3.2.1.3 Methods of studying cells

Question 1

Compare the description of optical microscopes, scanning electron microscopes and transmission electron microscopes

Answer 1

Optical
* Beam of light rays
* With a long wavelength

SEM and TEM
* Beam of electrons
* with short wavelength
* Can be focused with electromagnets

TEM
* Photomicrograph appears darker where electrons absorbed (denser=darker)

Question 2

Which microscope can view a 3d image of the specimens surface?

Answer 2

SEM

Question 3

Explain advantages and limitations of using transmission electron microscopes to investigate cell structure

Answer 3

• TEM use a bean of electrons
• These have short wavelength
• Allow high resolution
• Electrons scattered by molecules in air
• Vacuum established
• Can’t view living specimens
• Lots of preparation
• May alter appearance/ artefacts

Question 4

What organelles are visible with a light microscope?

Answer 4

Nucleus
Chloroplasts

Question 5

Compare the how optical microscopes, SEMs and TEMs view an image

Answer 5

Optical: passing light through specimen
SEM: electrons reflecting off metal coated specimen
TEM: electrons passing through specimen

Question 6

Resolution definition

Answer 6

Ability of a microscope to distinguish between 2 objects that are close together

Question 7

Magnification definition

Answer 7

Number of times an object is enlarged

Question 8

How can you increase contrast?

Answer 8

Staining

Question 9

Compare the resolution of optical microscopes, scanning electron microscopes and transmission electron microscopes

Answer 9

Optical 0.2 micrometers
SEM 20nm
TEM 0.1nm

Question 10

Contrast definition

Answer 10

Difference in shade between lightest and darkest parts of an image

Question 11

Disadvantages of each microscope

Answer 11

Optical- lower resolution, low magnification, only see little detail
TEM and SEM - needs vacuum (electrons absorbed by air molecules), no live specimens
SEM - Lower resolution than TEM
TEM - needs very thin specimen, 2d image, complex staining process, image may contain artefacts

Question 12

What are the advantages of each microscope?

Answer 12

Optical- live specimen
SEM- higher resolution than optical, sample can be thick, 3d image
TEM- higher resolution than SEM and optical

Question 13

Why must the sample for an optical microscope be thin?

Answer 13

One cell thick
So light is able to pass through

Question 14

How do you prepare a slide?

Answer 14

• Add a drop of water to a glass slide
• Obtain a thin section of the sample/ a single layer of cells and place on the water droplet
• Add a stain to create contrast (iodine)
• Slowly lower coverslip using a mounted needle to avoid air bubbles
• Blot away any excess stain/water with tissue/blotting paper

Question 15

What type of microscope can be used on live specimens? Why?

Answer 15

Optical (no vacuum)

Question 16

How would you find the average width of nuclei in plant cells?

Answer 16

• Calibrate the eyepiece graticule with the stage micrometer
• Measure the width of one nucleus in eyepiece units
• Convert this to micrometres using your calibration (eg mm X 1000 = um)
• Measure many more nuclei
• Calculate the mean