3.2.1.3 Methods of studying cells (3.2 Cells)

Question 1

Describe the difference between magnification and resolution

Answer 1

Magnification = Number of times greater image is than size of the real 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 microscope

Answer 2

Optical microscopes:
• light focused using glass lens
• Light passes through specimens different structures absorb different amounts and wavelengths
• Generates a 2D image of a cross sections
• Low resolution due to long wavelength of light
• Can’t see internal structures of organelles or ribosomes
• Specimen = thin
• Low magnification
• cant view slicing organisms
• simple preparation
• can show colour

Transmission electron microscope :
• Electrons focused using electromagnets
• Electrons pass through specimen , denser parts absorb more and appear darker
• Generates a 2D image of a cross section
• Very high resolution due to short wavelength of electrons
• Can see internal structures of organelles and ribosomes
• Specimen = Very thin
• High magnification
• Can only view dead / dehydrated specimens as uses a vacuum
• Complex preparation so artefacts often present
• Does not show colour

Scanning electron microscope:
• Electrons focused using electromagnets
• Electrons deflected off specimen surface
• generated a 3D image of surface
• Hugh resolution due to short wavelength of electrons
• Can’t see internal structures
• Specimen does not need to be thin
• High magnification
• Can only view dead / dehydrated specimens as uses a vacuum
• Complex preparation so artefacts often present
• Does not show colour

Question 3

Suggest how the scientific community distinguished between artefacts and cell organelles

Answer 3

• Scientists prepared specimens in different ways
• If an object was seen with one technique but not another , It was more likely to be an artefact than an organelle

Question 4

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

Answer 4

1) line up eyepiece graticule with stage micrometer
2) Calibrate eyepiece graticule - use stage micrometer to calculate size of decisions in eyepiece graticule
3) Take micrometer 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 eye piece graticule at different magnifications

Question 5

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

Answer 5

1) homogenise tissue / use a blender
• disrupt the cell membrane , breaking open cells to release organelles
2) Place in a cold , isotonic , buffered solution
• Cold to reduce enzyme activity so organelles are not 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 debris
4) Ultracentrifugation - separated organelles in order of mass
• centrifuge homogenate in tube at a low speed
• Remove pellet of heaviest organelle and respin supernatant at a higher speed
• Repeat at increasing speeds until separated out , each time the pellet is made of lighter organelles