Lesson video

Okay.

Hello, and welcome.

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: Including Standard Form.

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 what 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 it looks very different to light microscopes that we have today, essentially is very, very similar because in both modern microscopes or modern light microscopes and the light microscope that Robert Hooke used used 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, we can see the nucleus, those red structures, but we can't see any smaller subcellular 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 subcellular 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 details? 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 1, what is a light microscope? 2, define the term magnification, 3, define the term resolution, and 4, what is the maximum magnification and maximum resolution of a light microscope? So, pause the video, write down your answers, and I'll feedback in a moment.

Okay, so let's go through question 1 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 2, what does the term magnification mean? This means making small objects appear larger in order to see more detail.

3, define the term resolution.

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

And 4, 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 of magnification than the light microscope.

The reason being is that the wavelength of electrons are up to 100,000 times smaller than that of 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 2-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 3-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 of magnification of electron microscopes means that biologists can study cells in much finer detail, higher magnification and higher resolution.

This means that some 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 nucleoli, 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 feedback 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 feedback 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 electron microscope? Is it a, is it b, is it c? Make your selection.

Pause the video, make your selection, and we'll feedback 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 1, what type of microscope is used to create the image above? Can you explain your answer? Question 2, how was the image above produced? And question 3, 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 1, 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 2-dimensional sections of cells.

Question 2, 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 images produced based on the number of electrons that were transmitted through the sample.

So, electrons are fired through the sample, some passed through, some are absorbed by more dense structures.

And by collecting those electrons on a screen, an image could be produced like the one at the nucleus above.

So, here's the answers to question 3.

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, let's move on to the third part of today's lesson, which is the size and scale of cells.

So, to observe cells in detail, a microscope is necessary.

And we're gonna use the following units, we're gonna use the unit micrometre, which you can see there.

And the micrometre, the micro or mu symbol is a Greek letter, which looks like a u with a tail on it.

And we're also gonna be looking at nanometers, or nm, when measuring cells and cell structures.

So, if we look at the electron micrograph on the right there, we can see that there's a scale bar, and the scale bar represents 10 micrometres.

Question for you, why do you think it'd be better to use nanometers rather than micrometres to measure subcellular structures such as ribosomes? Okay, well, the answer really is that for very, very small structures, we need an appropriate unit of measurement.

And an appropriate unit of measurement for something as small as a ribosome will be the nanometer.

And for bigger structures, such as mitochondria or the nucleus, a micrometre unit or the micrometre unit is a better unit, a more appropriate unit to measure the size.

Okay, so let's look at some units used in microscopy in this table.

So, we have in the left-hand column, we've got the metre, the millimetre, the micrometre, and the nanometer.

Looking at the relationship to other units, you can read through there for yourself.

So, a metre is 1/1,000th of a kilometre.

A millimetre is 1/1,000th of a metre.

A micrometre is 1/1,000th of a millimetre, so if you imagine 1 millimetre on your ruler divided into 1,000 equal divisions, each 1 division represents 1 micrometre in length.

And the nanometer is 1/1000th of a micrometre.

So, if you look in the third column, this shows you how many there are in a metre.

So, a millimetre, there's 1,000 micrometre, there's a million, and a nanometer, there are an astonishing 1,000 million nanometers in 1 metre.

Now, what we're gonna address here is how to represent some of these numbers, either very large numbers or very small numbers in standard form.

And we can see in the right-hand column here, we can see the metre, 1 times 10 to the 0, millimetre, 1 times 10 to the -3 of a metre.

We've got micrometre, 1 times 10 to the -6 of a metre, so 1/1,000,000th of a metre.

And the nanometer at 1 times 10 to the -9, so we're talking extremely small structures when using the nanometer unit of length.

So, let's do a quick knowledge check.

What is the correct symbol for a micrometre? So, pause the video, make your choice, and I'll feedback the answer in a moment.

So, the correct symbol for a micrometre is d.

Well done if got that right.

Now, starting with the largest, can you sort the units of measurement into decreasing size order? So, pause the video, you know the score, and I'll be back in a short while to give you the answers.

Okay, so the answer is the metre, the largest unit of length, the millimetre, 1/1000th of a metre, the micrometre, 1/1000th of a millimetre, and finally, the nanometer at 1/1000th of a micrometre.

So, as we previously discussed, when working with very small or very big numbers, we use standard form.

So, how do we write standard form numbers? Well, standard form numbers are written as shown on this slide as A, a number, times 10 to the power n.

Now, that number has got to be greater than 1, but less than 10.

And the n is the power of 10.

So, let's look at this example here.

One of the bacteria in this image are 2.

6 micrometres in length.

Now, if we take 2.

6 micrometres and convert that into millimetres, we divide by 1,000, that equals 0.

0026 millimetres.

If we take 2.

6 micrometres and convert it into nanometers because there are 1,000 nanometers in a micrometre, we multiply 2.

6 by 1,000, so we get 2,600 nanometers.

Now, let's convert both of those into standard form.

So, 0.

0026 millimetres is equivalent to 2.

6 times 10 to the -3 millimetres in standard form.

2,600 nanometers is equal to 2.

6 times 10 to the 3 nanometers.

So, can you have a look at these numbers and answer which of the following are correct? So, I'll pause the video, make your selection, and I'll feedback the answers in a moment.

Okay, so which are correct? 1,000 equals 1 times 10 to the 3, so a is correct there.

0.

066 is 6.

6 times 10 to the -2.

Now, 476,000 is not equal to 4.

76 times 10 to the -5, it would actually be equal to 4.

76 times 10 to the 5.

And finally, d is correct because that value there is equivalent to 5.

5 times 10 to the -5.

So, well done if you've got those answers.

Now, let's look about about how to convert between units.

So, let's review that again.

We've got the following units: millimetre, micrometre, nanometer, and the metre.

So, if we're converting metres to millimetres, we simply take the number that we have and multiply by 1,000.

Similarly, if we have 1 millimetre and we want to convert that into micrometres, we multiply that by 1,000 too.

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

Yep, we'd multiply by 1,000 again.

Now, going in the reverse direction, if we're converting nanometers into micrometres, we'd simply divide by 1,000, micrometres to milli, divide by 1,000, and millimetres to metres, we divide by 1,000.

And we can see the division of a metre of each of those units in standard form in the right-hand column.

So, metre is 1, millimetre, 1 times 10 to the -3 of a metre, micrometre is 1/1,000,000th, 1 times 10 to the -6.

If a metre and a nanometer is 1 times 10 to the -9 of a metre.

Okay, let's do a worked example.

So, let's look at the problem on the left hand panel of the slide.

A red blood cell has a diameter of 7.

5 micrometres, give your answers using standard form.

So, firstly, what is its diameter in nanometers? So, if we take 7.

5 micrometres, each micrometre has 1,000 nanometers in it.

So, we take 7.

5 and we multiply by 1,000, that gives us 7,500 nanometers.

Converting that to standard form gives us 7.

5 times 10 to the 3 nanometers.

Secondly, what is its diameter in millimetres? Well, if we take 7.

5 micrometres and we convert that to millimetres, we divide 7.

5 by 1,000.

That gives us a value of 0.

0075 millimetres.

Writing that in standard form gives 7.

5 times 10 to the -3 millimetres.

So, pause the video, have a go at the problems on the right-hand side, and I'll give you the correct answers in a moment.

Okay, let's look at the problem.

A salmonella bacteria has a width of 0.

5 micrometres.

Give your answer using standard form.

So, firstly, what is its diameter in nanometers? If we take 0.

5 micrometres and convert that to nanometers, we multiply by 1,000, that gives us 500 nanometers.

Writing that in standard form gives us 5 times 10 to the 2.

Second question, what is its diameter in millimetres? So, 0.

5 micrometres, if we convert micrometres to millimetres, we divide by 1,000, and that gives us 0.

0005.

Writing that in standard form, 5 times 10 to the -4 millimetres.

So, we can compare sizes of organisms or sizes of cells or other structures, and we've got some examples on how to do that here.

We've got the woodlouse, it's length is 7 millimetres.

Palisade cell, the length is 70 micrometres.

Red blood cell, the diameter is 7 micrometres.

Bacterium, really small, 1 micrometre in length.

And virus the smallest with a diameter of 100 nanometers.

So, we can ask the question, how many times bigger is a palisade cell compared to a red blood cell? How do we do that? Well, we take the 70 micrometres of the palisade cell, we divide 7 micrometres, which is the diameter of the red blood cell, and that gives us a value of 10 times.

So, it is really important when we're comparing sizes that we work in the same unit.

So, in this case, we're dividing both numbers by the same units, the unit being the micrometre.

So, let's do a worked example.

What is the length of the woodlouse compared to the length of the palisade cell? So, the woodlouse length is 7 millimetres, the palisade cell length is 70 micrometres.

So, we can't divide 7 millimetres by 70 micrometres because if we did this, we're gonna be out by an order of magnitude of 1,000 because there are 1,000 micrometres in a millimetre.

So, what we need to do, first of all, is to convert both to the same units.

In this case, we're converting both to micrometres.

So, if we take 7 millimetres and multiply that by 1,000, we get 7,000 micrometres.

If we divide that by the length of the palisade cell, which is 70 micrometres, the answer is that the length of the woodlouse is 100 times longer than the palisade cell.

So, pause the video and have a go at this problem, and I'll go through the correct answers in a moment.

Okay, so the question is asking us what is the length of the bacterium compared to the diameter of the virus? The bacterium, the length is 1 micrometre, the virus, the diameter is 100 nanometers.

So, we've got to get both of these, both the length of the bacterium and the diameter of the virus in the same units.

So, here what we can do is we can take the micrometre, 1 micrometre, and convert into nanometers by multiplying by 1,000.

All we need to do now is to take the 1,000 micrometres, which is the length of the bacterium, divided by 100 nanometers, which is the diameter of the virus, and that gives us a factor of 10.

So, the bacterium is 10 times longer than the diameter of the virus.

So, this gives us an idea of the relative scale of these organisms. Now, we can calculate magnification using the real size of an object and the image size of a magnified image using the following equation where magnification equals the size of the image divided by the size of the real object.

So, let's have a practise using that equation.

And here's a problem on the left-hand side to do as a work example.

Calculate the magnification of a red blood cell with a diameter of 8 micrometres in an image measuring 5.

6 millimetres.

So, the first thing we need to do is to get both of these numbers into the same units.

So, 5.

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

If we take the equation from the previous slide, magnification equals image divided by real, we take 5,600 micrometres divided by the diameter of 8 micrometres, and that gives a value of 700 times.

So, the red blood cell has been magnified 700 times by the microscope.

Now, you pause the video, have a go at working on the magnification from this information on the right hand panel.

So, pause the video, and we'll feedback in a moment.

Okay, so the question's asking, calculate the magnification of an E.

coli cell with a length of 1.

7 micrometres in an image measuring 10.

2 millimetres.

So, selecting the correct equation, we have magnification equals image divided by real.

Now, getting both of these numbers into the correct units.

So, 10.

2 millimetres equals 10,200 micrometres.

So, image size is 10,200 micrometres, divide that by 1.

7 micrometres, gives us a magnification of 6,000 times.

So, well done if you've got the correct answer there.

So, let's do another quick knowledge check, and let's do another worked example.

So, we've got calculate the real size of a mitochondrion where the image size is 0.

3 centimetres and it's magnified 5,000 times.

However, this time we're gonna give our answer in millimetres using standard form.

So, let's take 0.

3 centimetres and convert that into millimetres.

So, 0.

3 centimetres multiplied by 10 because there are 10 millimetres in 1 centimetre gives us 3 millimetres.

We take the equation and rearranging it there, the real size equals the image size divided by the magnification, which means we have 3 millimetres divided by 5,000 times.

That gives us an answer of 0.

0006, which in standard form equals 6 times 10 to the -4 millimetres.

Now, part 2, can you convert that answer to micrometres? Well, to convert to micrometres from millimetres, hopefully you've remembered now that we multiply by 1,000 we're taking 6 times 10 to the -4 multiplied by 1,000 gives us 0.

6 micrometres.

Now, have a go answering this question on the right hand panel.

Pause the video, and I'll give you the correct answer in a moment.

Okay, so the question is asking, calculate the real size of a coronavirus when the image size is 8.

4 times 10 to the -3 centimetres and it is magnified 7,000 times.

Give your answer in millimetres using standard form.

So, the first thing we need to do is to convert that into millimetres.

So, 8.

4 times 10 to the -3 centimetres, if we multiply that by 10, that will give us a value of 0.

84 millimetres.

Selecting the correct equation to use.

real size equals image size divided by magnification.

So, 0.

84 millimetres divided by 7,000 gives us a value of 0.

00012.

And writing that in standard form gives us a value of 1.

2 times 10 to the -4 millimetres.

Again, really well done if you got the correct answer.

Finally, can you convert your answer to micrometres? Well, let's see if we can do that.

So, taking 1.

2 times 10 to the -4 millimetres.

Hopefully, you've understood now that we take millimetres and we multiply by 1,000 because there are 1,000 micrometres per millimetre.

So, taking that value and multiplying by 1,000 gives us a value of 0.

12 micrometres.

Okay, let's have a look at a bit more information about the size and scale of cells.

So, what I'd like you to do is looking at this table, we've got metre, millimetre, micrometre, and nanometer.

There are some missing values there.

What I'd like to do first of all is to convert the units that you are given and use those to complete the entire table.

Secondly, can you answer question 2? So, pause the video, have a go at that, and I'll speak to you in a few moments.

Okay, let's have a look at the answers.

So, what you can do here is rather than me talking through all of these answers is pause and check your answers and then start the video when you're ready to continue.

Okay, so hopefully you got the correct answers there.

Okay, so answer the following three questions.

So, pause the video, answer the questions, and we'll feedback in a moment.

Okay, question 6, let's go through the answer.

Calculate the real size of a mite with an image size of 8.

75 times 10 to the -2 metres magnified 50 times.

Give your answer in metres using standard form.

So, firstly, 8.

75 times 10 to the -2 is equivalent to nor 0.

0875 metres.

If we divide that by the magnification, by rearranging the magnification equation, that gives us a value of 1.

75 times 10 to the -3 metres.

So, well done if you've got that answer.

Question 7, the length of a palisade cell is 300 times that of a ribosome.

The ribosome length is 20 nanometers, what is the length of the palisade cell? Give your answers in nanometers using standard form.

So, if we take 300 times magnification, multiply that by 20 nanometers, we get a value of 6,000 nanometers, standard form 6 times 10 to the 3 nanometers.

Finally, the palisade cell is magnified 16,000 times, give the image size in millimetres.

So, if we take 6,000, so we got 6,000 nanometers.

Now, tricky this one, but there are 1 million nanometers in 1 millimetre.

So, if we take 6,000, divide it by 1 million, we get no.

006 millimetres.

Now, if we take that value, 0.

006, multiply by 16,000 times magnification, that gives us a value of 96 millimetres.

So, that was a tricky one, well done if you got that one correct.

Okay, let's summarise today's lesson then.

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

Electron microscopes are different in so much that they use beams of electrons instead of rays of light.

And because they use electrons, they have a much higher resolution of magnification compared to light microscopes.

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

Electron microscopy has increased our understanding of subcellular structures.

And finally, the size and scale of cells and subcellular structures can be compared in micrometres, nanometers, and millimetres using standard form.

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