This lesson is called using a microscope to observe effects of osmosis in plant cells: practical.

And it's from the unit coordination and control, maintaining a constant internal environment.

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.

In our lesson today, we're going to use a light microscope to observe the effects of osmosis in plant cells.

Now we're going to cover a number of keywords in our lesson today, and they're listed up here on the screen for you now.

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

Now in our lesson today, we're going to first of all remind ourselves of how to use the light microscope before we observe the effects of osmosis on plant cells and work out how to explain what we are viewing.

So are you ready to go? I certainly am.

Let's get started.

So we know that cells are usually too small to see with the unaided eye, and we therefore need a microscope of some form in order to be able to see them.

Now we could use a light microscope to observe most cells, and on the left of the screen here, you can see a picture of plant cells being observed using a light microscope.

Now, a light microscope has various parts to it, some of which I'm sure you are familiar with.

The eyepiece lenses at the top of the microscope, the objective lens is below it, and there are usually three objective lenses present on a light microscope.

Below that is the stage, and below that is the light source.

There will also be a coarse focusing wheel and a fine focusing wheel.

In this microscope, they are combined, but in some microscopes they are separate.

Now these parts have different roles to play.

So the eyepiece lens is a viewing lens, and it normally has a times 10 magnification power.

The objective lenses, as I say, there are three of them usually, and they have different magnifications.

So when combined with the eyepiece lens power, makes the total magnification power of the microscope.

Now, the object that we are viewing sits on the stage and usually we have a specimen on a slide which gets put on here.

Underneath that is the light source which illuminates the specimen.

In this case it's a light built into the microscope, but in older microscopes it may be a mirror which is used to reflect lamplight.

And then there's the coarse focusing wheel, which the focus in larger increments and the fine focusing wheel, which adjust the focus in smaller increments.

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

Okay, so hopefully you've identified that both A and C are part of the microscope which magnifies the image, the lenses.

And which label or labels indicate part of the light microscope, which brings the image into focus? A, B, C, or D.

Again, I'll give you five seconds to think about it.

Okay, so hopefully you've decided that part D is the focusing wheels, well done.

So setting up a light microscope is a very particular process and there are a few safety issues that you need to be considerate of as well.

So firstly, you need to turn the objective lenses, that's part B on the diagram, to the magnification lens with the lowest magnification.

That's usually the shortest lens.

Then you need to place the slide on the stage, which is part C, and fasten it with the clips.

You then need to turn on the light source, which is D, and if the microscope has a mirror, you need to angle it so that it can reflect light up through into the specimen.

But please be careful, lamps get hot, and you must never use a mirror to reflect sunlight into the light microscope.

It's too bright and it may blind you.

Once you've done those things, you can then look to the side of the microscope and whilst turning the coarse focusing wheel, which is E on the diagram, you can move the stage up.

So it is as close to the objective lens as it will go without the slide touching the lens.

Now you need to look at the side to be able to do this so that you can stop moving the stage upwards so that it doesn't smash the slide into the objective lens itself and cause either the lens or the slide any damage image.

Then once you are set up, you can look into the eyepiece lens, which is part A on the diagram, turning the coarse focusing wheel to bring the specimen away and into focus.

And finally using the fine focusing wheel, which is part F, to make the image clearer if needed.

So take a look at this video to see how to safely and correctly use a light microscope.

Okay, so let's have a little look at how to use our microscope.

So if you turn it around to your face you like this, you should be able to see that curved piece of glass there in the top.

That's your lens, your eyepiece lens.

If we turn it sideways, it's easier for you to see all of the different parts.

Now if you look at the back, there might be a light switch or you might have a mirror at the bottom that you need to angle, but it's really important that that light passes through that hole in the stage in order for you to be able to view your specimen.

So this is your eyepiece lens that you're gonna look through, and these are your objective lenses, which you can change in order to change the magnification of your image.

Then you have got your coarse focusing wheel, which gets in roughly the right focus and then the fine focusing to make a real sharp image.

You can adjust the amount of light coming through, if there's too much or too little light, and you're gonna place your slide with your specimen underneath those clips on the stage.

So when you're ready, have a little go at making sure that that eyepiece lens is facing you, different microscopes have them in slightly different ways, and you're gonna get yourself a slide that's already prepared and you're going to put it on the stage underneath those clips.

Now it's really important when you do this that the specimen is over that little hole where the light is coming through so that you can visualise it.

You need to adjust your objective lens, so it's on the smallest one there, and then you're gonna turn your coarse focusing wheel to bring the stage as close as you can to that lens.

And then you're gonna look through your eyepiece lens here and you're gonna move that stage slowly away using the coarse focusing wheel until you can see a clear image.

You can sharpen up that image by just turning the fine focusing wheel there, which may be a separate wheel.

Once it's in focus, you can then increase the magnification by changing your objective lens.

And again, it should be at roughly the right place, but you can adjust your focusing wheel slightly whilst looking down the eyepiece lens to get it into view.

And finally, you may be able to use your highest magnification objective lens.

Again, slightly turning your coarse and find focusing wheels to get it in view.

So what I'd like you to do now is place these steps in the correct order to describe how to set up a light microscope.

So part A is looking from the side turn the coarse focus wheel to move the stage up so it is close to the objective lens.

B is looking into the eyepiece lens, turn the fine focus wheel to make the image as sharp and clear as possible.

C is use the lowest power objective lens.

Place the slide on the stage and switch on the light.

And D is looking into the eyepiece lens, turn the coarse focus wheel to bring your specimen into focus.

But which order do they go in? I'll give you a few seconds to think about it.

Okay, so you should have started with part C and then chosen part A, then part D and finishing with part B.

Did you get them in the right order? Well done if you did.

So what I'd like you to do next is to set up your light microscope on your desk, following the instructions that you've been given and familiarise yourself with how to use it.

Make sure you set the objective lens to the lowest power and position the stage so that it is as close to the objective lens as is safely possible.

Once you've done that, I'd like you to describe two safety precautions to take when using a light microscope and explain why these are important.

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

Okay, so hopefully you've got your microscope set up and the slide positioned on the stage and it's up as close to the objective lens as possible.

I then asked you to describe two safety precautions to take when using a light microscope.

So you might have written that you need to position the stage as close to the objective lens as safely possible.

This is then because when it is moved to focus, the slide will not damage either the lens or the slide.

You need to avoid direct sunlight as a light source because this could blind.

Instead, you should use a lamp.

You might have written that when handling slides, you need to be careful not to drop them as they are made of glass and could cause cuts if they become broken.

Or you might have written that if you are using stains that you need to wear safety glasses and wash off the stain off of your hands as soon as possible because some stains are irritants.

So just check over your answer and make sure you've written at least two of those, well done.

Okay, let's move on to the actual practical.

So what we're going to do is observe the effects of osmosis on plant cells.

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

Now, water moves around plants and it has to, because water enters the plant via the roots.

So if it didn't move around the plant, it would only stay in the roots and that would be a bit useless for the rest of the plant.

So it enters via the roots and then it travels up through the plant via tissue called xylem.

It can be lost via leaves in a process called transpiration, and it can move up and down the plant with dissolved sugars through vessels called phloem.

So water is constantly shifting around a plant.

Now if we look at plant cells, we can see that water is used in photosynthesis.

So that is being used within the green chloroplasts as you can see within this slide image.

And it's also being used within the vacuole to provide turgid pressure.

Essentially what it's doing is filling up the cell to make it nice and firm, and that maintains the shape of the cell and it also helps to maintain the shape of the plant.

So you can think about this a bit like, you know, when you're blowing up a balloon, the more air you put into it, the better the shape.

Or when you're blowing up tyres on a bike or a car, the more air you add, the better the shape it has and the more firm it feels.

And that is essentially what water is doing to plant cells.

Now water moves passively through a selectively permeable membrane from high to low concentrations of water, and this process is called osmosis.

So water is entering and leaving plant cells by osmosis.

So who has summarised details about water implants correctly and what mistakes have they made? So Andeep says, "Water is taken in by the leaves of plants and used in photosynthesis." Izzy says, "Water is moved through the plant by osmosis via the xylem to the roots." And Alex says, "Water fills the vacuole, which is where photosynthesis takes place." So what is correct and what mistakes have been made? I'll give you a few seconds to think about it.

Okay, let's have a look then.

Andeep has made some mistakes because Andeep should have said that water is taken in by the roots of plants.

Izzy has made some mistakes because she should have said that water moves through the plant by osmosis, up the xylem from the roots.

And Alex has made some mistakes because he should have said that instead of the vacuole being where photosynthesis takes place, the vacuole provides turgor pressure to support the plant.

So did you get those correct? Well done if you did.

Now, if the concentration of water molecules is greater outside the cell compared to inside the cell, then water will move into the cell from high to low concentrations.

And you can see that in the diagram, there's more water outside the cell than inside, and so it moves from high concentration to low concentration into the cell.

What this means is that the vacuole, that's the big blue blob in the middle of that diagram that fills with water and it swells up and presses the cell membrane against the cell wall in a really solid turgid cell.

And for the plant, that means the plant becomes turgid as well.

So the cell becomes turgid, the plant becomes strong and it stands upright.

Conversely, if the concentration of water molecules is greater inside the cell than outside, then water will move outta the cell from high to low concentration.

So you can see in the diagram that there is more water inside the cell than outside, and water moves down the concentration gradient from high to low concentration and leaves the cell.

So what this means is that the vacuole loses water and shrinks.

And because there is essentially less matter inside the cell, the cell membrane gets pulled away from the cell wall.

And you can see that in the diagram.

And what that means in terms of the plant is that because the cell has become flacid and weak, so the plant loses its structure as well.

So the plant wilts and it flops over.

And all of these things are being caused because water is either moving into the cell and causing it to stand upright or leaving the cell and causing the plant to wilt.

Now that process of pulling the membrane away from the cell wall is called plasmolysis.

And you can see in this sped up video, looking at red onions cells down the light microscope, you can see how the cell membrane is being pulled away as water leaves the cell and the vacuole shrinks.

You can see how the space which is pink, which is the cytoplasm in the vacuole, is getting smaller and smaller and smaller, and the cell is shrinking inside the cell membrane.

Now obviously this is much faster than it would normally happen, but it can happen really quite quickly if the conditions are right, if you like, but definitely wrong for the plant.

So in that animated example, the onion epidermal cells were sitting in very salty water.

So salty water is made of sodium and chloride ions with water molecules.

And outside the cell there were a lot of sodium and chloride ions, but very few water molecules compared with the inside of the cell.

And it's the water molecules that matter because in this example, there are many more water molecules inside the cell than there are outside the cell.

So there is a higher concentration of water molecules inside the cell than outside.

And therefore water leaves the cell, moves from higher to lower concentration by the process of osmosis, and that causes the cells to plasmolyse, the process of plasmolysis.

So what causes plasmolysis in plant cells? Is it A, large quantities of water entering the cell, B, large quantities of water leaving the cell, or C, large quantities of salt solution entering the cell? I'll give you five seconds to think about it.

So you should have said that plasmolysis is caused by large quantities of water leaving the cell, well done.

What I'd also like you to do is to write the sequence which shows the correct pathway that water will osmose through the cells.

So the higher the number, the higher the cell water concentration.

A is at 50, B is 70, C is 80, and D is 60.

Which order will water move through these cells by? Again, I'll give you five seconds to think about it.

Okay, so you should have said that water is starting at the point of highest water concentration, which is C at 80.

It'll then move to B, which is 70, and then it will move to A of 50 and D at 60.

But then water will also move from D to A, 60 to 50.

So it's a slightly more complicated picture than it might have otherwise looked.

But if you got all of that right, well done indeed.

So what I'd like you to do now is using your light microscope, I'd like you to observe the turgid and flacid plant cells.

And I would like you to, if you have time and can, I'd like you to make a line drawing of one or both of the samples following the success criteria which are listed on the screen.

Once you've done that, I'd like you to think about what you can conclude about the conditions that these plant cells were in and the responses that those cells have had to those conditions.

Once you've finished your practical and you've tied it away, then I would like you to have a consideration of these two scenarios.

So the first scenario is a plant that has been left in a pot on the window sill for a few days without watering.

And I'd like you to explain why the plant is wilted using your understanding of osmosis.

Then I'd like you to consider the next example, which is that that same plant is watered and within a few hours it is standing upright again.

And I'd like you to explain why this has occurred.

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

Okay, so you should have observed your turgid and flacid plant cells using the light microscope, and you may have drawn a diagram of one or both of those cells.

So the turgid plant cell, there really isn't very much to draw in a turgid plant cell because you can't actually see the cell membrane using the light microscope because it is pressed so firmly against cell wall.

So you should have at least listed the cell wall and the nucleus, added a title and shown the magnification, in this instance, magnification 40.

So lowest power and the eyepiece lens multiplied together.

Or you might have shown the flacid cell.

The flacid cell will have a little bit more detail in it because you will be able to see the cell membrane because it is pulled away from the cell wall.

So both the cell membrane and the cell wall should be added into your diagram and labelled along with the nucleus if it's visible.

And again, with a title and the magnification used.

Now drawing diagrams of things that you are viewing down the light microscope is actually really difficult.

So if you've even attempted that, very well done indeed because it's a really hard job to do.

Then you should have concluded about the state of these cells.

So what conditions were they in and what has happened? What is the response of the plant? So you should have said that the turgid plant cell has swollen with water because it must have been in a high water concentration or a low solute concentration environment, such as distilled water, for instance.

You also might have written for the flacid plant cell that the cell has shrunk and the cell membrane has come away from the cell wall, and therefore it must have been in a low water concentration or a high solute concentration environment such as salty water.

So just check over your responses, have you got them around the right way? Have you described what is happening for the turgid and the flacid plant cells correctly? And well done if you have done.

Then for the two scenarios.

So I asked you to explain why the plant is wilted using your understanding of osmosis.

So you should have written that the concentration of water molecules outside the plant cells was lower than that inside the plant cells.

So water travelled by osmosis down the concentration gradient and left the cells.

the emptying vacuole shrank back from the cell membrane causing plasmolysis.

This reduced strength of the plant so it wilted and flopped over.

And this is all because it'd been left on the windowsill for a good few days without watering.

Then in the next scenario, the plant is watered and within a few hours it is standing upright, but why? So you should have written that the concentration of water molecules outside the plant cells becomes higher than that inside the plant cells once it's watered.

And this means that water travelling by osmosis moves down the concentration gradient from high to low water concentration as usual, but in this case it enters the cell.

So the vacuole then fills with water, which pushes the cell membrane up against the cell wall, and that increases the strength of the plant, making it turgid and standing it back upright again.

So again, check over your answers.

Did you get all of that detail? Well done if you did, because that's quite complex explanation, good work.

So I hope you've enjoyed your lesson today, that mixture of practical, observation and explanation.

In our lesson today, we've seen that light microscope can be used to observe the effects of osmosis on plant cells and reminded ourselves that osmosis is the movement of water from an area of high concentration to an area of low water concentration through a selectively permeable membrane.

When plant cells gain water, the vacuole fills up and becomes turgid, and this gives the plant strength.

When the plant cell loses water, the cell membrane pulls away from the cell wall in a process called plasmolysis.

This causes the cells to become flacid and the plant wilts.

And these are examples of plant habits that we see fairly frequently and hopefully any wilted plants that you've got at home, you're gonna go home and give them a good water this evening.

Anyway, thank you very much for joining me.

I hope you've enjoyed today's lesson, and I hope to see you again soon, bye.