Biology // Module 2 // Cell Structure

Question 1

What is staining?

Answer 1

Artificial colouration of a substance to allow the examination of different tissues, microorganisms or other cells under a microscope.

Question 2

Why is staining important?

Answer 2

1) Increases contrast
2) Makes cells and organelles found inside cells visible
3) Differentiate between different cells and the different organelles inside them

Question 3

What is differential staining?

Answer 3

Staining with the aim of identifying differences.

Question 4

What is gram staining used for?

Answer 4

It is used to determine different classes/types of bacteria.

Question 5

Characteristics of gram-positive bacteria

Answer 5

-Thick peptidoglycan layer (means it can retain more dye)
-Turns purple after crystal violet dye
E.g Staph infection

Question 6

Characteristics of gram-negative bacteria

Answer 6

-Thin peptidoglycan layer (means it cannot retain as much dye)
-Outer lipid membrane making it more resistant to antibiotics
-Counterstains pink after dye due to not being able to retain purple dye
E.g E coli or Salmonella

Question 7

What is the acid-fast technique?

Answer 7

Apply primary stain, apply heat, apply decolouriser, and apply methylene blue.
If acid-fast, it stains red, If non acid-fast, it turns blue/green with counterstain

Question 8

What are the three types of staining techniques used for eukaryotic organelles?

Answer 8

Basic stains, Acidic stains, Negative stains

Question 9

What are the specific dyes used for basic stains, as well as their purpose and outcome?

Answer 9

-Methylene blue, crystal violet
-Stains negatively charged molecules and structures such as nucleic acids and proteins
-Positive stain

Question 10

What are the specific dyes used for acidic stains, as well as their purpose and outcome?

Answer 10

-Congo red, eosin
-Stains positively charged molecules and structures such as proteins
-Can be either a positive or negative stain depending on the cell’s chemistry.

Question 11

What are the specific dyes used for negative stains, as well as their purpose and outcome?

Answer 11

-India ink. nigrosin
-Stains background and not the specimen
-Dark background with a light specimen

Question 12

What is resolution?

Answer 12

Ability to distinguish two individual objects as separate entities. However this is limited by diffraction of light waves as it passes through the specimen and lenses.

Question 13

What is the equation for Image size? (microscopy)

Answer 13

Image size (mm)= Actual size (micrometres) X Magnification(x)

Question 14

Why do microscopes need to be calibrated?

Answer 14

Ensures accuracy, standardisation and repeatability. Ensures that the same sample when assessed with different microscopes will provide the same results.

Question 15

How many subdivisions does one eyepiece graticule have?

Answer 15

100.

Question 16

What is the scalar value of an eyepiece graticule?

Answer 16

It does not have a scale.

Question 17

What is the main limitation of the light microscope?

Answer 17

Visible light has a resolution of 200nm meaning that the smallest ‘gap’ it could fit through is 200nm. However, cells often have gaps in the cells that are much smaller and the wavelength of light, limiting the image that can be seen.

Question 18

What is the resolution of an electron?

Answer 18

Less than 1nm

Question 19

What is the electron said to have?

Answer 19

Higher resolving power

Question 20

What is the magnification of the electron microscope?

Answer 20

500,000x

Question 21

Describe how an electron microscope works?

Answer 21

Uses electron beams rather than rays of light. Lenses are electromagnets which focus the beams of electrons. Electrons have a shorter wavelength than light meaning that they have higher resolution. However the specimen must be kept within a vacuum.

Question 22

What are the two types of electron microscopes?

Answer 22

Transmission electron microscope (TEM), Scanning electron microscope (SEM).

Question 23

Describe how a Transmission electron microscope (TEM) works

Answer 23

-Beams of electrons are passed through a thin specimen.
-Parts of the specimen absorb electrons and appear dark, other parts allow the electrons through and appear light.
-Black and white 2D image is produced (500,000x magnification), however it has very high resolution
E.g used in fractures, tissue infections.

Question 24

Describe how a Scanning electron microscope (SEM) works

Answer 24

-Beams of electrons are sent across the surface not the interior of the specimen and reflected electrons are collected.
-A computer produces a 3D image black and white image of the specimen.
-Resolution is lower than TEM but the 3D detail gives more information about each specimen.

Question 25

What are the advantages of electron microscopy?

Answer 25

-Can see the entire 3D structure
-Higher magnification and resolution
-Helps in the medical profession to detect problems at a structural level.

Question 26

What are the disadvantages of electron microscopy?

Answer 26

-It is very expensive (up to £1,000,000)
-Specimens must be sliced extremely thinly
-Only able to be used on non-living tissue/organisms
-Only black and what images
-Artefacts can be left on images

Question 27

What is an artefact?

Answer 27

Damage that occurs during the making of a specimen.
E.g damage to membranes, distortion of organelles, bubble on a temporary mount.

Question 28

What is Laser Scanning Confocal Microscopy? (LSCM)

Answer 28

Use of a high-intensity laser beam which scans the surface of a very thin specimen. The specimen has been treated with a fluorescent dye. The dyed parts of the specimen reflect light which is picked up by a detector and processed through a computer. The other parts of the specimen do not reflect light so the image produced is much sharper.

Question 29

What is the value of using LSCM and fluorescent dyes?

Answer 29

The fluorescent dye can be genetically engineered to target specific proteins in certain cells, tissues or organs. Multiple fluorescent tags can be used to produced detailed images. This is vital in medical imaging.

Question 30

What are the advantages of LSCM?

Answer 30

-Non-invasive and currently used in specimens, e.g diagnosis of eye disorders, skin cancer without the need for a biopsy
-Can see different layers of a sample

Question 31

What are the disadvantages of LSCM?

Answer 31

-Very expensive (£750-£90,000)
-Limited by the number of wavelengths available with a dye.
-Hard to detect some complex interconnected structures