methods of studying cells

Question 1

Describe the difference between magnification and resolution

Answer 1

● Magnification = number of times greater image is than size of the real (actual) object
○ Magnification = size of image / size of real object
● Resolution = minimum distance apart 2 objects can be to be distinguished as separate objects

Question 2

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of how they work

Answer 2

optical - light focused using glass lenes

TEM - electrons focused using electromagnets

SEM - electrons focused using electromagnets

Question 3

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in tems of how a picture is produced

Answer 3

optical - light passes through specmen different structures absorb different amounts of wavelengths

TEM - electrons pass through specimen denser parts absob more and appear darker

SEM - electrons deflected off specimen surface

Question 4

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of image created

Answer 4

optical - 2D image of cross section

TEM - 2D image of cross section

SEM - 3D image of surface

Question 5

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of resolution

Answer 5

optical - low due to long wavelength of light

TEM - very high due to short wavelength of electrons

SEM - high due to short wavelength of electrons

Question 6

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of internal structures

Answer 6

optical - cant see of organelles or ribosomes

TEM - can see of organelles and ribosomes

SEM - cant see

Question 7

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of specimen thickness

Answer 7

optical - thin

TEM - very thin

SEM - doesnt need to be thin

Question 8

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of magnification

Answer 8

optical - low x1500

TEM - high x1,000,000

SEM - high x1,000,000

Question 9

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of specimen living or dead

Answer 9

optical - living

TEM - only dead as uses a vacuum

SEM - only dead as uses a vacuum

Question 10

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of preparation

Answer 10

optical - simiple

TEM - complex so often artefacts

SEM - complex so often artefacts

Question 11

Compare the principles and limitations of optical microscopes,
transmission electron microscopes and scanning electron microscopes in terms of colour

Answer 11

optical - colour

TEM - no colour

SEM - no colour

Question 12

how did scientists distinguish artefacts and organelles

Answer 12

prepare specimens in different ways
if object was seen on one technique but not another it is more likely an artefact

Question 13

List the steps in calculations involving magnification, real size & image size

Answer 13

1 Note formula / rearrange if necessary (I = AM)
2 Convert units if necessary - image and actual size
must be in same unit
3 Calculate answer and check units required or if
standard form etc. is required

Question 14

Describe how to convert between different units

Answer 14

m –> mm –> um –> nm

–> x1000
<– /1000

Question 15

Describe how the size of an object viewed with an optical microscope can be
measured

Answer 15

1. Line up (scale of) eyepiece graticule with (scale of) stage micrometre
2. Calibrate eyepiece graticule - use stage micrometre to calculate size of divisions on eyepiece graticule
3. Take micrometre away and use graticule to measure how many divisions make up the object
4. Calculate size of object by multiplying number of divisions by size of division
5. Recalibrate eyepiece graticule at different magnifications

Question 16

Describe and explain the principles of cell fractionation and
ultracentrifugation as used to separate cell components

Answer 16

1. Homogenise tissue / use a blender
   ● Disrupts cell membrane, breaking open cells and releasing contents / organelles
2. Place in a cold, isotonic, buffered solution
   ● Cold to reduce enzyme activity → so organelles not broken down / damaged
   ● Isotonic so water doesn’t move in or out of organelles by osmosis → so they don’t burst
   ● Buffered to keep pH constant → so enzymes don’t denature
3. Filter homogenate
   ● Remove large, unwanted debris eg. whole cells, connective tissue
4. Ultracentrifugation - separates organelles in order of density / mass
   ● Centrifuge homogenate in a tube at a high speed
   ● Remove pellet of heaviest organelle and respin supernatant at a higher speed
   ● Repeat at increasing speeds until separated out, each time pellet made of lighter
   organelles (nuclei → chloroplasts / mitochondria → lysosomes → ER → ribosomes)