2.1 Cell Structure- Methods of study

Question 1

What are the three main types of microscope used to study cells?

Answer 1

Optical Microscope (light)
Scanning Electron Microscope
Transmission Electron Microscope

Question 2

How much can you see through as optical microscope?

Answer 2

The nucleus, cell membrane and cytoplasm.

Question 3

What are the advantages of an optical microscope?

Answer 3

They’re cheap, small and portable
They’re simple to prepare and use
A vacuum isn’t required
The natural colour of the sample is maintained
Living and dead material can be viewed
Unaffected by magnetic fields

Question 4

What are the disadvantages of an optical microscope?

Answer 4

Lowest resolution

The magnification is only up to 2000

Question 5

How do scanning electron microscopes work?

Answer 5

A beam of electrons are fired through the specimen (from above)
The electrons scatter depending on the contours of the specimen surface
A 3D image is produced by analysis of the pattern of scattered electrons

Question 6

What are the advantages of scanning electron microscopes?

Answer 6

Election beam has short wavelength so the resolution is high
Electrons are negatively charged so the beam can be focused using magnets

They produce a 3D image
5000 times magnification
Sample can be any thickness

Question 7

What are the disadvantages of scanning electron microscopes?

Answer 7

The specimens are killed
They’re more expensive
The image is in black and white
A vacuum is required
Preparation of the specimen can produce artefacts
Carries a small risk of radiation
Specimen must be conductive

Question 8

How do transmission electron microscopes work?

Answer 8

A beam of electrons is focused onto the specimen (from below) by a condenser electromagnet
Some electrons are absorbed and others pass through
This produces an image than can my photographed to give a photomicrograph

Question 9

What are the advantages of transmission electron microscopes?

Answer 9

Highest resolution
Highest magnification at 1 million times or more
Scientific, educational and industrial uses
Easy to operate with good training
Information gathered on surface features, shape, size and structure

Question 10

What are the disadvantages of transmission electron microscopes?

Answer 10

Vacuum required 
Black and white image
Samples are killed
Samples must be thin
Image may contain artefacts

Question 11

What is the calculation for magnification?

Answer 11

Magnification=image size/size of real object

Question 12

What is cell fractionation?

Answer 12

The process where cells are broken up and the different organelles they contain are separated out
Processes such as respiration and photosynthesis have been studied this way

Question 13

Why is tissue cut up and kept in a cold, isotonic, buffered solution before cell fractionation?

Answer 13

Cold- reduces enzyme activity that might break down the organelle
Isotonic- prevent organelles bursting or shrinking as a result of osmosis
Buffered solution- keeps the pH the same so organelle structure is unchanged and enzyme function in unaffected

Question 14

What is homogenation?

Answer 14

Blending of cells
Releases organelles from the cell by breaking down cell wall
Produces homogenate
This is filtered to remove any complete cells or large debris

Question 15

How are organelles separated in cell fractionation?

Answer 15

The homogenised tissue is spun in an ultracentrifuge at low speed for 10 minutes to force the heaviest organelles to the bottom.

Question 16

List the main components of a cell, heaviest to lightest?

Answer 16

Nuclei, mitochondria, lysosomes, endoplasmic reticulum, ribosomes.

Question 17

What can be done to make a simple convex glass lens more effective?

Answer 17

Using them in pairs as in a compound light microscope

Question 18

What do you call the material put under the microscope?

Answer 18

The object

Question 19

What is the equivalent (in meters) of the following measurements? 
km
m
mm
um
nm

Answer 19

km- 10 ^3
m-   1
mm-10 ^-3
um- 10 ^-6
nm- 10 ^-9

Question 20

Define resolution.

Answer 20

The minimum distance apart that two objects can be in order for them to appear as separate items
Higher resolution produces a clearer and more precise image

Question 21

What does resolution depend on?

Answer 21

The wavelength or form of radiation used

Question 22

What would a resolution of 0.2um mean?

Answer 22

Any objects that are 0.2um apart or more will be seen separately

Question 23

Define ultracentrifugation.

Answer 23

The process by which the fragments in the filtered homogenate are separation in a centrifuge
It spins tubes of homogenate at high speeds to create a centrifugal force
The heaviest organelles move to the bottom

Question 24

Describe the process of ultracentrifugation.

Answer 24

Filtrate spun slowly so heavy organelles (nuclei) form thin sediment at the bottom
Supernatant fluid is removed, transferred to a tube and spun at a higher speed to sediment the next heaviest organelle (mitochondria)

Question 25

Why is a vacuum required for electron microscopes?

Answer 25

Electrons are absorbed or deflected by molecules in the air so a vacuum is needed for it to work effectively

Question 26

What is an artefact?

Answer 26

Things that result from the way the specimen is prepared

Appear on photomicrograph but not part of the natural specimen

Question 27

What is the significance of the sample for TEM being thin?

Answer 27

Allow electrons to penetrate
Produces 2D image
Multiple section can be taken to build up a 3D image

Question 28

What is used to measure length directly through a microscope and how?

Answer 28

Eyepiece graticule

Calibrated for a particular objective lens with a stage micrometer