Lesson video

Hello.

Today's lesson is from the unit Classification in modern biology, and the lesson title is Electron microscopy, and the size and scale of cells.

Hello, my name's Dr.

Pugh and I'll be taking you for today's lesson.

So by the end of today's lesson, you should be able to describe the size and scale of cells and cell structures and explain how electron microscopy has increased our understanding of sub-cellular structures.

So let's look at today's lesson keywords.

We've got four lesson keywords there.

We've got light microscope, electron microscope, magnification, and resolution.

Now, here are the descriptions of those keywords.

Now, I'm not gonna go through those with you now, but if you'd like to pause the video and make notes or copy them down, you can do so now.

Okay, let's look at today's lesson outline.

Now, the lesson's gonna come in three parts.

The first part is gonna be dealing with light microscopes, the second part with electron microscopes, and the third part with the size and scale of cells.

So let's look at light microscopes first of all.

Now, most cells are too small to see without a microscope, and our knowledge of cells started with the invention of the light microscope in the late 1500s.

One pioneer was the English scientist Robert Hooke, who in 1665 published his book "Micrographia," which contained images of the objects he was observing using a light microscope.

So let's look at one those images.

Here you can see one of the images from his book.

And he was the first person that we know of to use the word cell, because what he saw reminded him of cells that you find in honeycomb, and you can see similarities of honeycomb or honeycomb-like structures with the image that he's drawn here.

Now, his light microscope, although one looks very different to light microscopes that we have today, essentially is very, very similar, because both modern microscopes or modern light microscopes and the light microscope that Robert Hooke used use glass lenses to magnify images.

Now, modern light microscopes have got a greater magnification and resolution than those used by Robert Hooke, and it's very important that we get to grips with these two words.

So the first question is, what does the word magnification actually mean? Well, magnification means making a small object appear larger in order to see more detail.

Let's have a look at the word resolution.

Now, resolution is the minimum distance apart that two points of the specimen can be and still be clearly seen as separate objects.

Okay, so let's look at two images here which have been produced by a modern professional light microscope, and these microscopes have got a maximum magnification of around about 2,000 times.

That means they will enlarge a small image 2,000-fold.

And a maximum resolution of 0.

2 micrometres, and we'll look at that unit, the micrometre, a little bit later on in the lesson.

And this is enough to see some details of animal and plant cells, and we can see two examples here.

We can see animal cells on the left and plant cells on the right.

Now, the animal cells on the left may appear quite similar to cheek cells that you've observed in a practical class at school.

And we can see the outline of the cell membrane, (clears throat) excuse me, we can see the nucleus, those red structures, but we can't see any smaller sub-cellular structures, such as mitochondria, using this microscope.

Similarly, the plant cells viewed under the light microscope here on the right, we can see the outline of the cell, the cell wall and cell membrane very close together, but we can't resolve the difference between them because they're too close together to be resolved using the light microscope.

However, we can see some small sub-cellular structures.

In this case, we can see the chloroplasts in these cells.

So let's have a quick knowledge check.

Which student is correctly explaining the word resolution? Now, Izzy says, "It's the amount of detail that can be seen in the image measured by the smallest distance between two points that can be distinguished from one another." Andeep says, "It's making an object look larger than it is so we can see more detail." So pause the video, make your choice, and I'll go through the correct answers in a moment.

Okay, so let's see which student is correctly explaining resolution.

And the correct answer is Izzy.

It is the amount of detail that can be seen in the image measured by the smallest distance between two points that can be clearly distinguished from one another.

And another quick knowledge check.

Which of the following is a correct definition of magnification? Is it a, making large objects appear smaller in order to see the whole structure, b, altering the orientation of an object in order to see it from a different perspective, or c, making a small object appear larger in order to see more detail? So pause the video, make your choice, and I'll go through the correct answer in a moment.

Okay, so the correct answer is c, magnification is making a small object appear larger in order to see more detail.

Now some practise questions.

So question one, what is a light microscope? Two, define the term magnification.

Three, define the term resolution.

And four, what is the maximum magnification and maximum resolution of a light microscope? So pause the video, write down your answers, and I'll feed back in a moment.

Okay, so let's go through question one first of all.

What is a light microscope? A light microscope is a type of microscope that uses visible light and a system of lenses to generate magnified images of small objects.

Number two, what does the term magnification mean? This means making small objects appear larger in order to see more detail.

Three, define the term resolution.

The minimum distance at which two distinct points of the specimen can still be seen.

And four, what is the maximum magnification and maximum resolution of a light microscope? Maximum magnification is 2,000 times and the resolution is 0.

2 micrometres.

Okay, that brings us on to the second part of today's lesson, which is the electron microscope.

So the key difference between electron microscopes and light microscopes is whereas light microscopes use light to see a specimen, electron microscopes use beams of electrons instead of light rays.

Now, what benefit this gives the electron microscope is that this means that they have a higher resolution and magnification than the light microscope.

The reason being is that the wavelength of electrons are up to 100,000 times smaller than that a visible light.

And you can see on the right hand here we've got a photograph of a transmission electron microscope.

Now, let's look at electron microscopes.

There are two types of electron microscopes.

There is the transmission electron microscope, or TEM, and this uses beams of electrons, and it will fire beams of electrons through an extremely thin slice of the specimen.

And you can see an image of a cell, it's an image of a nucleus which has been generated using a TEM.

The second type of electron microscope is the scanning electron microscope, or SEM.

Now, this is slightly different, in so much that it will use beams of electrons, but the beams of electrons will scan across the surface of the specimen.

And you can see an SEM micrograph of red blood cells on the right-hand side.

So let's look at these two types of microscopes in a little bit more detail.

So, the transmission electron microscope produces flat, two-dimensional images, and it's often used to look at sections through cells.

In contrast, the scanning electron microscope, because it scatters beams of electrons from the surface of the specimen, it can be used to construct three-dimensional images, and therefore it's really useful for looking at whole cells.

So the transmission electron microscope can look inside the cell and the scanning electron microscope can look at the cell in its entirety.

So both are used in combination with each other to build up images, detailed images, high-resolution images of cells.

So the greater resolution and magnification of electron microscopes means that biologists can study cells in much finer detail, higher magnification and higher resolution.

This means that sub-cellular structures which we either couldn't see with the light microscope or we could see in low detail are now visible in much higher detail, an example of which is shown here.

So we've got the ultrastructure of a nucleus on the right.

So what can we see? We can see the nuclear membrane, and we can actually see there that the nuclear membrane is appearing as a double membrane, which it actually is.

Now, in the light microscope, although you can see the layer surrounding the nucleus, you can only see that as a single membrane.

Another structure which is shown in great detail here is the nucleolus.

And again, in the light microscope you can see a structure which is the nucleolus, but you can't see it in such detail.

So, let's have another knowledge check.

So true or false, electron microscopes have a higher resolution than light microscopes.

Is that true, is it false, and can you justify your answer? So read a and b, pause the video, answer the question, and I'll feed back the answers in a moment.

Okay, let's go through the answers now.

So true or false, electron microscopes have a higher resolution than light microscopes.

That's true.

Now, justify your answer.

You have two selections: a, the wavelength of electrons is much smaller than the wavelength of visible light, or b, the wavelength of electrons is much greater than the wavelength of visible light.

And the correct answer is a, the wavelength of electrons are much smaller than the wavelength of visible light.

Okay, another check for you.

Which of these images is most likely to have been taken using a scanning electron microscope? So pause the video, make your selection, and I'll feed back the correct answer in a moment.

Okay, hopefully you put down a as the correct answer.

Next question.

Which of the images is most likely to have been taken using a transmission electronic microscope? Is it a, is it b, is it c? Make your selection, pause the video, make your selection, and we'll feed back the correct answer in a moment.

Okay, the correct answer is c, c was made using a transmission electron microscope.

Okay, let's do some practise questions.

Let's run through those questions now.

Question one, what type of microscope was used to create the image above? Can you explain your answer? Question two, how was the image above produced? And question three, why do electron microscopes have a higher magnification than light microscopes? So pause the video and answer your questions now.

Okay, let's go through the answers.

So, let's look at question one.

What type of microscope was used? The answer being a transmission electron microscope.

Explanation: the image was too detailed to be a light microscope, so it must be from an electron microscope.

More information is that the image is in two dimensions, which is indicative of using a transmission electron microscope, 'cause the transmission electron microscope takes images of very thin two-dimensional sections of cells.

Question two, how is this image produced? The image was produced by firing a large beam of electrons through a very thin section of a specimen.

The image is produced based on the number of electrons that were transmitted through the sample.

So electrons are fired through the sample, some pass through, some are absorbed by more dense structures, and by collecting those electrons on a screen an image can be produced like the one of the nucleus above.

So here's the answers to question three.

Why do electron microscopes have a higher magnification than light microscopes? Electron microscopes use electrons instead of visible light.

Electrons have a shorter wavelength than visible light.

Electron microscopes have a resolution of 0.

002 micrometres compared with 0.

2 micrometres of a light microscope.

And the higher resolution means that images of a higher magnification can be viewed more clearly.

Okay, so let's get onto the third part of the lesson now, which is the size and scale of cells.

So how can we use light microscopes and electron microscopes to give us some information about size and scale? So in order to observe cells in detail, a microscope is necessary, and we're gonna use two units of length.

The first unit of length is the micrometre, and that is given the symbol of the Greek letter mu, which is a bit like a U with a tail on it.

You can see in brackets there, micrometre.

And also in nanometers, and we're gonna use these units of length as appropriate units to look at the size of sub-cellular structures and even cells themselves.

And you can see a scanning electron micrograph here on the right, and you can see a scale bar, and that scale bar has a length of 10 micrometres.

So when looking at objects the size of pollen grains, a micrometre is a suitable unit of length to use.

However, why might it be better to use nanometers rather than micrometres to measure the size of other sub-cellular structures, such as ribosomes? Well, the reason being that the units of length that is the micrometre is actually too large for some of the smallest sub-cellular structures, so we need a more appropriate units of measurement.

And in the case of really small sub-cellular structures like ribosomes, the nanometer is a more appropriate unit of length.

So let's look at the units used in microscopy in more detail.

So the units we've got here are the metre, the millimetre, the micrometre, and the nanometer.

Let's look at the relationship to other units.

So if we take the metre, there are 1,000 metres in one kilometre.

We have 1,000 millimetres in one metre.

We've got 1,000 micrometres in one millimetre.

So if we take one millimetre and divide that into 1,000 equal divisions, each one of those divisions is one micrometre in length.

Similarly, if we take one micrometre and divide that into 1,000 equally-spaced divisions, each one of those divisions will be 1/1,000 of a micrometre in length.

So finally, let's just compare and see how many of those units that are in one metre.

Well, if we look at the millimetre, there are 1,000.

Micrometre, one million.

And in the case of the nanometer, the smallest scale division that we have in this table, there are 1,000 million nanometers in one metre.

And that gives you some indication of the relative sizes of these units of length.

Okay, let's do a quick knowledge check now.

So, can you write down which of these letters represents the correct symbol for a micrometre? So pause the video, make your choice, and we'll go through the answer in a moment.

Okay, so hopefully you put down the correct symbol for a micrometre as d.

Well done if you got that.

Let's do another one.

Starting with the largest, sort the units of measurement into decreasing size order.

So we've got a, micrometre, b, metre, c, millimetre, and d, nanometer.

So pause the video, organise those units, and we'll feed back in a moment.

Okay, let's go through the answers now.

So, decreasing size order, so we're gonna start off with the metre being the largest unit of length.

Next unit of length is the millimetre, then the micrometre, then the nanometer.

Well done if you got those in the correct sequence.

Okay, now, bit more tricky now is converting between units, because when we do some calculations a little bit later, it's important that we get all our measurements in the same unit, and that's gonna require some conversion.

So, let's look at the unit in the left-hand column.

So we've got a metre, millimetre, micrometre, and nanometer.

So, if we want to take a metre and convert it into a millimetre, all we need to do is to multiply by 1,000.

Similarly, as we go down the table, if we want to convert millimetres to micrometres, we multiply that by 1,000.

And I'm sure you can guess what we do to convert micrometres into nanometers now.

Yep, we multiply it by 1,000.

Now, slightly trickier really is going from small to large again, right? So if we take the nanometer and convert it into micrometres, we have to divide by 1,000 this time.

Similarly, micro to millimetre, divide by 1,000.

And finally, milli to metre, we divide by 1,000 again.

Okay, so let's look at this.

Let's do an example.

A red blood cell has a diameter of 7.

5 micrometres.

So what is its diameter in nanometers? So if you remember from the previous slide, if we go from micrometre to nanometers, what we need to do is multiply by 1,000, because there are 1,000 nanometers in one micrometre.

So if we take 7.

5 micrometres and multiply by 1,000, we get 7,500 nanometers.

Now, what would its diameter in millimetres be? Let's look at this example.

So if we take 7.

5 micrometres, in order to convert that to millimetres, we need to divide by 1,000.

So 7.

5 divided by 1,000 equals 0.

0075 millimetres.

Okay, let's do a check to see how you can get on with these unit conversions.

So, let's look at the following problem.

A salmonella bacterium has a width of 0.

5 micrometres.

What is its diameter in nanometers and what is its diameter in millimetres? So you can use the worked example on the left side of the slide to help you answer this question.

So pause the video, calculate the diameter in nanometers and diameter in millimetres, and we'll feed back in a moment to see how you get on.

Okay, so let's look to see how you got on.

So, diameter in nanometers.

So if we've got 0.

5 micrometres, what do we need to convert this value into nanometers? We need to multiply it by 1,000.

So 0.

5 micrometres is the equivalent of 500 nanometers.

Now, the diameter of that bacterium in millimetres, right, if we go from micrometres to millimetres, just like the example on the left panel, we take 0.

5 divided by 1,000, and that will equal 0.

0005 millimetres.

Okay, so once we've got some idea about the the relative sizes of objects, let's compare different organisms and different cells.

And we can see here we've got a selection of organisms. We have the woodlouse, we have a palisade cell, we have a red blood cell, a bacterium, and we've also got a virus there on the right.

So let's look at a woodlouse.

So a woodlouse is about seven millimetres in length.

Palisade cells are around about 70 micrometres.

Red blood cell, about seven micrometres.

Bacteria, really small, one micrometre in length.

And the smallest here is the virus, which isn't strictly speaking an organism.

However, it's got the smallest size and it has a diameter of 100 nanometers, so really, really small.

Now, if we're comparing sizes, we might want to work out how bigger one object is compared to another.

So if we look at a palisade cell, okay, so a palisade cell is 70 micrometres.

If we want to know how much bigger or how many times bigger the palisade is compared to a red blood cell, all we need to do is to divide the large by the small in this case.

So length of the palisade cell being 70, length of the red blood cell, seven micrometres, 70 micrometres divided by 7 micrometres gives us a factor of 10.

So the palisade cell is 10 times larger than a red blood cell.

Well, clearly, looking at that, if we're comparing sizes, it's really important that we get the objects that we're trying to compare in exactly the same unit or we're gonna be a factor of 1,000 out in our calculations.

So, let's have a look at this worked example and you can have a go at calculating a similar problem.

So what is the length of the woodlouse compared to the length of the palisade cell? So the woodlouse is seven millimetres.

The palisade cell is 70 micrometres.

Now, we can't divide 7 millimetres by 70 micrometres because we're a factor of 1,000 out in each of these units, because there are 1,000 micrometres in one millimetre.

So, what we can do here is we can convert everything, or the millimetres, I should say, into micrometres.

So, a woodlouse, seven millimetres long, is 7 times 1,000.

That is 7,000 micrometres.

We know from this light micrograph the length of the palisade cell is 70 micrometres.

Therefore, 7,000 divided by 70 equals 100.

So we can say that the woodlouse is 100 times longer than the palisade cell.

So, using this worked example as a guide, let's see if you can answer or work out the following problem.

So what is the length of the bacterium compared to the diameter of the virus? The bacterium, the length is one micrometre.

The virus, the diameter is 100 nanometers.

So pause the video, have a go answering the question using the left-hand panel of the slide as a guide, and I'll go through the correct answers in a moment.

Okay, so what are we gonna do to these units? Well, the bacterium is in micrometres, the virus is in nanometers, so we've got to get them both into the same unit.

So we can do here is we can convert both into nanometers.

So, if we take one micrometre, there are 1,000 nanometers in one micrometre.

So therefore, if we multiply 1 by 1,000, that gives the number of nanometers.

In this case it's 1,000.

So the length of the bacterium is 1,000 nanometers.

We know that the diameter of the virus is 100 nanometers.

So dividing 1,000 by 100, we get the value of 10, and we can therefore say that the bacterium is 10 times longer than the diameter of the virus.

Well done if you got the correct answer there.

Okay, so what we can do now is that we can calculate magnification using the real size of an object and the image size of a magnified image using the following equation.

And that equation is that magnification equals the size of the image, so that's the size of the magnified image, divided by the size of the real object.

So let's just see how we can use that equation now.

So, calculate the magnification of a red blood cell with a diameter of eight micrometres in an image measuring 5.

6 millimetres.

So, first thing we need to do is to get both of these values in the correct units.

So here what I'm gonna do is I'm gonna convert both of these lengths into micrometres.

So 5.

6 millimetres equals 5,600 micrometres, because there are 1,000 micrometres in every millimetre.

If I take the magnification equation from the previous slide, magnification equals the image length divided by the real length.

So if we take 5,600 micrometres and divide that by eight micrometres, we come up with the value of 700 times, okay? So the magnification of a red blood cell is 700 times.

Now, you have a go at the following problem.

Again, use the worked example as a guide to help you answer the question.

And the question is, calculate the magnification of an E.

coli cell with a length of 1.

7 micrometres in an image measuring 10.

2 millimetres.

and the equation is given to you there, that magnification equals the image size divided by the real size.

So pause the video, calculate the answer, and we'll work through that question in a moment.

Okay, let's see how you got on.

So, firstly, 10.

2 millimetres.

Let's convert 10.

2 millimetres into micrometres so we've got all the values, all the numbers in the correct units.

So, 10,200 micrometres, which represents the image size.

The real size is 1.

7.

So all we need to do now is to take 10,200, divide it by 1.

7, and that will give us the value of 6,000 times.

So the magnification is 6,000 times.

Okay, let's do some practise questions.

So we've got a table here and we've got some values in the table.

What I'd like you to do is convert the following to complete the table.

So we have 0.

33 metres, 4.

5 millimetres, and 67 micrometres.

So complete that table for question one.

Question two, a cell has a diameter of 75 micrometres.

A mitochondrion has a diameter of 0.

6 micrometres.

What is the diameter of the cell compared to the diameter of the mitochondrion? So pause the video, calculate those answers, and I'll go through those in a moment.

Okay, let's go through question one.

Convert the following to complete the table.

So, let's look at the first row.

So in metre, 0.

33 metres.

That represents 330 millimetres, because don't forget, as we go from big to small, we multiply it by 1,000 each time.

So because there are 1,000 millimetres in a metre, if we multiply this value 0.

33 by 1,000, we get 330 millimetres.

330,000 micrometres, because there are 1,000 micrometres in a millimetre.

And finally, how many nanometers have we got? Well, if we take the number of micrometres and multiply by 1,000 again we have 330 million nanometers.

Let's look at the second row.

So we've got 4.

5 millimetres.

Well, how many metres is that? Bit more tricky this time, because when we're going from millimetres to metres we have to divide by 1,000.

So 4.

5 millimetres is 0.

0045 metres.

Now, for milli to micro we multiply by 1,000, so we have 4,500 micrometres.

And finally, in terms of nanometers, 4,500 micrometres are 4,500,000 nanometers.

Okay, last one.

If we start off with 67 micrometres, how many millimetres is that gonna be? Well, if we're going from micro to milli, we need to divide by 1,000.

So 67 micrometres is the same as 0.

000067 metres.

0.

067.

So in each case from the micrometre, we divide it by 1,000 first of all to get millimetres, and then take 0.

067 and divide by 1,000 again to get metres.

If we take micrometres and convert that to nanometers, we just need to multiply by 1,000 in this case.

So the number of nanometers is 67,000.

Question two, a cell has a diameter of 75 micrometres, mitochondrion a diameter of 0.

6 micrometres.

What's the diameter of the cell compared to the diameter of the mitochondrion? The answer being, if we take 75 and divide by 0.

6, that equals 125 times larger.

Now, we didn't need to do any unit conversions there because both values were in micrometres.

So well done if you got those answers correct.

Okay, let's do some more practise questions.

Question three, a human cheek cell has a diameter of 60 micrometres.

M.

tuberculosis cell has a diameter of 0.

3 micrometres.

How many times bigger is the human cheek cell than the TB cell? Question four, calculate the magnification of a mitochondrion with a length of 0.

8 micrometres in an image measuring 14 millimetres.

And question five, calculate the magnification of a flea's leg with a length of 12 millimetres in an image measuring 15 centimetres.

So, you're gonna need your magnification equation for some of these questions, okay? So pause the video, have a go at working those out, and I'll give you the right answers in a moment or two.

Okay, so let's look at the answers.

Here we go.

Okay, question three.

So, we've got two numbers here.

We've got 60 micrometres, we've got 0.

3 micrometres.

Now, because they're both in the same units, we don't need to do any unit conversions.

So if we take 60, the 60 micrometres which represents the diameter of the human cheek cell, and divide that by 0.

3 micrometres, which represents the diameter of the TB cell, we get a value of 200 times.

So the human cheek cell is 200 times larger.

Let's look at question four.

We have a mitochondrion with a length of 0.

8 micrometres in an image measuring 14 millimetres.

So, calculating the magnification, well what we need to do here is take the image length and divide that by the real length.

That's from your magnification equation that we looked at before.

However, we've got different units.

So, what we can do here is we can convert everything into micrometres, okay? So, 0.

8 micrometres, 14 millimetres.

Every millimetre, remember, represents 1,000 micrometres.

So if you multiply 14 by 1,000, that gives us the image size in micrometres, and that's 14,000.

If we divide that by 0.

8, that gives us a value of 17,500 times magnification.

Okay, question five, calculate the magnification of a flea's leg with a length of 1.

2 millimetres in an image measuring 15 centimetres.

Okay, slightly tricky now because we're using centimetres and millimetres and we haven't looked at that so far, okay? So, easiest way of doing this is to leave 1.

2 in millimetres.

If we take 15 centimetres.

Now, don't forget, there are 10 millimetres in every centimetre.

So 15 centimetres is the same as 150 millimetres.

So, if we take 150 millimetres and divide by 1.

2 using the magnification equation again, we end up with a value of 125 times magnification.

So well done if you've got those unit conversions correct and you got the right answer.

If you didn't, make sure that you go through these a little bit later on, understanding how we go from milli to micro, micro to nano, and also the slightly awkward centimetre as a unit of length for such conversions.

Okay, let's summarise what we've learned today.

So the whole lesson has been based on light microscopy, electron microscopy, and the size and scale of cells and using some maths to work out to compare the relative sizes and magnification of objects that can be observed by both light microscopes and electron microscopes.

So the light microscope uses light and a system of glass lenses to generate magnified images of small objects.

And using the light microscope, we can see some sub-cellular structures, such as the nucleus, such as chloroplasts, the cell membrane, et cetera.

Electron microscopes use beams of electrons instead of light rays, and because they use electrons they have a much higher resolution and magnification compared to light microscopes.

And this is because the wavelength of electrons is up to 100,000 times smaller than that of visible light.

And using electron microscopes, this really has revolutionised biologists' understanding of sub-cellular structures in both eukaryotic cells, so plant, animal, fungi, protist cells, and also bacteria and viruses.

Now, the size and scale of cells and sub-cellular structures can be compared using appropriate units of length.

And those units that we've looked at today are the millimetre, the micrometre, and the nanometer.

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

Goodbye.