This lesson is called Observing xylem and phloem using a light microscope: practical, and is from the unit Transport and exchange surfaces implants.

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 use a light microscope to observe xylem and phloem vessels within a plant.

And we're also going to calculate the actual size of these structures.

So I hope you're looking forward to a practical lesson today.

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

You may wish to pause the video to make a note of them, but I will introduce them to you as we come across them.

So in our lesson today, we're going to first of all just refresh our memory about how to use a light microscope before we use it to observe xylem and phloem.

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

So let's get started.

So cells are usually too small to see with the unaided eye.

There are some cells which are large enough that we can see them individually, but most cells, we would need to use a light microscope to see them.

And you can see plant cells in that micrograph image quite clearly, having been magnified using the light microscope.

Now a light microscope has various parts to it.

It has an eyepiece lens and objective lenses.

It has a stage, a light source, a coarse focus wheel, and a fine focus wheel.

And each of these parts have different roles.

So the eyepiece lens has a times 10 magnification and is the part of the microscope that you look through.

There are three objective lenses in a light microscope, usually with times four, times 10, and times 40 magnifications, which allows us to view an image at different magnifications.

The coarse focus wheel allows us to adjust the image in large increments.

And the fine focus wheel allows us to adjust in small increments.

The light source be that an inbuilt lamp or a mirror to reflect an external lamp provides a light source which is shone up through the specimen and through the eyepiece lens so that we can see the specimen.

And the stage is where the specimen is put so that we can view it.

So which label or labels indicate a part of the light microscope that magnifies the image? I'll give you five seconds to think about it.

So you should have said that parts A and C both include magnification lenses.

Well done.

Now when you are using a light microscope, you need to set it up in a particular way.

So let's have a look at the method.

So first of all, you need to turn the objective lenses, which are part B in the image, to the lowest magnification which is the shortest lens.

Then you need to put your specimen on its slide onto the stage, part C, and fasten it with the clips.

And then you need to turn on the light source.

Now if your microscope has a mirror, you'll need to angle it so that it's reflecting the external light source up through the microscope.

But remember, you must never reflect direct sunlight because that will damage your eyes.

And also remember that lamp sources will get hot.

So just be careful once you are handling them after they've been on for a while.

Once you've got the specimen on the stage, you can then turn and look from the side and turn the coarse focus wheel so that the stage moves up as close to the objective lenses as possible without the stage and the specimen crashing into the objective lens.

So if you look from the size, you'll be able to see when they're nearly touching and be able to stop in advance so you don't smash the two together and damage the specimen or the objective lens.

Then once it's at its highest point, you can then use the coarse focus and turn it so that the stage moves away from the objective lens to bring it into focus and then use the fine focus wheel to bring it into really sharp focus in order to be able to view it really clearly.

So watch this video to see how to safely and correctly use the light microscope.

So when you're using a light microscope, when you're trying to make the image sharp and clear, you adjust the fine focus first, and then the coarse focus.

True or false? So you should have said that is false, but why? Well you should have explained that by saying that you first need to look from the side and turn the coarse focus wheel to move the stage up towards the objective lens.

And then looking through the eyepiece lens, you use the coarse focus wheel again to bring the image into focus.

And you only use the fine focus wheel to very finely adjust the focus if required.

Well done if you got that correct.

So what I'd like you to do is to firstly identify each part of the light microscope in the image and then explain what each part is used for.

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

Okay, let's check our work.

So you should have identified part A as the eyepiece lens and said that this is used to view the specimen with a times 10 magnification.

That part B is the objective lenses, and there are three lenses each with different magnification, which are usually times four, times 10, and times 40.

You might not have included that, but you can always add that now if you wish.

Part C is the stage and this is where the slide is placed and secured with clips.

Part D is the light which is used to illuminate the specimen.

Part E is the coarse focus wheel, which is used to adjust the focus in large increments.

And part F is the fine focus wheel, which is used to adjust the focus in small increments.

So I hope you got all of those correct.

Well done.

Okay, let's move on to the practical part of our lesson then, which is about observing the xylem and phloem in a plant specimen.

So xylem is involved in the transportation of water and dissolved mineral ions.

So water leaves the plant via a process called transpiration from the leaves.

And by losing water from the leaves, this pulls water up through the plant via the xylem vessels.

And this pulls water into the plant via the roots through a process of osmosis at the root hair cells.

So xylem runs throughout a plant in the root, the stem, and the leaves, and is responsible for the transportation of water and dissolved mineral ions.

Meanwhile, sugars and other dissolved nutrients are transported around a plant via phloem vessels.

So sugars are made in cells which photosynthesize, for instance, in the leaves.

And sugars are loaded in from the source into the phloem vessels, and then transported through the plant to a sink cell where those nutrients are required.

And when it reaches its destination, it is unloaded into that sink cell.

So phloem is responsible for transporting sugars and dissolved nutrients around the plant.

Now we can see these vessels relatively easily within various different specimens such as celery and bananas because these vessels are quite chunky.

They are vascular tissue.

And like the veins in our body, they're quite easy to see.

So the string parts in celery are xylem and the stringy fleshy parts in banana are phloem.

But although we can see the macrostructure of these vessels whilst looking at regular pieces of fruit and vegetables, we need to look at them under a light microscope if we want to see any detail in them.

So individual xylem and phloem vessels cannot be seen with the unaided eye.

True or false? So you should have said that that is true, but why? So you should have explained that by saying that bundles of xylem and phloem vessels can be easily seen with the unaided eye in examples such as celery, bananas, you might even have mentioned carrots.

But in order to be able to see the detail of those individual xylem and phloem vessels rather than just the bundle together, we need to magnify the sample using a light microscope.

Well done.

So a light microscope can be used to observe xylem and phloem in cross sections or slices of plants, stems, roots, and leaves.

So this micrograph shows a slice through, a cross-section through the stem of a plant.

And is at a magnification of times 40, so a really low powered magnification.

Whereas this sample is a cross-section view through a leaf, and this is at times 400, so much greater magnification at the highest magnification possible using a light microscope.

And when we are viewing the specimens of the stem or the leaf, we can see the vascular bundles which contain both xylem and phloem.

But it's actually quite straightforward to identify the difference between xylem and phloem because phloem are the fairly densely packed, relatively narrow tubes, whereas xylem are the very large wide gaping tubes.

So you can see actually there's quite a significant difference in the diameter of the tubes of xylem and phloem.

And that is what makes it so straightforward to identify the difference between them.

Now when you're taking an image, using a light microscope is always a good idea to indicate the magnification that that image was taken at.

So you can see in this picture, it has a magnification of times 400.

And that's useful because what it does is provide a sense of scale for anyone who's viewing the specimen.

So they get a sense of scale and size.

They get a rough idea of how big that object really is in real life.

Now to calculate magnification using the microscope, you need to times the eyepiece lens magnification by the objective lens magnification.

So the eyepiece lens magnification is usually times 10.

And then you need to identify which objective lens you've used.

Is it the times four, times 10, or times 40 magnification? And then whichever one that is, you multiply that by the eyepiece lens to get the total magnification.

So the total magnification is calculated by multiplying the magnification of the eyepiece lens, which is usually times 10, by the magnification of the objective lens.

So which objective lens was used to produce this image, which has a times 400 total magnification? Is it A, times four, B, times 10, or C, times 40? I'll give you five seconds to decide.

So you should realise that if the eyepiece lens is contributing times 10 magnification, then the objective lens that was used to make times 400 magnification must have been the times 40 objective lens, the highest power.

Well done if you got that right.

Now we can calculate the actual size of a structure in a microscope image by using an equation.

The actual size of the object is the size of the object in the image that we are viewing it in divided by the magnification of that image.

So if we take this cross-sectional view of a leaf, we can measure the width of the xylem vessel as 15 millimetres, and it has a magnification of times 400.

So if we put those numbers into our equation, the actual size of the xylem vessel is 15 millimetres divided by the magnification, which is times 400, and that makes an actual size of 0.

0375 millimetres, which we can convert into micrometres is 37.

5 micrometres.

So 0.

0375 millimetres and 37.

5 micrometres are equivalent values, just ones in millimetres and ones in micrometres.

And that does add an extra layer of complexity when calculating magnification, but it is a useful thing to be able to do.

So the actual size of a structure in a microscope image is calculated using the equation, actual size equals the size of the object in the image divided by the magnification.

So what is the actual size of the xylem vessel that appears in this image to be 12 millimetres across and is being viewed at a magnification of times 400? I'll give you five seconds to think about it.

So you should have realised that what we need to do is divide 12 millimetres, the size in the image, by 400, the magnification, and therefore the first answer A, 0.

03 millimetres is the correct answer.

Well done if you work that out correctly.

So what I'd like you to do now is to first of all, collect a slide that has a cross-section of a plant shoot or leaf on it.

And follow the method on the worksheet to observe the specimen on the slide using the light microscope.

Can you identify the xylem and the phloem tissue based on what you know they look like from the images that I've already shown you? Then once you've identified them, what I'd like you to do is calculate the actual size of the vascular bundle as shown in the image on the screen.

So the whole vascular bundle appears to be 54 millimetres wide.

What I'd like you to do is calculate its actual size using the equation actual size equals size in the image divided by magnification, where magnification is times 400.

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

So I hope you've enjoyed looking down the microscope and viewing the xylem and phloem vessels.

They're really fascinating to look at, and often stain very well in prepared light microscopes, so it can be very pretty to look at as well.

Now you might have observed something like these images, either at low magnification or high magnification, and either stem or leaf.

Either way, hopefully you've identified the vascular bundles, those collections of both xylem and phloem.

And were also able to identify the phloem vessels which are more densely packed compared to the xylem vessels, which are really quite large gaping holes in the stem.

So well done if you spotted them.

The other thing you might have noticed and we're able to use that to discern the xylem vessels from the other cells within the stem or the leaf, is that their cell wall is really quite a lot thicker than the cell wall around ordinary cells.

So that's another clue that you can use to identify xylem as opposed to just a large packing cell within the stem or the leaf.

Then I asked you to calculate the actual size of the vascular bundle in the image.

Now the vascular bundle measures 54 millimetres across when we are viewing it in the image and is at a magnification of times 400.

So if we take the equation actual size equals size in the image divided by the magnification and plug in the numbers, we'll find 54 millimetres divided by 400 magnification, and that equals an actual size of 0.

135 millimetres.

So well done if you calculated that correctly and especially well done if you showed your workings.

Remember, that's a very important thing to do whenever you're using an equation.

So we've come to the end of our lesson today, and I hope you've enjoyed using the light microscope to view the xylem and phloem vessels.

We've seen today how the light microscope has two lenses and two focusing wheels, which enables us to view specimens really clearly and at high magnification.

And we've seen how we can view xylem and phloem vessels as cross-sectional slices through plant stems or leaves using the light microscope.

Now xylem and phloem vessels are found together within a plant as a vascular bundle.

We can calculate the total magnification being used to observe a specimen by multiplying the eyepiece lens by the objective lens that is being used.

And if we use that value and the size of the image that we've viewed, we can calculate the actual size of the vascular bundle or xylem and phloem vessels within it.

So if we calculate the actual size, we can do that using the equation actual size equals the size in the image divided by magnification.

So I hope you found that a really interesting lesson.

Thank you very much for joining me today and I hope to see you again soon.

Bye.