2.1.1: Microscopes

Question 1

What is magnification?

Answer 1

How many times larger the image is compared to the object

Question 2

What is resolution?

Answer 2

The minimum distance between two objects in which they can still be viewed as separate
Optical microscope - determined by the wavelength of light
Electron microscope - determined by the wavelength of the beam of electrons

Question 3

What are the types of microscopes?

Answer 3

Opitical (light) microscopes
Laser scannig microscopes
Electron microscopes
* Trasmission electron micoscope
* Scanning electron microscope

Question 4

What are the 4 types of sample preparations?

Answer 4

Dry mount
Wet mounts
Squash slide
Smear slide

Question 5

What is dry mount?

Answer 5

When thin slices of whole specimens are viewed, with just the coverslip places on top e.g plant tissue or hair

Question 6

What is wet mount?

Answer 6

When the specimens are added to water before lowering the coverslip with a mounted needle to prevent air bubbles from forming. Aquatic organisms could be viewed this way

Question 7

What are squash slide?

Answer 7

Wet mounts which you then push down on the coverslip to squash the sample to ensure you have a thin layer to enable light to pass through. This is used when creating a root tip squash sample to view the chromosomes in mitosis

Question 8

What are smear slides?

Answer 8

Created using the edge of a slide to smear the sample across another slide to create a smooth, thin, even coated specimen. A cover slip is placed on top after smearing. This is used when examining blood cells in a blood sample.

Question 9

What is differential staining?

Answer 9

A technique which involves many chemical stains being used to stain different parts of a cell in different colours

Question 10

What is Crystal violet and methylene blue?

Answer 10

They are two stains commonly used
Positively charged and therefore attracted to and stain negatively charged materials

Question 11

What is nigrosin and congo red?

Answer 11

They are negatively charged and therefore cannot enter the cells as cytosel repels them. This creates a stained background and the upstairs cells then stand out

Question 12

What is gram staining?

Answer 12

Another common use of differential staining
Two different stains are used crystal violet and safranin
* Crystal violet is added, then iodine to fix the stain, and alcohol is used to wash away any unbound stain
Gram positive bacteria appear blue/purple as the stain is retained due to the thicker peptidoglycan cell wall later absorbing the dye
Gram negative cannot absorb cystal violet stain as their peptidoglycan cell wall is thin, so they do not retain the stain due to thinner cell walls
* Safranin is used as a counter stain, turning them red

Question 13

What are features of an optical microscope?

Answer 13

Uses light to form an image
This limits the resolution
* Using light it is impossible to distinguish between two objects that are closer than half the wavelength of light

Optical microscopes have a maximum resolution of around 0.2 micrometers
* They can be used to observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts
* Cannot be used to observe smaller organelles such as ribosomes, ER or lysosomes
Maximum magnification = x1500

Question 14

What are features of electron microscopes?

Answer 14

It has a high resolution because:
* A beam of electrons has a very short wavelength so small organelles and internal structures can be visualised
* Image is created using an electromagnet to focus the beam of negative charged electrons

It must be in a vaccum because electrons are absorbed by air
* Only non-living specimens can be examined

It must be stained because the images are black and white

Question 15

What are the two types of electron microscopes?

Answer 15

Transmission electron microscopes
Scanning electron microscope

Question 16

What are features of transmission electron microscopes?

Answer 16

Extremely thin specimens are stained and placed in a vaccum
Electron gun produces a beam of electrons that passes through the specimen
Some parts of the specimen absorb the electrons and this makes them appear darker
The image produced is 2D and shows detailed images of the internal structure of cells

Question 17

What are features of scanning electron microscope

Answer 17

Specimen does not have to be thin as the electrons are not transmitting through
Electrons are beamed onto the surface and electrons are scattered in different ways depending on the contours
This produces a 3D image

Question 18

What are features of a laser scanning confocal microscope?

Answer 18

A type of flourescent microscope
The image is created using a very high light intensity to illuminate the specimen stained with a flourescent dye
Combines the benefits of high resolution optical imaging with depth security
Enables scientists to view sections of tiny structures that would be challenging to physically section off, such as embryos, and creates a 3D image
* The image is created as the microscope scans the specimen point by point using a focused laser beam to create a 2D image, or a 3D image in different focal planes. As the light is emitted from the specimen it causes the flourescence