2.1.1 - Cell Structure (set D - Microscopes)

Question 1

Define magnification?

Answer 1

Magnification tells you how many times bigger the image produced by the microscope is than the real-life object you are viewing

Question 2

Define resolution - give an exmaple?

Answer 2

Resolution is the ability to distinguish between objects that are close together (eg the ability to see two structures that are very close together as two separate structures)

Question 3

Give three different types of microscopes?

Answer 3

• optical microscopes or light microscope
• electron microscopes
• laser scanning confocal microscopes

Question 4

Give the equation for magnification?

Answer 4

Magnification = size of image / actual size of object

Question 5

Explain how optical/light microscopes work?

Answer 5

Use light and objective lens to form an image

• max magnification is x1500

Question 6

Explain the main disadvantage of light microscopes

Answer 6

limited resolution - due to using light, it’s impossible to distinguish between two objects that are closer then the wavelength of light (500-650 nm)

• cannot be used to observe smaller organelles such as ribosomes, the endoplasmic reticulum or lysosomes

Question 7

Explain the advantages of light microscopes?

Answer 7

• can be used to observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplast
• produces a colour image
• can examine both dead and live specimens

Question 8

Explain how electron microscopes work?

Answer 8

They use electrons to form an image, which greatly increases the resolution compared to optical microscopes, giving a more detailed image

• magnification is around 1,500,00 x

Question 9

Explain how electron microscopes have a much higher magnification then optical microscopes?

Answer 9

A beam of electrons have a much smaller wavelength than light so the microscope can distinguish between two objects that are extremely close together

• can observe ribosomes, endoplasmic reticulum and lysosomes

Question 10

Give both types of electron microscopes?

Answer 10

• transmission electron microscopes (TEMs)
• scanning electron microscopes (SEMs)

Question 11

Explain how transmission electron microscopes work (TEMs)?

Answer 11

Uses electromagnets to focus a beam of electrons which is transmitted through the specimen - denser parts of the specimen absorb more electrons so will appear darker on the image produced, producing a contrast between different parts

• focus image onto fluorescent screen or photographic plate using magnetic lenses

Question 12

Give two advantages of transmission electron microscopes (TEMs)?

Answer 12

• give high-resolution images which are more detailed
• allows the internal structures within cells (or organelles) to be seen

Question 13

Give three disadvantages of transmission electron microscopes (TEMs)?

Answer 13

• can only be used with very thin specimens or thin sections of the object being observed
• cannot be used to observe live specimens - due to the vacuum and all water being removed from the specimen
• do not produce colour image’s unlike optical microscopes

Question 14

Explain how scanning electron microscopes work (SEMs)?

Answer 14

SEMs scan a beam of electrons across the specimen - the beam bounces of the surface of the specimen and the electrons are detected forming an image

• produces three-dimension images that show the surface of the specimen

Question 15

Give two advantages of scanning electron microscopes (SEMs)?

Answer 15

• can be used on thick 3D specimens
• they allow the external 3D structures of the specimen to be observed

Question 16

Give three disadvantage of scanning electron microscopes (SEMs)?

Answer 16

• give lower resolution images then TEMs
• cannot be used to observe live specimens
• also do not produce a colour image

Question 17

Explain how laser scanning confocal micrscopes work?

Answer 17

Involves the cell being stained with fluorescent dyes and being scanned with a laser beam which is reflected by the dye - multiple depth of the tissue scanned builds up the image

Question 18

What are the advantages of laser scanning confocal microscopes?

Answer 18

• can be used on thick or 3D specimens
• allow external 3D structures of the specimen to be observed
• clear image due to the high resolution (laser beam can be focused at a very specific depth) - can see structures of the cytoskeleton

Question 19

What are the disadvantages of laser scanning confocal microscopes?

Answer 19

• it is a slow process and take a long time time to obtain an image
• laser has potential to cause photo damage to the cells

Question 20

Give some of the tools used in the preparation of a microscope slide?

Answer 20

• forceps
• scissors
• scalpel
• coverslip
• slides
• pipettes
• staining solution

Question 21

Give the function of the turret on a light microscope?

Answer 21

Turret rotates to bring the objective lenses into place

Question 22

Give the function of the stage on a light microscope?

Answer 22

Where the slide being observed is placed

Question 23

Give the function of the condenser on a light microscope?

Answer 23

Used to vary the intensity of light reaching the object

Question 24

Give the function of the coarse focus on a light microscope?

Answer 24

Used to focus the low and medium power objective lenses

Question 25

Give the function of the fine focus on a light microscope?

Answer 25

Used to focus the high power objective lens

Question 26

Explain how to prepare a liquid specimen for a slide?

Answer 26

1) add a few drops of the sample to the slide using a pipette 2) cover the liquid with a coverslip - lowering it down gently at a 90 degrees angle 3) gently press down to remove air bubbles

Question 27

Explain how to prepare a solid specimen for a slide?

Answer 27

1) use scissors to cut a small sample of the tissue 2) peel away a very thin layer of cells from the tissue using a scalpel and forceps 3) place the thin layer on the slide and apply a stain 4) gently place a coverslip on top and press down to remove air bubbles

Question 28

Why do we add a drop of water with a pipette to the cells on the slide?

Answer 28

The thin layers of material on the slides can dry up rapidly so by adding a drop of water to the specimen (beneath the coverslip) we can prevent the cells being damaged by dehydration

Question 29

Why do you first observe the specimen with a low power objective lens?

Answer 29

- its easier to find what you are looking for in the field of view - helps prevent damage to the lens or coverslip in case the stage is raised to high

Question 30

Explain 3 reasons and fixes which would cause unclear or blurry images?

Answer 30

- switching to a lower power objective lens and using the coarse focus can give a clearer image - wether the specimen is thin enough to allow light to pass though - might be cross contamination with foreign cells or bodies

Question 31

Define sectioning when is it used?

Answer 31

Involved in the preparation of solid specimens - is the process of cutting the specimen into very thin slices using a sharp blade

Question 32

What is a dry mount?

Answer 32

When the specimen is placed directly on the slide - used for samples like pollen, hair, feathers and plant material

Question 33

What is a wet mount?

Answer 33

When the specimen is suspended in a liquid such as water or an immersion oil - used for aquatic samples and living organisms

Question 34

Explain what squash slides are, and what they’re used for?

Answer 34

A wet mount is first prepared and then the sample is squashed by pressing down the cover slip with a lens tissue - helps obtain very thin samples - used to look at cell division in root tips

Question 35

Explain what a smear slide is, what samples it’s used for?

Answer 35

Involves the edge of a slide being used to smear the sample which creates a thin, even coating - a coverslip is then placed over the sample - used when observing samples of blood

Question 36

Why are samples stained when preparing a slide?

Answer 36

To enhance the visualisation of the cell or certain cellular components - stains increase contrast as different components within a cell take up stains to different degrees, which is important as the cytosol of cells and other cellular structures are often transparent and therefore hard to view

Question 37

Explain what differential staining is?

Answer 37

Specimens are stained with multiple dues to ensure different tissues within the specimen show up - this works as certain tissues absorb certain dyes due to their chemical nature

Question 38

Explain two common dyes used and what colour they stain?

Answer 38

- toluidine blue - turns cells blue - phloroglucinol - turns cells red/pink

Question 39

Explain how staining is done for electron microscopes and why these dyes are used?

Answer 39

Specimen must be stained to absorb the electrons, heavy metal compounds are often used as dyes because they absorb electrons well - this results in the tissue showing up black or different shades of grey - colour can be added with software later

Question 40

Give an exmaple of two dyes used for electron microscopes?

Answer 40

- osmium tetroxide - ruthenium tetroxide

Question 41

Explain what fixing is in regard to preparation of microscope slides?

Answer 41

Involves chemicals like formaldehyde being used to preserve specimens in a near-natural state

Question 42

What is a graticule?

Answer 42

Small disc that has an engraved ruler, it can be placed into the eyepiece of the microscope to act as a ruler in the field of view - has no fixed units so it must be caliberated

Question 43

Explain how the graticule is calibrated?

Answer 43

Calibrated for the objective lens that is in use by using a scale engraved on a microscope slide (stage micrometer) - by using both, the number of micrometers each graticule unit is worth can be worked out - allows the graticule to be used as a ruler

Question 44

What is a stage micrometer used for?

Answer 44

Used to find out how many micrometers each graticule unit represents