Glucose Active Symporter Model: Videos & Practice Problems

The sodium-glucose symporter exemplifies secondary active transport in intestinal epithelial cells. It utilizes the sodium gradient established by the sodium-potassium pump, which transports 3 sodium ions out and 2 potassium ions in, creating a high extracellular sodium concentration. This gradient drives the co-transport of sodium and glucose into the cell, allowing glucose to move against its concentration gradient. Subsequently, glucose exits into the bloodstream via the GLUT2 uniporter, facilitating its distribution throughout the body. This process highlights the interplay between active transport mechanisms and nutrient absorption.

Glucose Active Symporter Model

So now that we've covered secondary active transport, in this video, we're going to talk about a specific example of secondary active transport, and that is the glucose active symporter model. Now, this is a classic and relevant example of secondary active transport. And more specifically, we're going to be looking at the intestinal epithelial sodium glucose symporters. And so just to get a better idea of exactly what we're talking about when we refer to intestinal epithelial cells, let's take a look at our image down below. And so notice that we've got our anatomical figure over here and we're zooming into this specific region. And, you can see that we have the stomach, the pancreas, and the intestinal lumen right here. And so if we zoom into the intestinal lumen wall right here in this region, what you'll notice is that the wall is made up of these villi structures that you see right here, these curved structures that give more surface area to the intestinal lumen and allow for better absorption of materials that we eat. And so what you'll notice is that these villi are lined with individual intestinal epithelial cells. And also these villi are closely associated with our bloodstreams, which we have right here in the villi. And so if we zoom into this particular region over here on the right-hand side of our villi, notice that, again, zooming in will show a bunch of individual intestinal epithelial cells. And these little structures that you see here would be the microvilli. Again, allowing cells to increase surface area and increase their absorption of materials that get into our intestines. And so notice that now we're zooming into one individual intestinal epithelial cell here, and that is going to be this image that you see down below. And so really, this image down below is the main image for this lesson, and all of the other images are just there to give you guys a little bit of context. 3 important components that you guys should know. And we've labeled each of these three components as a, b, and c, and you'll be able to see each of these 3 components corresponds with the images that we have down below. So you can see component a is right here, component b is over here, and component c is right here. And so, of course, we know from our previous lesson videos that secondary active transport is indirectly driven by primary active transport. And so that's exactly where we start with component a is primary active transport. And more specifically, it's actually the sodium potassium pump that is going to maintain a transmembrane sodium gradient. And this transmembrane sodium gradient, we know is going to be really high on the outside of the cell, and really low on the inside of the cell. And so if we take a look at our image down below at component a over here, notice that it's just the sodium potassium pump that we talked about from our previous lesson videos. And so we know that it hydrolyzes ATP, and in the process, it transports 3 sodium ions to the outside of the cell as it transports 2 potassium ions to the inside of the cell. And so again, this is establishing a concentration gradient for sodium that is really high on the outside of the cell and really low sodium on the inside of the cell. And so that transitions us directly into the secondary active transport part of this lesson, and that is exactly where the sodium glucose symporters come into play. And so the sodium glucose symporters are going to co-transport, as their name implies, sodium and glucose in the same direction across the membrane. And that's what makes them symporters because the 2 molecules are being transported in the same direction. And so really, what powers the transport of this co-transport here is that there are 2 sodium ions that are going to be imported down their concentration gradients or with their concentration gradients from high to low. And, these 2 sodium ions being imported down their concentration gradients is really what powers the movement of 1 glucose molecule being imported into the cell against its concentration gradient from an area of low concentration to an area of higher concentration. And so if we take a look down below at our image, notice that we have component b up here at the top right, and this is our sodium glucose symporter, our secondary active transporter model. And so notice that no ATP hydrolysis is directly involved here with this, sodium glucose importer. So we know it's gotta be a form of secondary active transport, and it's utilizing the gradient that was established by the sodium potassium pump, the sodium gradient that was established. So there's low sodium on the inside and high sodium on the outside. And so as sodium gets imported down its concentration gradient from high to low, it's powering the movement of glucose from the intestinal tract into the intestinal epithelial cell against its concentration gradient. And so now that transitions us to the last part here of our lesson, part c. And that is just to know that as glucose is pumped into the intestinal epithelial cell from the intestinal tract to the inside of the cell, it's also simultaneously being moved into the bloodstream via a GLUT2 uniporter. And of course, we know uniporters will transport 1 molecule in one direction across the membrane, and that is going to be glucose. And so if we take a look at our image down below, notice down below part c here, we have the GLUT2 uniporter, which is going to transport glucose, in one direction across the membrane so that it can diffuse into the blood. And of course, once glucose is in the blood, that glucose can diffuse to pretty much every cell in our bodies. And so one thing to note here is that the sodium glucose symporter that we talked about in part b and this, GLUT2 uniporter that we just talked about in part c are actually operating on opposite sides of the intestinal epithelial cells. And so if we take a look back down below, notice again that the sodium glucose importer is operating on the intestinal tract, side of the cell. And notice that the GLUT2 uniporter is operating on the opposite side of the cell that's closer to the bloodstream. And so all of these systems here are operating together to allow glucose that gets into our digestive system when we eat foods and transport that glucose from our intestinal tract into our intestinal epithelial cells, and then through, the intestinal epithelial cell into the blood where, again, it can be transported to every cell in our bodies. And so really, it's this sodium glucose importer here that is our example of secondary active transport. And really, this is the conclusion of our lesson on glucose active importer model, and we'll be able to get a bunch of practice in our next couple of videos. So I'll see you guys there.

The Na⁺-Glucose symporter transports the two molecules into the cell, while the Na⁺-K⁺ ATPase uses ATP to transport Na+ ions out of the cell. What would be the result of a mutation leading to a nonfunctional Na⁺-Glucose symporter?

• A. Increased levels of intracellular K⁺.
• B. Increased levels of intracellular glucose.
• C. Decreased activity of the Na⁺-K⁺ ATPase.
• D. Overactivation of the Na⁺-K⁺ ATPase.
• E. Increased levels of ATP.

Answer: (C)

Imagine that you perform a series of experiments to test the rate of glucose transport (V₀) into epithelial cells using the Na⁺-Glucose symporters. These experimental epithelial cells contain no intracellular Na⁺ but have the same glucose concentration as their surroundings. In experiment #1, you transfer your cells to test tubes that contain different extracellular [Na⁺] & then measure the rate of glucose transport (V₀). In experiment #2, you introduce leakage Na+ channels into the cell membranes & then repeat the same experiment. Label the data on the plot below as showing results to either Experiment #1 or Experiment #2.

Alright. So here we have a word problem that looks a lot more difficult than it actually is. It asks us to imagine that you perform a series of experiments to test the rate of glucose transport or \( v_0 \) into epithelial cells using the sodium-glucose symporters that we talked about in our last lesson video. Although this does not apply to actual epithelial cells, this problem tells us that these particular experimental epithelial cells contain no intracellular sodium ion. As long as we add a little bit of sodium ion on the outside of the cell, we've got ourselves a sodium ion gradient. But notice that they have the same glucose concentration as their surroundings, and so technically because there's the same glucose concentration, a significant amount cannot be transported unless active transport is implemented, specifically secondary active transport using the sodium-glucose symporters. Now, in experiment number 1, it says that you transfer your cells to test tubes that contain different extracellular sodium ion concentrations, creating, of course, a sodium ion gradient since there's no intracellular sodium ion concentration, but there is extracellular sodium ion concentration. You then measure the rate of glucose transport or \( v_0 \). We can take that data and plot it onto a graph like the one that we have down below where notice on the y-axis we have the initial rate of glucose transport into the cell or \( v_0 \), and then on the x-axis we have the extracellular sodium ion concentration. Again, on the inside of the cell, there's no sodium ions. Any extracellular sodium ion concentration will create a sodium ion gradient. In experiment number 2, you introduce leakage sodium ion channels, which recall from our previous lesson videos always remain open. We introduce these leakage sodium ion channels into the cell membranes of these experimental epithelial cells, and then we pretty much repeat the same exact experiment as we did in experiment number 1. All this problem wants us to do is to label the data on the plot down below as showing results to either experiment number 1 or experiment number 2. You can see these blanks that we have down below where we can label the data as either coming from experiment number 1 or experiment number 2. We need to determine that. Now, what we've done is drawn ourselves a little sketch of experiment number 1 and experiment number 2. On the left over here, notice that we're showing you experiment number 1. What we have is a test tube, just like what we mentioned in experiment number 1. On the inside of the test tube, we have our experimental epithelial cell. This red structure that you see right here represents the sodium-glucose symporters. Recall that on the inside of these cells, there's absolutely no intracellular sodium. By adding extracellular sodium ion concentration, we're creating a sodium gradient. A sodium gradient definitely does exist in experiment number 1. Recall that the sodium-glucose symporter is powered by a sodium gradient that does exist. Two sodium ions are able to be transported down their concentration gradient into the cell as a glucose molecule is transported against its concentration gradient into the cell. Now, when we compare this with experiment number 2, like what we have over here on the right-hand side, notice that we have our test tube once again, and it's pretty much the same exact experiment as experiment number 1, but the only difference is that we're introducing leakage sodium ion channels, which recall remain open. These leakage sodium ion channels, because they remain open, will allow sodium ions to diffuse down their concentration gradient, and so, they will also allow sodium ions to diffuse back. These leakage sodium ion channels, essentially what they're going to do is disrupt the sodium gradient. They're going to quickly create an equal sodium gradient on the inside as it is on the outside. In experiment number 2, there will be no sodium gradient. Without a sodium gradient, the sodium-glucose symporter that we have here in red is not able to use a gradient to power the transport of glucose. In experiment number 2, we should expect that there is no initial rate of glucose transport since there's no sodium gradient that can power the transport of the glucose molecules. Essentially, what you'll notice is that this line down here is showing essentially no glucose transport. And so this would be associated with experiment number 2. Again, since there's no sodium gradient to power the glucose transport, there will be no glucose transport. This other line that we see here, this curve that we see here, must be the data for experiment number 1. We can label it as experiment number 1. This is all that this practice problem wanted us to do: label the data as experiment number 1 or experiment number 2. Now that we've done that, we've completed this word problem. I'll see you guys in our next video.

Here’s what students ask on this topic:

What is the role of the sodium-potassium pump in the glucose active symporter model?

The sodium-potassium pump plays a crucial role in the glucose active symporter model by establishing a sodium gradient across the intestinal epithelial cell membrane. This pump uses ATP to transport 3 sodium ions out of the cell and 2 potassium ions into the cell, creating a high concentration of sodium outside the cell and a low concentration inside. This gradient is essential for the secondary active transport of glucose. The sodium-glucose symporter utilizes the energy from sodium ions moving down their concentration gradient to co-transport glucose into the cell against its concentration gradient. This process is vital for efficient glucose absorption in the intestines.

How does the sodium-glucose symporter facilitate glucose absorption in the intestines?

The sodium-glucose symporter facilitates glucose absorption in the intestines by coupling the transport of glucose with sodium ions. This symporter is located on the apical side of intestinal epithelial cells. It uses the energy from sodium ions moving down their concentration gradient, established by the sodium-potassium pump, to transport glucose into the cell against its concentration gradient. As sodium ions flow into the cell, they drive the uptake of glucose from the intestinal lumen into the epithelial cells. Once inside, glucose is then transported into the bloodstream via the GLUT2 uniporter, ensuring its distribution throughout the body.

What is the difference between primary and secondary active transport in the context of glucose absorption?

Primary active transport directly uses ATP to move ions against their concentration gradients. In the context of glucose absorption, the sodium-potassium pump is an example of primary active transport. It hydrolyzes ATP to transport 3 sodium ions out of the cell and 2 potassium ions into the cell, creating a sodium gradient. Secondary active transport, on the other hand, does not directly use ATP. Instead, it relies on the energy stored in the ion gradients established by primary active transport. The sodium-glucose symporter is an example of secondary active transport, using the sodium gradient to co-transport glucose into the cell against its concentration gradient.

What is the function of the GLUT2 uniporter in glucose transport?

The GLUT2 uniporter plays a critical role in the final step of glucose transport from the intestinal epithelial cells into the bloodstream. After glucose is co-transported into the epithelial cells by the sodium-glucose symporter, the GLUT2 uniporter facilitates the passive transport of glucose across the basolateral membrane of the epithelial cells into the blood. This uniporter operates by allowing glucose to move down its concentration gradient, ensuring that glucose absorbed from the intestines can enter the bloodstream and be distributed to various tissues and organs throughout the body.

How does the sodium-glucose symporter model exemplify secondary active transport?

The sodium-glucose symporter model exemplifies secondary active transport by using the energy stored in the sodium gradient, established by the sodium-potassium pump, to drive the uptake of glucose into the cell. The sodium-potassium pump, a primary active transporter, creates a high concentration of sodium outside the cell. The sodium-glucose symporter then uses this gradient to co-transport sodium and glucose into the cell. Sodium ions move down their concentration gradient, providing the energy needed to transport glucose against its concentration gradient. This indirect use of ATP, via the sodium gradient, is characteristic of secondary active transport.