Erythrocyte Facilitated Transporter Models: Study with Video Lessons, Practice Problems & Examples

GLUT1 is a key erythrocyte glucose uniporter that facilitates passive transport by undergoing conformational changes to move glucose down its concentration gradient. This process is crucial as it maintains low intracellular glucose levels for energy metabolism. Additionally, the chloride-bicarbonate antiporter plays a vital role in the chloride shift, exchanging chloride and bicarbonate ions to enhance carbon dioxide transport from tissues to lungs. Understanding these transport mechanisms is essential for grasping cellular metabolism and gas exchange processes in the body.

Erythrocyte Facilitated Transporter Models

Alright. So here we have our membrane transport map of our lesson. And so far we've explored our left branch with molecular transport and we've talked a lot about passive transport differentiating simple versus facilitated, and in our last lesson video we talked about the differences between carrier/transporters and pore/channels. And so in this video, we're going to talk about where we're headed next, which is talking about very specific types of carriers/transporters. And we're going to talk about the erythrocyte glucose uniporter GLUT1 in our very next video. And then after that, we'll talk about the erythrocyte chloride bicarbonate antiporter. Then after that, we'll zoom out and talk about some specific types of pore/channels. And so this here concludes this video, and I'll see you guys in our next video where we'll talk about the erythrocyte glucose uniporter GLUT1. See you guys there.

Glucose transport into erythrocytes (not into intestinal epithelial cells) is an example of:

• A. Primary active transport.
• B. Secondary active transport.
• C. Facilitated symport.
• D. Facilitated Uniport.
• E. Simple Diffusion.
• F. None of the above.

Which of the following correctly ranks the steps of erythrocyte glucose transport by GLUT1?

I. A conformational change exposes glucose to the opposite site of the membrane.
II. Glucose binds to the transporter on one side of the membrane.
III. The GLUT1 transporter reverts back to its initial conformation.
IV. The glucose molecule has a weakened affinity to GLUT1 and dissociates from the transporter.

• A. II, I, III, IV.
• B. II, I, IV, III.
• C. IV, III, II, I
• D. I, II, IV, III.
• E. IV, II, III, I

Erythrocyte Facilitated Transporter Models

So a classic biological example of facilitated passive transport are erythrocyte or red blood cell glucose transporters called GLUT 1. And so GLUT 1, because it is a transporter, we already know that it must undergo conformational changes in order to transport a molecule across the membrane, and that is exactly what GLUT1 does. So GLUT1 conformationally changes as it transports glucose down its concentration gradient across the membrane, and GLUT 1 is able to do this as a uniporter, which you might recall just means that it transports one molecule at a time in a specific direction. Now due to glucose metabolism inside of cells, glucose is constantly being broken down and used to create energy. And so glucose concentration inside of cells generally is going to be kept relatively low with respect to the blood glucose concentration.

And so if we take a look at our image down below right here, notice we're showing you the GLUT1 uniporter. And of course, we're zooming into the plasma membrane of the erythrocyte or the red blood cell. And in the plasma membrane of the erythrocyte or red blood cell, that's where we can find this GLUT1 uniporter. And so notice that on the inside of the cell, which we have marked with the yellow background, there is a lower concentration of glucose. Again, due to glucose metabolism inside of cells that glucose concentration is kept low as we indicate here. And of course, on the outside of the cell up here, there is a higher concentration of glucose in the blood. And so because, again, GLUT 1 is a transporter, we know that it's going to bind to one of the glucose molecules and then undergo a conformational change here to allow the glucose molecule to be released to the inside of the cell. And of course once it releases that glucose molecule to the inside of the cell, it can revert back to its original position where it can again continue this process and take in another glucose molecule.

Now what's also important to note is that there are actually several different types of glucose transporters that exist, and they exist in different tissues with varied functional roles. And so as we move forward in our course, we will talk about other types of glucose transporters as well. Here in this video, we talked about the GLUT1 transporter. And so in this table you can see the type of glucose transporter in this column, the tissue that the glucose transporter is expressed in this column, and the biological role that the transporters have in those tissues. And so the GLUT 1 transporter, notice that it is actually ubiquitously expressed, and so it is going to be expressed pretty much everywhere in all cell types including erythrocytes, our red blood cells, and its biological role is for basal glucose uptake, essentially just bringing in glucose into the cell.

Now moving forward in our course later, in our course, we'll also talk about other glucose transporters like GLUT 2 and GLUT 4. And so here, we're just introducing that there are other glucose transporters, and there are, of course, more than just GLUT 2 and GLUT 4, but moving forward, GLUT 2 and GLUT 4 are the ones that we're specifically going to talk about later in our course. And so, don't worry too much about the tissue expression and the biological roles, this is just here for context, and, we will revisit GLUT 2 and GLUT 4 again later in our course. For now, what I want you guys to see is that GLUT 1 is a classic example of facilitated passive transport, and it is found in erythrocytes, as a glucose uniporter. And so this here concludes our lesson on the erythrocyte glucose uniporter GLUT1, and we'll be able to get a little bit of practice in our next video. So I'll see you guys there.

Erythrocyte Facilitated Transporter Models

So another classic example of facilitated passive transport are erythrocyte or red blood cell, chloride bicarbonate antiporters. Now, before we talk more about these chloride bicarbonate antiporters, I first want to point out that this image that we have down here should look familiar to you guys from our previous lesson videos. Specifically, where we talked about hemoglobins binding activity in the tissues versus in the lungs. And so, if this image does not look familiar to you guys, be sure to go back and check out those older lesson videos before you continue here. Now that being said, there are a few things that we're going to review in this video from those older lesson videos. And so, the first thing that I want you guys to recall from those older lesson videos is that CO₂ or carbon dioxide that's produced by our respiring tissues is going to diffuse into our erythrocytes, and on the inside of our erythrocytes is where we can find an enzyme called carbonic anhydrase that will convert the CO₂ and water into the bicarbonate anion that we see here HCO₃^- which plays a really big role in the chloride bicarbonate antiporter that we're going to introduce here shortly.

Let's take a look at our image down below to remind ourselves of a few things. First, I'm going to remind you that the entire left-hand side of the image over here is pretty much dedicated to our blood near the tissues. And so here in the middle on the left-hand side, we can label this as being near the tissues. And of course, the entire right-hand side of the image all the way over here is dedicated to our blood near the lungs, and so over here on the right-hand side, we can label this as near the lungs. You can see that this dotted black line that we see here is really separating what we want to focus on and separate the blood near the tissues versus the blood near the lungs.

As we mentioned above, our respiring tissues are producing lots and lots of CO₂, and so there's a high concentration of CO₂ in our tissues. And of course, this CO₂ is going to diffuse out of the tissues and into the blood and make its way into our erythrocyte, which is our red blood cell right here. And once the CO₂ is inside our erythrocytes, that's where the enzyme carbonic anhydrase can convert the CO₂ and water into carbonic acid, which will dissociate and break apart into the bicarbonate anion and a hydrogen ion. And, of course, the bicarbonate anion here is going to play a really big role in the chloride bicarbonate antiporter, which is this blue structure that we see right here that we'll talk about here very shortly.

Chloride Bicarbonate Antiporters are going to passively transport, as their name implies, chloride and bicarbonate in opposite directions. We know that's exactly what antiporters do. They take two molecules and transport them across the membrane in opposite directions. As the chloride is being transported in one direction and the bicarbonate in the opposite direction across the membrane, this shift of chloride and bicarbonate is commonly referred to as just the chloride shift, even though bicarbonate still plays a big role in this process.

The chloride shift refers to the phenomenon of chloride and bicarbonate exchange near the tissues and near the lungs. The way this chloride bicarbonate exchange works is going to be different near the tissues and near the lungs. But we'll talk more about exactly how the chloride shift works in more detail once we get to this section down below.

The Chloride anion acts as a counterion to help balance the charge across the membrane when the bicarbonate is pumped across the membrane. It's the bicarbonate that has the most important functions of the chloride shift. The bicarbonate acts as a buffer to maintain blood pH, which is important for maintaining the structure of enzymes in our blood. Also, the chloride shift and the bicarbonate shifting have another more important role, which is that it increases the blood's capacity to transport carbon dioxide from the tissues to the lungs. This increase in the blood's capacity to carry carbon dioxide is the main function that the chloride shift provides.

Let's focus more on how exactly this chloride shift works so we can help clarify some of the ideas that we've talked about. Near the tissues, as mentioned, bicarbonate anion is going to be produced, and so there's a high concentration of bicarbonate anion inside of the cell near the tissues. The high concentration of bicarbonate is going to diffuse down its concentration gradient and make its way to the outside of the cell. You can see that the bicarbonate anion here again, which is in high concentration on the inside of the cell near the tissues, is going to diffuse to the outside of the cell here, near the tissues. It's going to do that via the chloride bicarbonate antiporter, which is this blue structure that we see right here. And as the bicarbonate gets pumped out of the cell, the chloride anion is being pumped into the cell in a one-to-one ratio.

Near the lungs, this all works a little bit differently. Near the lungs, there's actually a low concentration of CO₂ which is the opposite of what we have near the tissues. What happens is this reaction that's catalyzed by carbonic anhydrase is going in the opposite direction as it was here. You can see that the bicarbonate is actually being converted to CO₂, and the CO₂ is making its way to the lungs where it can be exhaled. What needs to happen near the lungs is that the bicarbonate anion is actually going to be transported into the erythrocyte where it can be converted into the CO₂ and then ultimately exhaled. As the bicarbonate gets shifted into the cell near the lungs, the chloride anion that was originally shifted into the cell is now going to be shifted back out of the cell, reversing essentially what happened near the tissues. This can happen in a cycle. Essentially what happens near the lungs is pretty much the complete opposite of what happens in the tissues, and the opposite events occur.

This concludes our introduction to the erythrocyte Chloride Bicarbonate Antiporter and the chloride shift. We'll be able to get some practice applying

The Chloride-Shift occurs when:

• A. Carbon dioxide moves into the RBCs.
• B. Hemoglobin binds carbon dioxide.
• C. Oxygen moves into the red blood cells.
• D. Bicarbonate moves out of the red blood cells.
• E. Chloride shifts across the nuclear membrane.

Which of the following statements is FALSE concerning the chloride-bicarbonate exchanger?

• A. The exchanger increases the rate of bicarbonate transport across the membrane.
• B. The exchanger uses ATP as an energy source to drive bicarbonate transport.
• C. The exchanger transports chloride ions across the membrane.
• D. The exchanger is classified as an antiporter.

In the "chloride shift' diagrams below, label each scenario (A & B) as occurring in either the tissues or lungs:

Alright. So this practice problem says, in the chloride shift diagrams down below, label each scenario a and b, where a is over here on the left-hand side and b is over here on the right-hand side, as occurring in either the tissues or the lungs. And so there are actually several different ways that we can actually answer this problem, but one of the easiest ways is actually just to focus on oxygen, and the direction that it's being transported. And so what you'll notice is that on the inside of both of these cells over here, you can see the enzyme carbonic anhydrase, and so we know that these cells right here represent erythrocytes, our red blood cells. And so, of course, here we can see oxygen is being transported into our red blood cells to bind to the hemoglobin. And of course, we know that in our lungs, there are a lot and lot of oxygen, there's a lot of oxygen and that oxygen is going to diffuse into our red blood cells. And so just by looking at oxygen diffusing into our red blood cells over here, we could identify that this is going to be the scenario that's in the lungs. And just by looking at the oxygen leaving our red blood cells over here, we know that that is consistent with what happens in the tissues, and so we know that hemoglobin will deliver oxygen to the tissues, and so oxygen diffusing out is consistent with what we see in the tissues, and really, these are the correct answers to each of these scenarios. Now other ways that we could have answered this and determine this is by looking at the CO2. And, of course, we know that in our lungs, we are going to breathe out CO2. And so CO2 is going to get transported out of the red blood cell into our lungs so that we can exhale that CO2. And so the CO2 leaving the red blood cell here in this fashion is consistent with the lungs as well. And over here, when we take a look at the CO2, noti

Here’s what students ask on this topic:

What is the role of GLUT1 in erythrocytes?

GLUT1 is a glucose uniporter found in erythrocytes (red blood cells) that facilitates passive transport of glucose across the cell membrane. It operates by undergoing conformational changes to move glucose down its concentration gradient, from areas of high concentration in the blood to lower concentrations inside the cell. This process is crucial for maintaining low intracellular glucose levels, which is essential for energy metabolism within the cell. GLUT1 ensures a continuous supply of glucose for cellular activities, making it vital for the proper functioning of erythrocytes.

How does the chloride-bicarbonate antiporter function in erythrocytes?

The chloride-bicarbonate antiporter in erythrocytes facilitates the exchange of chloride (Cl^-) and bicarbonate (HCO₃^-) ions across the cell membrane. This process, known as the chloride shift, is essential for transporting carbon dioxide (CO₂) from tissues to the lungs. In tissues, CO₂ diffuses into erythrocytes and is converted to bicarbonate by the enzyme carbonic anhydrase. The antiporter then exchanges intracellular bicarbonate for extracellular chloride. Near the lungs, the process reverses: bicarbonate enters the erythrocytes, is converted back to CO₂, and is exhaled. This mechanism helps maintain blood pH and enhances CO₂ transport.

What is the significance of the chloride shift in gas exchange?

The chloride shift is crucial for efficient gas exchange and maintaining acid-base balance in the blood. It involves the exchange of chloride (Cl^-) and bicarbonate (HCO₃^-) ions across erythrocyte membranes. In tissues, CO₂ produced by cellular respiration diffuses into erythrocytes and is converted to bicarbonate, which is then exchanged for chloride. This process helps transport CO₂ in the form of bicarbonate to the lungs. Near the lungs, bicarbonate re-enters erythrocytes, is converted back to CO₂, and is exhaled. This exchange mechanism increases the blood's capacity to carry CO₂ and helps maintain blood pH.

What are the different types of glucose transporters and their roles?

Glucose transporters (GLUTs) are proteins that facilitate the transport of glucose across cell membranes. There are several types, each with specific roles and tissue distributions. GLUT1 is ubiquitously expressed and responsible for basal glucose uptake in most cells, including erythrocytes. GLUT2 is found in liver, pancreas, and kidney cells, playing a role in glucose sensing and homeostasis. GLUT4 is insulin-responsive and primarily found in muscle and adipose tissues, where it regulates glucose uptake in response to insulin. Each GLUT type ensures that glucose is efficiently transported to meet the metabolic needs of different tissues.

How does GLUT1 undergo conformational changes to transport glucose?

GLUT1 transports glucose by undergoing conformational changes that allow it to move glucose across the cell membrane. Initially, GLUT1 binds to a glucose molecule on the extracellular side of the membrane. This binding induces a conformational change, shifting GLUT1 to an inward-facing state, which releases the glucose into the cytoplasm. After releasing the glucose, GLUT1 reverts to its original outward-facing conformation, ready to bind another glucose molecule. This cycle of conformational changes ensures continuous glucose transport down its concentration gradient, facilitating passive transport without the need for energy input.