Vernier Software and Technology
Vernier Software & Technology

Investigate Transpiration with the Gas Pressure Sensor — Tech Tips

John Melville, Ph.D. introduces the experiment, Transpiration, from Advanced Biology with Vernier.

Follow along as our Director of Biology, Dr. John Melville, walks through how to measure pressure changes as a plant takes up water into the stem. "Transpiration of Plants" is an experiment available in our Investigating Biology through Inquiry and Biology with Vernier lab books.

Hello. I'm John Melville from Vernier Software & Technology. And today, I'm going to be showing you how to conduct a transpiration experiment or to measure transpiration from a plant. This is a very popular lab that can be found in Biology with Vernier, Advanced Biology with Vernier, and also our Investigating Biology with Vernier lab book.

Now, the most important thing you need to know to get this lab to work, is you need to know how to properly set it up. So that's what I'm going to be focusing on. Now, one of the most important things you need to know is to get this lab to get to work properly, you really do need to use a woody stemmed plant, not a tomato or potato plant, but a woody stemmed plant. This is a fresh cut limb, small branch, from a cherry tree.

The next thing you need to know is that you need to make a very fresh cut of the woody stem, and I like to use a razor blade. And you want to cut that at an angle. Then you can just place it in some water and let it equilibrate and sit, and it should be fine. Now, the reason why I say using a razor blade is because you can make an angled cut with a razor blade, and you're not going to crush the limb. If you use a scissors, you're going to crush it, and that can be a real problem.

The other thing that you're going to need to do this lab is a gas pressure sensor, a syringe, some tubing, a little stopcock, some hose clamps, and then two utility clamps, a stand, and in this case, I'm using a LabQuest 2. Now, all of these smaller items here, the syringe, and the tubing, and the luer lock, and the hose clamps with the stopcock, they all come with the gas pressure sensor. You can also buy them separately as the gas pressure sensor accessory kit. So you should have all of these items laying around if you have a gas pressure sensor.

The next thing that I'm going to show you is really important, and that's actually how to place the stem into the tubing. So first thing that we're going to do, is we're going to fill the tubing with some water. So I'm going to remove one of these little pieces from the tubing, and then I'm going to fill it with water by placing the syringe into the other end of the tubing. And I'm going to place the stopcock on the other end of the tubing. I'm just going to make sure it's open, and then I'm going to press water through the tubing until it flows out the other end. And I want to make sure there are no air bubbles in it, so I'm just going to gently press through until water begins to flow through it. It's dripping through there, and then you could see a nice little bead of water on the top. And then I'm going to close the lock. And then now what I'm going to do is I'm going to gently press on the syringe to make sure that there is no air bubble in the tip here either. So, as I begin to pull off, I'm going to just gently press down on the syringe so that I make sure that this fills up with some water. And then just gently twist off. There we go. It's okay. You might get a little bit wet. You just want to make sure you have a nice bead on the end there.

Next, what I'm going to do is I'm going to take one of this hose clamps, and I'm going to place it on the other end of the tubing. And now I'm ready to insert the stem with all the leaves into the tube. So, right here, I'm going to take the stem, place it into the tubing, press it in until it's nice and tight. Now, if you see on the end here, what's really important is that there is no air bubble that's formed right there, not on the side or below it, and that's because the stem is actually cut perfectly. It's got the right diameter. It can't be too big, and it can't be too small.

And the next thing I'm going to do is I'm going to just take this clamp and gently lock it on the end. I'm just going to clamp it down until it clicks once. There we go. The reason why you don't want to clamp down on it too hard is if you do that, once again, you'll crush the stem, and then transpiration won't happen. Now what I'm going to do is I'm going to take the stem, and I'm going to attach it to the rest of the apparatus, but first I have to get everything set up. So, I'm going to take one clamp, put it down low, going to take another clamp, put it up high. Then I'm going to take the gas pressure sensor, and I'm going to attach it to the clamp that's placed high. Then I'm going to attach it to the LabQuest 2. And next, I'm going to attach the tubing to the gas pressure sensor, and then I'm going to place the plant here on the lower clamp. Connect this to the gas pressure sensor. Just turn this around for you so you can see that, and then I'm going to place this down here. Here we go, and now we're ready to begin the experiment.

Now, once I open this valve, what will happen is these leaves will begin to transpire. The plant will begin to transpire, and water will be pulled from this column up the stem, and then will be released out these leaves, so that means that the pressure should actually drop as a function of time. So, let's just begin the experiment. I'm going to open the valve. You can see that the pressure drops a little bit. I'm just going to wait a few seconds, and then I'm going to begin the experiment, and we can see that the pressure is decreasing.

And then just to show you that it really is decreasing, I'm going to autoscale the graph very quickly so we can see and/or you could show your students that the pressure's going down, and yes, the plant is transpiring. So, I'm going to click on Graph, Autoscale Once, and there you can see the graph is decreasing, so transpiration is happening.

Now, so if you follow all of these little tricks that I've shown you, you should be able to get transpiration to work every single time, and it should work this easily. Just remember, the key things are you do need to use a woody-stemmed plant. You need to cut it fresh. Remember to use a razor blade and not a scissors when you cut the stem, make sure that the stem is the right diameter so it doesn't creat an air bubble when you place it into the tubing, and just make sure that there are no air bubbles in the tubing, and then last, remember that the gas pressure sensor needs to be placed higher than the branch. If you do all that, you should get it to work every single time.

So, if you have further questions, you can check out our website, or you can just email biology@vernier.com.

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