This lesson is called, "The effect of light on water uptake by a plant: Practical," and is from the unit, "Transport and exchange services in plants." Hi there, my name's Mrs. McCready, and I'm here to guide you through today's lesson.

So thank you very much for joining me today.

In our lesson today, we're going to look at how we use a simple potometer to investigate the effect of light on the water uptake of plants.

So we've got a kind of practical coming up, but a potometer is a fairly tricky piece of equipment to use, so this may well just be a demonstration instead.

Now, in our lesson today, we're gonna come across a number of key words, and they're listed up here on the screen for you now.

You may wish to make a note of them by pausing the video first, but I will cover them as we come across them, so you don't have to do that.

Now in our lesson today, we're gonna first of all look at transpiration, photosynthesis, and water uptake in plants before we look in detail at how we use a potometer to measure the rate of transpiration and water uptake.

And then we're going to have a look at how we calculate that water uptake using the data that we obtained from the potometer.

So, are you ready to go? I certainly am.

Let's get started.

Now, water is an essential resource for plants, and plants use water for many, many different things, including in photosynthesis, as a component of cytoplasm where it acts as a solute for many reactions that take place, and to provide turgid cells to fill up the cells and give the cells its shape, and that gives the plant its shape.

So there are many reasons why water is essential to plants.

Now, plants lose water and they do that through the process of transpiration.

And most of the water that a plant loses through this process of transpiration is lost via open stomata in the leaves.

And you can see stomata in the electron micrograph image there on the screen.

They are very small holes in the leaf and they're open there so that carbon dioxide can come in and waste oxygen can leave the leaf, and water can leave this way as well, in this process called transpiration.

Now, the transpiration stream, which is the supply of water up through the plant, supplies water to all the cells that require it, including in the use of photosynthesis.

Remembering that in photosynthesis water is combined with carbon dioxide to form glucose with waste oxygen.

Now, plants take in as much water as they need to replace the water that they have lost.

And by measuring the rate of water uptake, we can get an idea of how much transpiration is happening within the plant and also the rate of photosynthesis.

So measuring the rate of water uptake gives us an indication of both of these values, the rate of transpiration and the rate of photosynthesis.

So, let's quickly check ourselves on that.

By which methods do plants lose water? A, photosynthesis, B, respiration, C, transpiration, and D, translocation? What do you think? I'll give you five seconds to think about it.

Okay, so you should have said that plants lose water through photosynthesis and transpiration.

Well done.

So, what I'd like you to do is to summarise what we've covered so far.

And to do that, I'd firstly like you to describe the process by which plants lose water, and then I'd like you to explain why potted plants require more watering on a sunny day than on a cloudy day.

So, pause the video and come back to me when you're ready.

Okay, let's see what you've written.

So firstly, I asked you to describe process by which plants lose water.

And you should have written that plants lose water vapour through transpiration.

This is when water moves out of the leaves through open stomata.

Then I asked you to explain why potted plants require more watering on a sunny day than on a cloudy day.

And you should have written that plants require more watering on a sunny day because they will have lost more water for photosynthesis than on a cloudy day.

They will also have lost more water through transpiration because the stomata and the leaves are open due to photosynthesis occurring.

This means that that water will need to be replaced and therefore plants will need more watering on a sunny day than on a cloudy day.

Okay, let's move on to the practical part of our lesson today.

Now, as I said, it's very likely that this is actually just going to be a demonstration, and I hope by the time we've gone through the method you will see why.

So let's have a look at how we use a potometer.

Now we can measure the effect of changing conditions on the rate of water uptake by using a piece of equipment called a potometer.

And a potometer provides essentially a single source of water to the plant from which we can then measure the volume of water that has been taken up by the plant in a certain length of time.

And you can see on the screen there a very simple potometer with the water gauge on the left where we can measure how much water has been taken up by the plant, by how much it goes down, and then the plant connected on the other end of the tube joining the two parts together, and that's the bit obviously where the water is being taken up by.

So your school potometer may well look different from the image here.

That's perfectly okay, but the principle is the same.

We have water in one part of the potometer, and the plant attached to the other end, and the two are joined together with a tube.

So a simple potometer has several little parts to it.

It has the leafy shoot that we insert into one end of it.

It has watertight seals between the leafy shoot and the tube, and the water cylinder and the tube so that no water can escape through a gap without us realising.

It has a water-filled tube, and it has a water-filled graduated pipette of some form.

Now, some potometers, like this example, measure the water uptake by measuring how much volume of water has been taken up by the plant in the course of the experiment.

In other words, by how much the water has moved down the pipette.

Now other potometers measure how far an air bubble has moved along a scale, and either will give us the same types of measurements of water uptake over a certain length of time.

Now, as I said, setting up a potometer is a tricky business.

It's super fiddly and it's very difficult to get right.

Now, the key to setting up a potometer well is to do it under water.

Because by setting up a potometer under water, you prevent air bubbles from entering the equipment and getting trapped.

And if you have air bubbles within the equipment, what this will lead to are random errors in your measurements.

So you have to set up a potometer under water to avoid including air within the equipment that should not be there.

So, why should the potometer be set up under water? Is it A, to remove contaminants which may reduce the rate of transpiration, B, to prevent air bubbles getting inside the equipment, or C, to ensure the plant is clean to maximise photosynthesis? I'll give you five seconds to think about it.

Okay, so hopefully you spotted that B, preventing air bubbles from getting inside the equipment is the reason why the potometer should be set up under water.

Well done.

So a potometer can be used to investigate the water uptake by a leafy shoot at different light intensities.

So we're going to watch a demonstration of this, but let's look at the method for doing this before we carry on.

So the independent variable for this experiment will be the light intensity.

This is the value that we are deliberately choosing to change.

The dependent variable for this experiment, the value that we are measuring, is the volume of water uptake by the plant.

That's what we use in the potometer for.

And the control variables, the ones we're keeping the same, include the surface area of the leaves, the temperature, and the air movement around the plant.

So, first part of the method is about assembling the potometer under water.

So, firstly, you need to fill a sink with water and then place the equipment that you need to put together into the water.

So that's the tubing and the pipette under water and fill them with water, making sure that any trapped bubbles are removed.

Then whilst you're keeping everything fully submerged, you need to put the two pieces of equipment together by inserting the end of the pipette into one end of the tubing.

Then under water also, you put the bottom of the stem that you're going to be inserting into the equipment, and under water, you cut off the end of the stem, then insert the cut stem into the potometer, making sure it is tight and the stem is fully inserted into the tubing of the potometer.

So this is all done under water.

Then, now that it's all assembled, you can then set it up so you can then remove all of that equipment, keeping it all together, and believe me, that is fiddly, especially if it's just loose pieces of tubing, and the pipette and the plant attached to it, they all wobble around all over the place.

So it's really, really tricky.

But anyway, you need to remove the potometer with the leafy shoot and the measuring cylinder attached, keeping them all together and clamp them into position.

Then you need to pat the equipment dry and dry the leaves carefully on the shoot so that the plant is as dry as you can make it whilst not disturbing it within the equipment.

Now, you also then need to check the seals to make sure that they are nice and tight.

And if they aren't, then you can seal all of them using some petroleum jelly or waterproof jelly to make sure that they are completely tight and air cannot get in and water cannot get out at those points.

So, how might you ensure the joints between the parts of the equipment and the plant are tight and fully sealed? Might you, A, wiggle the pieces and see if they come apart easily, B, pat the leaves and equipment dry and check for leaks, or C, seal the joints with waterproof jelly? What do you think? I'll give you five seconds to think about it.

Okay, so to check that the joints are tight and fully sealed, you need to seal the joints with waterproof jelly.

Well done if you got that right.

Now, this experiment is going to measure the effect of changing light intensity, that's the independent variable, on the rate of water uptake.

So, we're going to use a lamp at different distances from the shoot in order to change the light intensity.

And the further away the lamp gets, the lower the light intensity.

Now, the lamp, as well as being a source of light, it is also a source of heat.

So we need to manage the heat being radiated onto the plant in order to keep the temperature the same.

Now, the temperature is therefore a control variable.

We need to keep it the same in all of the instances of the experiment, otherwise, it may well affect water uptake.

And to do this, what we're going to do is place a beaker of water on a tripod between the lamp and the leafy shoot, so that the water can absorb the heat.

It doesn't absorb the light, but it will absorb the heat, and that will keep temperature constant throughout the experiment.

So, changing the light intensity is the next step of our method.

So, you need to set up a lamp facing the shoot, and then control the temperature by placing a beaker of water between the light and the shoot.

Then you can vary the light intensity by changing the distance between the lamp and the shoot, and you can measure this using a ruler.

So, how might the light intensity be varied during the investigation? A, by changing the distance between the lamp and the shoot, B, by changing the water in the beaker between the lamp and the shoot, or C, by choosing days with different weather on which to complete the practical? I'll give you five seconds to think about it.

Okay, so you can vary the light intensity by changing the distance between the lamp and the shoot.

Well done.

Now, the final part of our very long method is to measure the water uptake.

So, you need to wait until the meniscus, that's the skin of the water, in the pipette, has moved down to the next line on the pipette.

That means that you know that the plant is taking up water and the experiment is properly set up.

Once that has happened, you can then record the starting volume and start a timer.

Then you need to leave the experiment for at least 30 minutes.

And after the period of time that you are leaving it for at least 30 minutes, you can then record the amount of time that is passed and the volume of water that has been uptaken from the pipette.

So you're taking a measurement of the starting volume and the end volume, plus the time that has elapsed.

Then you can change the distance between the lamp and the shoot and repeat the experiment again.

So I hope you can see that with all the fiddly bits of setting this equipment up, plus the length of time in which you need to leave the experiment, you can see hopefully why this is not necessarily a practical that you are going to have a go at doing yourself today.

So let's watch a video to see how the potometer can be used.

Okay, so which measurements must you record in a table? A, the start and end volumes, B, the temperature of water in the beaker, C, the distance between lamp and plant, and D, the time taken? I'll give you five seconds to decide.

So you must record the start and end volumes, the distance between the lamp and the plant, and the time taken.

Well done if you selected all three of those.

So, what I would like you to do now is to summarise the method of using a potometer by completing the cartoon framework to describe in both words and diagrams, how to set up a potometer and take measurements.

Now, remember, your diagrams do not need to be fancy, they just need to be scientific outlines.

So don't feel that you have to be super artistic, but you do need to be accurate with your method.

So pause the video and come back to me when you're ready.

Okay, let's see what you might have included within your cartoon.

So for setting up the equipment, you should have included a picture of a sink filled with water, a pipette, and a piece of rubber tubing, and written that the equipment has to be assembled under water, and checked to make sure that no air bubbles are present, and remove them if they are.

Then, we're inserting the stem.

So your diagram should include the stem attached to those pieces of equipment, also under water, with a note to say that you're cutting the plant stem under water and inserting it into the potometer, also under water.

Then we're taking the equipment out.

So the next step is to dry the equipment.

So you should then have drawn a picture of the setup, potometer out of water this time, and noted that we're removing the equipment from the water and patting it dry.

We then need to dry the plant, and this has to be done carefully and completely over all of the plant leaves.

Then we're setting up the other equipment.

So this includes positioning the lamp, using a ruler, and placing a beaker of water between the lamp and the plant.

And hopefully your diagram shows all of those items as well.

And then our final step is to take the measurements where we're recording the start and end pipette measurements and the time taken.

So, have a review of your method.

Have you got all of the detail in against each of those stages, and well done.

As you can see, it's really quite a fiddly and tricky experiment to conduct.

So let's move on to the last section of our lesson, which is about calculating the rate of water uptake.

Now, what we have measured is the volume of water uptake, but this is not the rate of water uptake.

A rate is a measure of how much change occurs per unit of time.

And just saying how much water has been uptaken is therefore not a measure of rate.

Now in this experiment, we can calculate the rate of water uptake using the amount of water that has been uptaken divided by the length of time over which that uptake occurred.

So let's look at that in a bit more detail.

To calculate the rate of water uptake, we are looking at the pipette, and we're seeing what it started off at, in this case, 0 centimetres cubed, and what it finishes at, in this case, 1.

3 centimetres cubed.

So to calculate the rate of water uptake in centimetres cubed per minute, we are calculating the change of water volume in centimetres cubed, divided by the time taken in minutes.

Now, in this experiment, we have got a volume uptake of 1.

3 centimetres cubed, and that took 30 minutes to occur.

So we're doing 1.

3 centimetres cubed divided by 30 minutes, and that gives us a rate of water uptake of 0.

04 centimetres cubed per minute.

Now, when you've calculated rate for any situation, not just a potometer, look at the value that you are given.

Does it make sense? 0.

04 centimetres cubed per minute times by 30 minutes, that gives 1.

3 centimetres cubed in total.

Yes, that makes sense.

Yes, it's a relatively small number, but it makes sense.

But if you've multiplied the two together or if you've divided time by the change in volume, you'll get a completely different number.

And if you work that back, does it make sense? If it doesn't, where have you gone wrong? So always check your work over with a little bit of thought and see, you know, "Does this number actually makes sense?" And if it doesn't, start again.

So, let's check ourselves on this.

Now, in this experiment lasting 35 minutes, the starting reading was 0.

3 centimetres cubed, and the end reading was 2.

8 centimetres cubed.

So, if we calculate the rate of water uptake in this experiment, we can state firstly the equation that we're using, change in water volume divided by time taken, plug in our numbers, 2.

5 centimetres cubed divided by 35 minutes, and then come up with our answer, 0.

07 centimetres cubed per minute.

So this was slightly faster than the previous example.

So that's my demonstration.

What I'd like you to do now is to calculate the rate of water uptake for this experiment.

Now this experiment lasted 25 minutes.

Its start reading was 0.

6 centimetres cubed, and the end reading was 3.

4 centimetres cubed.

So I'll give you five seconds to work that out.

Okay, let's work our way through this method then.

So firstly, we're writing out the equation, that is always good practise because it helps to build some muscle memory in as you're writing it out.

Always good for exam practise.

So we're calculating change in water volume divided by time taken.

Then we're plugging in the numbers, 2.

8 centimetres cubed is the change in water volume.

That's 3.

4 minus 0.

6 divided by 25 minutes, and that makes a reading of 0.

11 centimetres cubed per minute, slightly faster still.

Did you get that calculation correct? Well done if you did.

Did you show your workings? Well done again.

And separately, if you did, make sure that you always show your workings so that if you have made a mistake, you can work out where it's gone wrong, and an examiner can work out what you have done as well.

Okay.

What I would like you to do now is to calculate the rates of water uptake at the following distances from the lamp.

So you can see all that data in the table, and your job is to calculate the rate of water uptake.

Then I would like you to describe the effect of distance on the rate of water uptake and suggest why this may have occurred.

So, show your workings, remember? Take your time, pause the video, and come back to me when you're ready.

Okay, let's check our workings then.

So to calculate the rate of water uptake, remember that we are doing the change in volume divided by the time taken.

So for 10 centimetres distance between the lamp and the plant stem, we've got 7.

9 minus 0.

5 divided by 30 minutes, gives a rate of 0.

25.

For a distance of 20 centimetres, it's 2.

3 minus 0.

3 divided by 31 minutes, giving a rate of 0.

06 centimetres cubed per minute.

At a distance of 30 centimetres, we've got 1.

2 minus 0.

2 divided by 32 minutes, giving a rate of 0.

03 centimetres cubed per minute.

And at a distance of 40 centimetres, we've got 0.

9 minus 0.

4 divided by 29 minutes, and that gives a rate of 0.

02 centimetres cubed per minute.

Did you get all of those calculations correct? Well done if you did.

Have you shown your workings? Especially well done if you have.

Then I asked you to describe the effect of distance on the rate of water uptake and suggest why this may have occurred.

So, you should have written that as the distance doubles, the rate approximately quarters.

And so we can show that mathematically by showing that the rate of water uptake at 10 centimetres was 0.

25 centimetres cubed per minute, which is approximately four times higher than at 20 centimetres, which is 0.

06 centimetres cubed per minute.

Now, this is because as the lamp moved closer, the relative brightness increased, and this increased the rate of photosynthesis and the rate of transpiration, and therefore increased the rate of water uptake too.

Now, when you're dealing with data, it's always worth including some data calculations as part of your argument to show that you've really considered it well.

So check over your answer, add anything that you've missed, and well done for having a go at that.

Okay, we've come to the end of our lesson now, and what we've done today is see how a simple potometer can be used to measure the rate of water uptake by a leafy shoot from a plant, and how the water uptake varies depending on the rates of transpiration and photosynthesis.

Now, setting up a potometer is a fairly tricky business, and firstly, it must be assembled under water to ensure that no air bubbles get into the equipment which can cause random errors.

Now, we can change the light intensity by varying the distance that a lamp is placed facing the leafy shoot, and changing that distance in the experiment.

And if we're using a lamp to change the light intensity, we need to control the temperature because of the amount of heat radiated from the lamp, and we can do that by placing a beaker of water between the lamp and the plant, which will absorb that heat from the lamp and control the temperature.

Then we can measure the rate of water uptake by dividing the change in volume by the time taken.

So I hope you've enjoyed our lesson today, and I hope to see you again soon.

Bye.