3.2.1 Cell structure microscopes

Question 1

Describe how optical microscopes work

Answer 1

Lenses focus rays of light and magnify the view of a thin slice of specimen
Different structures absorb different amounts and wavelengths of light
Reflected light is transmitted to the observer via the objective lens and eyepiece

Question 2

Outline how a student could prepare a temporary mount of tissue for an optical microscope

Answer 2

Obtain thin section of tissue
Place plant tissue in a drop of water
Stain tissue on a slide to make structures visible
Add coverslip using mounted needle at 45 degrees to avoid trapping air bubbles

Question 3

Advantages and limitations of using an optical microscope

Answer 3

A: Colour image, can show living strucutes
L: 2D image, lower resolution than SEM/TEM

Question 4

Describe how a TEM works

Answer 4

Transmits a high energy beam of electrons through thin slice of specimen
More dense structures appear darker since they absorb more electrons
Generates an image on the computer

Question 5

Suggest the advantages and limitations of using a TEM

Answer 5

A: Very high resolution and magnification (electrons have shorter wavelength than light)
L: 2D images only, dead specimens only, can contain artefacts, no colour

Question 6

Describe how a scanning electron microscope (SEM) works

Answer 6

An electron beam is scanned across a specimen and knocks electrons off the specimen and gathers in a cathode ray tube to form an image

Question 7

Advantages and limitations of using SEM

Answer 7

A: 3D image, high resolution and magnification (electrons have shorter wavelength),
L: Can contain artefacts, no colour image

Question 8

Define magnification

Answer 8

Factor by which the image is larger than the actual specimen

Question 9

Define resolution

Answer 9

How well a microscope can distinguish between 2 points close together

Question 10

Explain how to use an eyepiece graticule and stage micrometer to measure the size of a structure

Answer 10

1. Place micrometer on stage to calibrate eyepiece graticule
2. Line up scales on graticule and micrometer. Count how many graticule divisions are in 100 micrometres on the micrometer
3. Length of 1 eyepiece division = 100 micrometres / number of divisions
4. Use calibrated values to calculate actual length of structure

Question 11

Microscope equation

Answer 11

Image size = actual x magnification