Imagine that you perform a series of experiments to test the r...

Question

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.

Graph showing glucose transport rates for two experiments with Na+-Glucose symporters.

Accepted Answer

Alright. So here we have a word problem that looks a lot more difficult than it actually is. And so it wants us to imagine that you perform a series of experiments to test the rate of glucose transport or v naught into epithelial cells using the sodium glucose importers that we talked about in our last lesson video. And so although this does not apply to actual epithelial cells, in this problem it's telling us that these particular experimental epithelial cells contain no intracellular sodium ion concentration. And so as long as we add a little bit of sodium ion on the outside of the cell then we've got ourselves a sodium ion gradient. But notice that they do have the same glucose concentration as their surroundings. And so technically because there's the same glucose concentration, on the inside and the outside, then glucose should not be transported in either direction across the membrane in a significant amount unless active transport is implemented, specifically secondary active transport using the sodium glucose importers. 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. And then you measure the rate of glucose transport or v naught. And so 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 naught, and then on the x axis we have the extracellular sodium ion concentration. And again on the inside of the cell there's no, sodium ions. And so, any extracellular sodium ion concentration will create a sodium ion gradient. And so in experiment number 2, you actually introduce leakage sodium ion channels which recall from our previous lesson videos always remain open. And 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. And so really 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. And so 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. And so, here what we've done is we've drawn ourselves a little sketch of experiment number 1 and experiment number 2. And so, on the left over here, notice that we're showing you experiment number 1. And so what we have is a test tube right here, just like what we mentioned in experiment number 1. We're transferring ourselves to test tubes. And so on the inside of the cell, what we on the inside of the test tube what we have is our experimental epithelial cell. And of course this red structure that you see right here represents the sodium glucose importers. And so we recall that on the inside of these cells there's absolutely no intracellular sodium. And so again by adding extracellular sodium ion concentration we're creating a sodium gradient. And so a sodium gradient definitely does exist in experiment number 1. And recall that the sodium glucose importer is powered by a sodium gradient that does exist. And so 2 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. And so, 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 these leakage sodium ion channels, which recall remain open. And so our leakage sodium ion channel is this blue structure that we have down here, and because they remain open they're going to allow sodium ions to diffuse down their concentration gradient, and so, they will also allow sodium ions to diffuse, backwards as well. And so these leakage sodium ion channels, essentially what they're going to do is disrupt the sodium gradient. And so they're pretty much going to create quickly create an equal sodium gradient on the inside as, it is on the outside. And so in experiment number 2 there will be no sodium gradient. And 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. And so 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. And so essentially, what you'll notice is that this line down here is showing essentially no glucose transport. And so this would be with experiment number 2. Again, since there's no sodium gradient to power the glucose transport, and so there will be no glucose transport. And of course, this other line that we see here, this curve that we see here, must be the data for experiment number 1. And so we can label it as experiment number 1. And so really this is all that this practice problem want us wanted us to do is to label the data as experiment number 1 or experiment number 2. And so now that we've done that, we've completed this word problem and so I'll see you guys in our next video.

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