3A Observing cells

Question 1

How does light microscope work? (4)

Answer 1

• thin slice of biological material placed on stage
• illuminated from bellow
• objective lens produces magnified and inverted image
• eyepiece lens focuses image at eye

Question 2

Example of staining (2)

Answer 2

• Methylene blue

- acetocarnine

Question 3

Advantages of light microscope (3)

Answer 3

• can see living plants and animals
• cheap
• light and portable

Question 4

Disadvantages of Light microscope (2)

Answer 4

• preserving and staining can produce artefacts

- limited powers of resolution and magnification

Question 5

Artefacts

Answer 5

Things observed in a scientific investigation that are not naturally present

Question 6

Why are specimens stained for electron microscopes?

Answer 6

to improve scattering of electrons, which produces an image with more contrast, so it is easier to interpret

Question 7

Resolution

Answer 7

how far apart two points must be before we see them as seperate objects

Question 8

Transmission electron micrographs (2)

Answer 8

• two dimensional images

- similar to light microscope

Question 9

Scanning electron micrograph

Answer 9

lower magnification, 3-D

Question 10

Advantages and disadvantages of electron microscope (6)

Answer 10

Ad: huge powers of magnification and resolution
Dis: - vacuum so dead
- artefacts
- expensive
- large
- must be kept at constant temp and pressure + internal vacuum

Question 11

Why does it need a vacuum?

Answer 11

Otherwise air would scatter the electrons and produce a blurred image

Question 12

Why is a certain organism not visible under the light microscope? (2)

Answer 12

• not enough mag

- organism too small

Question 13

how is the total magnification of specimen calculated with light microscope?

Answer 13

magnification of objective lens x magnification of eyepiece lens

Question 14

How does an electron microscope work? (6)

Answer 14

• beams of electrons are used to form an image
• electrons are scattered by the specimen
• the electrons behave similarly to the light waves, but with a very tiny wavelength
• electromagnetic or electrostatic lenses focus the electron beam to form an image
• resolving power increases as the wavelength gets smaller
• electron microscope can resolve detail 1000 times better than light microscope

Question 15

what processes are involved in the preparation of a specimen for the electron microscope? (5)

Answer 15

• chemical preservation
• freeze-drying
• dehydration
• sectioning and mounting on a metal grid
• often stained using heavy metal ions such as lead and uranium