carbonic acid, H2CO3. And then here we have a brush border enzyme, okay, carbonic
anhydrase, that’s going to break down carbonic acid into water and carbon dioxide. So carbon
dioxide then can use simple diffusion to get into the cytoplasm of the cell. Then it can
combine with your water. Okay, we know water can move into the cell as well, correct? Then
that’s going to form, once again, using carbonic anhydrase we’re going to form carbonic
acid, which readily dissociates into your hydrogen ions and your bicarbonate. So we’ve
already talked about what happens to these hydrogen ions; now let’s focus on these
bicarbonate ions. So we know that bicarbonate then
is moving from a high concentration toward a low concentration and we’ve already said
that sodium is low inside the cell and it’s moving from low to high. So this type of transport
here, that we’ll label number four across the basolateral membrane. So number four is
sodium bicarbonate co-transport. Then we also have bicarbonate being reabsorbed
by using a chloride transporter. So again, we have a high concentration bicarbonate.
Chloride ions, then, must be moving from a low concentration toward a high concentration
inside the cytoplasm of the cell. So that means step five here is called your chloride
bicarbonate counter-transport. There’s two more ways that you can get carbon
dioxide inside of that cell. One of those, if you remember, is via cellular respiration.
So CO2, remember, is a product of cellular respiration. So that gives us a second mechanism by which we can get carbon dioxide inside of our cells. The other way is to think about
where this is. This is the plasma; it’s a peritubular capillary. When you reabsorb,
you reabsorb into the venous end of the capillary. So if this is the venous part of the capillary,
it’s going to have a relatively high partial pressure of carbon dioxide. Inside the cell
we’re going to have a low partial pressure of carbon dioxide. So that means that carbon
dioxide can follow its pressure gradient and move into the cell via simple diffusion.