1: Microscopy

Question 1

What does cell theory state?

Answer 1

• Both plant and animal tissue is composed of cells
• Cells are the basic unit of all life
• Cells only develop from existing cells

Question 2

Advantages of light microscopy

Answer 2

• Easily available
• Relatively cheap
• Can be used in the field
• Can be used to observe living and dead organisms

Question 3

What are the two lenses a compound light microscope has? What do they do?

Answer 3

The objective lens - placed near to the specimen and and produces a magnified image
The eyepiece lens - through which the specimen is viewed, magnifies the magnified image from the objective lens

Question 4

How do you prepare a dry mount? Give examples

Answer 4

Cut solid specimens into very thin slices with a sharp blade (sectioning). Place the specimen on the centre of the slid and place a cover clip over the sample.
Examples: hair, insect parts can be viewed whole; muscle tissue and plants can be sectioned

Question 5

How would you prepare a wet mount? Give examples

Answer 5

Specimens are suspended in a liquid such as water or oil. A cover slip is placed on from an angle.
Examples: aquatic samples, and other living organisms

Question 6

How would you prepare a squash slide? Give examples

Answer 6

First prepare a wet mount, then use a lens tissue to gently press down the cover slip
Examples: root tips to look at cell division

Question 7

How would you prepare a smear slide? Give examples

Answer 7

Use the edge of a slide to smear the sample to create a thin, even coating on another slide. Place a cover slip over the sample
Examples: blood (to view cells)

Question 8

Why is staining used in microscopy?

Answer 8

Stains increase contrast as different components within a cell take up stains to different degrees. Increase in contrast allows components to become visible so they can be identified

Question 9

What are positively charged dyes? Give examples

Answer 9

Attracted to negatively charged materials in the cytoplasm leading to staining of cell components.
Examples: crystal violet, methylene blue

Question 10

What are negatively charged dyes? Give examples

Answer 10

Repelled by the negatively charged cytosol so stay outside cells, leaving the cell unstained, which then stand out against the stained background (negative stain technique)
Examples: nigrosin, Congo red

Question 11

What is Gram stain technique used for?

Answer 11

To separate bacteria into Gram-positive bacteria and Gram-negative bacteria

Question 12

How do you carry out the Gram stain technique?

Answer 12

1. Apply crystal violet to the bacterial specimen, then iodine, which fixes the dye
2. Wash the slide with alcohol
3. Gram-positive bacteria retain crystal violet so appear blue/purple. Gram-negative bacteria have thinner cell walls and therefore lose the stain
4. Gram-negative bacteria are then stained with safranin dye (counterstain) so they appear red

Question 13

What is the acid-fast technique used for?

Answer 13

Differentiate species of Mycobacterium from other bacteria

Question 14

What stages are involved in making pre-prepared slides?

Answer 14

1. Fixing
2. Sectioning
3. Staining
4. Mounting