Microscopy (a)

Question 1

Magnification

Answer 1

How much bigger a sample appears under the microscope compared to real life (human eye)

Question 2

Resolution

Answer 2

The ability to distinguish between two points on an image (I.e the amount of detail)

Question 3

Overall magnification =

Answer 3

Objective lens x eyepiece lens

Question 4

Light microscope

Answer 4

Specimens are illuminated with light, which is focused using glass lenses viewed with either the eye or the photographic film. Specimens can be living or dead, but need to be stained with a coloured dye (iodine). Specific dues allow certain ultrastructure to be shown eg, the cytoskeleton , DNA, lipids.All compound molecules so they use many different magnifications. Resolution is about 200nm which is good enough to see but not specific structures in organelles

Question 5

Electron microscope

Answer 5

Uses electron beams, rather than electromagnetic radiation to illuminate the specimen . Electrons behave like waves so can easily be produced (using a hot wire) focused (using electromagnets) and detected using a (photographic film). A beam of electrons has an effective wavelength of less than 1nm so can be used to resolve small sub-cellular ultrastructure.

Question 6

Transmission electrons

Answer 6

Pass a beam of electrons through a specimen . Electrons pass through are detected by the specimen through a fluorescent screen which the images are displayed.
Thin sections of specimen are needed for transmission electron microscopy as the electrons have to pass through a specimen for the image to be produced .
Most common form and has best resolution

Question 7

Scanning electron microscopes

Answer 7

Pass a beam of electrons over the surface of the specimen in the form of scanning a beam.
Electrons are reflected off the surface of the specimen as it has previously been coated in heavy metals.
It is these heavy electron beams that are focused of the fluorescent screen in order to make an image up.
Larger thicker structures can thus be seen under as the electrons don’t have to pass through a sample in order to form the image given an excellent 3 dimensional images of surfaces. However resolution is lower

Question 8

Comparison of the light microscope and the electron

Answer 8

Cheap to purchase (£100 – 500) Expensive to buy (over £ 1 000 000).
Cheap to operate. Expensive to produce electron beam.
Small and portable. Large and requires special rooms.
Simple and easy sample preparation. Lengthy and complex sample prep.
Material rarely distorted by preparation. Preparation distorts material.
Vacuum is not required. Vacuum is required.
Natural colour of sample maintained. All images in black and white.
Magnifies objects only up to 2000 times Magnifies over 500 000 times.
Specimens can be living or dead Specimens are dead, as they must be fixed in plastic and viewed in a vacuum
Stains are often needed to make the cells visible The electron beam can damage specimens and they must be stained with an electron-dense chemical (usually heavy metals like osmium, lead or gold).