studying cells

Question 1

Define Magnification

Answer 1

how much bigger the image of a sample is compared to its actual size

Question 2

Define Resolution

Answer 2

The minimum distance that can be observed between two points

Question 3

What is the Magnification Equation?

Answer 3

Magnification = Size of Image / Actual Size of Object

Question 4

Optical Microscope [4]

Answer 4

• Use light to form a 2D image
• Produces coloured images
• Visible light longer wavelength so lower resolution (200nm)
• Low magnification (x1500)

Question 5

Give 2 advantages of an Optical Microscope

Answer 5

• Can see living organisms
• Produces coloured images

Question 6

Give 4 disadvantages of an Optical Microscope

Answer 6

• 2D image
• Only used on thin specimens
• Low magnification
• Low resolution; can’t see internal structures of organelles
  or smaller organelles

Question 7

Scanning Electron Microscopes [4]

Answer 7

• Use electrons to form a 3D image
• Beams of electrons scan surface, knocking off electrons from the specimen, which are gathered in a cathode ray tube to form an image
• Electrons shorter wavelength so higher resolution (0.2nm)
• High magnification (x1,500,000)

Question 8

Give 3 advantages of a Scanning Electron Microscope

Answer 8

• 3D image
• High magnification
• High resolution

Question 9

Give 2 disadvantages of a Scanning Electron Microscope

Answer 9

• Specimen must be viewed in a vacuum; can’t see living
  organisms
• Can’t see internal structures

Question 10

Transmission Electron Microscope [4]

Answer 10

• Use electrons to form a 2D image
• Electromagnets focus beam of electrons onto specimen, transmitted onto photographic plate, more dense = more absorbed = darker appearance
• Electrons shorter wavelength so higher resolution (0.2nm)
• High magnification (x1,500,000)

Question 11

Give 3 advantages of a Transmission Electron Microscope

Answer 11

• High magnification
• High resolution
• Can see internal structures of organelles

Question 12

Give 2 disadvantages of a Transmission Electron Microscope

Answer 12

• Specimen must be viewed in a vacuum; can’t see living
  organisms
• Very thin samples must be used

Question 13

How do you use a Graticule? [5]

Answer 13

• Line up eyepiece graticule with stage micrometer
• Use stage micrometer to calculate the size of divisions on eyepiece graticule at a particular magnification
• Take the micrometer away and use the graticule to measure how many divisions make up the object
• Calculate the size of the object by multiplying the number of divisions by the size of divisions
• Recalibrate eyepiece graticule at different magnifications

Question 14

How do you prepare a Temporary Mount Slide? [3]

Answer 14

• Use tweezers to place a thin section of specimen on a water drop on a microscope slide
• Add a drop of a stain (e.g. iodine)
• Add a cover slip by carefully tilting and lowering it, trying not to get any air bubbles

Question 15

Cell Fractionation [9]

Answer 15

Homogenise tissue using a blender
- Disrupts cell membrane
- Releasing contents / organelles
Place in a cold, isotonic, buffered solution
- Cold reduces enzyme activity, organelles aren’t broken down
- Isotonic so water doesn’t move in/out of organelles by osmosis so they don’t burst / shrivel
- Buffered keeps pH constant so enzymes don’t denature
Filter homogenate
- Remove large, unwanted debris (e.g. whole cells, connective tissue)

Question 16

Cell Ultracentrifugation [4]

Answer 16

• Centrifuge homogenate in a tube at a low speed
• Remove pellet of heaviest organelle and spin supernatant at a higher speed
• Repeated at higher and higher speeds until organelles separated out
• Separated in order of mass/density: nuclei → chloroplasts → mitochondria → lysosomes → endoplasmic reticulum → ribosomes

Question 17

What is an Artefact?

Answer 17

Something that appears in the electron micrograph then isn’t a part of the specimen