Ocr A AS Biology - Microscopes

Question 1

Define the term ‘Magnification’

Answer 1

Magnification is the number of times larger an image appears compared to the objects actual size.

Question 2

Define the term ‘Resolution’

Answer 2

Resolution is the ability of an optical microscope to produce an image that shows fine detail clearly.

Question 3

Give the names of the 5 different microscopes

Answer 3

Optical Microscope
Laser Scanning Microscopes 
Electron Microscopes
Transmission Electron Microscope
Scanning Electron Microscope

Question 4

List the properties of an Optical Microscope

Answer 4

Easy to use
Cheap
Mag - x1500 - x2000 > enables us to see clearly some of the larger structures inside cells
Resolution is limited
Portable
Use visible light - WL: 400 - 700nm - Ribosomes can’t be seen ( 20nm in diameter)
Used to study living cells

Question 5

Define the term ‘Photomicrograph’

Answer 5

Photograph of images seen using an optical microscope.

Question 6

List the properties of a Laser Scanning Microscope

Answer 6

Have depth selectivity, can focus on structures at different depths within a specimen.
Also known as Confocal Microscopes
Images are of a high resolution and show a high contrast
Used in many branches of biological research
Use laser light to scan an object, point by point and assemble by computer. The pixel information is displayed on a computer screen.

Question 7

List the properties of an Electron Microscope

Answer 7

Use a beam of fast-travelling electrons, which are fired from a cathode.
Has a WL of 0.004nm —> much greater resolution. Can give highly magnified images.
Fast beam of electrons have a WL of 125 000, smaller than the central part of the visible light spectrum

Question 8

Properties of SEM

Answer 8

Developed in the 1960’s
Cause secondary electrons to ‘bounce off ‘ and focused on a screen
Electrons don’t pass through a specimen
3D image produced (Grey scale) —> False colour can be added
Magnification of about x15 - x 200 000

Question 9

Properties of TEM

Answer 9

Magnification of up to 2 million times

Form a 2D grey scale image, known as an electron micrograph.

Question 10

Both SEM and TEM are:

Answer 10

Large and Expensive

Specimens must be dead —> SEM - Vaccuum

Question 11

Observing unstained specimens

Answer 11

Some microscopes use light interference rather than absorption, in order to produce a clear image without staining.
Some use a dark background, which the illuminated specimen shows up.

Question 12

Staining specimens

Answer 12

Stains are coloured chemicals that bind to molecule in/on the specimen making it easy to see
Methylin blue - all purpose stain
Differential staining - When some stains bind onto specific cell structures.

Question 13

What does Acetic Orcein stain?

Answer 13

Binds to DNA and stains chromosomes dark red

Question 14

What does eosin stain?

Answer 14

It stains Cytoplasm

Question 15

Sudan Red stains…

Answer 15

Lipids

Question 16

Iodine stains…

Answer 16

The cellulose in cell walls yellow and starch granules - blue/black

Question 17

Process of Prepared specimens

Answer 17

Dehydrating the specimen
Embedding them in wax to prevent distortion during slicing
Using a special instrument to make very thin slices (Sections) - these are stained and mounted in a special chemical to be preserved.

Question 18

Eyepiece Graticule?

Stage Graticule?

Answer 18

Measuring device, which is placed in the eyepiece of a microscope, acts as a ruler.
It’s a precise measuring device. A small scale is placed on a microscope stage and is used to calibrate the value of eyepiece divisions at different magnifications.