Electron microscopes

Question 1

what are the types of microscopes apart from light microscopes

Answer 1

electron and laser scanning

Question 2

what is special about each type of microscope

Answer 2

they have strengths and weaknesses and are used to observe different things

Question 3

how does an electron microscope work

Answer 3

uses a beam of fast travelling electrons with wavelength of 0.004nm. the electrons are fired from a cathode and focused on a screen or photographic plate

Question 4

why does an electron microscope have a better resolution

Answer 4

Because electrons are smaller than photons of light, electron microscopes have a much greater resolution than optical microscopes.

Question 5

what is special about the electrons in an electron microscope

Answer 5

the fast moving electrons have a wavelength of 125000x smaller than central part of visible light spectrum resulting in a better resolution.

Question 6

why can’t electron microscopes be seen in colour

Answer 6

unlike light microscopes, there is only one wavelength transmitting electrons. the colour has to be determined by educated guess work and then added

Question 7

what are the two types of electron microscope

Answer 7

transmission and scanning

Question 8

how is the specimen prepared in a transmission electron microscope

Answer 8

covered in resin to form a solid block and cut into thin slices. the specimen is chemically fixed by dehydration and stained with heavy metals salts.

Question 9

what happens to electrons in a TEM

Answer 9

parts of specimen that are heavily stained absorb the electrons. electrons that pass through and around the specimen giving contrast are focused by electromagnets onto fluroscent screens or photographic film

Question 10

what are the details of the image shown in TEM

Answer 10

the image is 2D with magnification of x500 000 up to 2 millions times. details of internal subcellular structures can be seen.

Question 11

how is the specimen prepared in a scanning electron microscope

Answer 11

specimen placed in a vacuum and coated with a thin layer of metal to improve conductivity and contrast

Question 12

what happens to the electrons in a SEM

Answer 12

electrons dont pass through the whole speciumen but cause secondary electrons to reflect off the surface of the specimen. they can be focused on a screen

Question 13

what the details shown in the images in a SEM

Answer 13

3D images of whole cells, tissues and organisms. magnification of x100 000. computer software programmes add false colours.

Question 14

what is similar about both electron microscopes

Answer 14

both are large, expensive and need great deal of skill and training to use.
specimens even whole ones in SEMs have to be dead as they are viewed while in a vaccuum. metallic salt stains are potentially hazardous.

Question 15

compare the initial cost between light and electron microscopes

Answer 15

light - cheap (£100-150)
electron - expensive (over £50,000)

Question 16

compare the operation cost between light and electron microscopes

Answer 16

light - inexpensive to operate
electron - expensive to produce electron beam

Question 17

compare the size and ease of use between light and electron microscopes

Answer 17

light - small and portable
electron - large and needs to be installed. requires special rooms

Question 18

compare the specimen preparation between light and electron microscopes

Answer 18

light - simple and easy and the material is rarely distorted by the preparation
electron - complex and lengthy preparation which distorts material

Question 19

compare the requirement of a vacuum between light and electron microscopes

Answer 19

light - not required
electron - required

Question 20

compare the colour of the sample between light and electron microscopes

Answer 20

light - natural colour of sample is seen (stain is used)
electron - black and white images produced which can be coloured digitally

Question 21

compare the magnification between light and electron microscopes

Answer 21

light - up to x2000
electron - over x500000

Question 22

compare the resolution between light and electron microscopes

Answer 22

light - 200nm. limited so cannot see cell detail
electron - TEM = 0.5nm about 1nm. excellent resolution and can see most minute cell details. SEM = 3-10nm

Question 23

compare whether specimens need to be dead or alive in light and electron microscopes

Answer 23

light - you can see live and dead
electron - only dead as they must be fixed in plastic and viewed in a vaccuum

Question 24

compare whether staining is necessary in light and electron microscopes

Answer 24

light - stains are needed to make cells visible
electron - electron beam can damage specimens so they must be stained with an electron dense, heavy metal salt

Question 25

what are the advantages of electron microscopes

Answer 25

• resolution is high (0.2nm)
• we can see structured detailed images of organelles inside the cell
• the SEM produces 3D images that can uncover details of contorts and cellular tissue arrangement - not possible with TEM microscopes

Question 26

what are the disadvantages of electron microscopes

Answer 26

• electron beams are defelected by air molecules so the sample has to be in a vacuum - so the sample has to be dead
• electron microscopes are very expensive (minimum of £250,000)
• preapring slides and samples for electron microscopes requires a high level of skill and training as the slides have to be very thin