Cell Structure and Division - Analysis of Cell Components

Question 1

How do you work out magnification?

Answer 1

size of image / size of real object

Question 2

What are the two main types of microscope and the differences between them?

Answer 2

Light / optical and electron.

LIGHT / OPTICAL
- Use LIGHT to form an image
- Have a max resolution of 0.2 micrometres
(so can’t be used to view organelles smaller than this e.g. ribosomes / lysosomes / RER and SER) – you may be able to make out mitochondria but not in perfect detail – you can see nuclei
- Have a max magnification of x 1500

ELECTRON:
- Use ELECTRONS to form an image
- Have a max resolution of 0.0002 micrometres
(higher resolution than light microscope – about 1000 times higher – so give a more detailed image and can be used to look at more organelles)
- Have a max magnification of x 1,500,000

Question 3

How do you convert nanometres to millimetres?

Answer 3

Divide by 1000

Question 4

How do you convert millimetres to nanometres?

Answer 4

Multiply by 1000

Question 5

What are the different types of electron microscopes and what are the differences between them?

Answer 5

Transmission Electron Microscopes (TEM)
and
Scanning Electron Microscopes (SEM)

TEM:

• use electromagnets to focus a beam of electrons, which is then transmitted through the specimen.
• denser parts of the specimen absorb more electrons, which make them look darker on the image you end up with
• good because they give you high resolution images, so you see the internal structure of organelles like chloroplasts
• but they can only be used on thin specimens

SEM:

• scan a beam of electrons across the specimen – this knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image
• the images you end up with show the surface of the specimen and they can be 3D
• good because they can be used on thick specimens
• but they give lower resolution images than TEMs

Question 6

What is meant by ‘resolution’

Answer 6

RESOLUTION:

How detailed the image is.
More specifically, how well a microscope distinguishes between two points that are close together.
If a microscope lens can’t separate two objects, then increasing the magnification won’t help.

Question 7

Describe the process and its 3 stages you’d have to carry out before looking at organelles under an electron microscope.

Answer 7

Process: CELL FRACTIONATION

Step 1 - HOMOGENISATION - Breaking Up the Cells:

Homogenisation can be done in several different ways
e.g. by vibrating the cells or by grinding the cells up in a BLENDER. This BREAKS UP the PLASMA MEMBRANE and RELEASES the ORGANELLES into solution.

The solution must be kept ICE COLD, to reduce the activity of enzymes that break down organelles.

The solution should also be ISOTONIC – this means that it should have the same water potential / concentration of chemicals as the cells being broken down, to prevent damage to the organelles through osmosis.

A BUFFER SOLUTION should be added to maintain the pH.

Step 2 - FILTRATION - Getting Rid of the Big Bits

The homogenised cell solution is FILTERED through a gauze to separate any large cell debris or tissue debris, like connective tissue, from the organelles.
The organelles are much smaller than the debris, so they pass through the gauze.

Step 3 - ULTRACENTRIFUGATION - Separating the Organelles

After filtration you’re left with a solution containing a mixture of organelles. To separate them, use ultracentrifugation:

1 - The cell fragments are poured into a tube.
The tube is put into a centrifuge and is spun at a low speed.
The heaviest organelles (e.g. nuclei) get flung to the bottom of the tube.
They form a thick sediment at the bottom – a pellet.
The rest of the organelles stay suspended in the fluid above the sediment – the supernatant.

2 - The supernatant is drained off, poured into another tube and spun in the centrifuge at a higher speed.
Again, the heaviest organelles (this time the mitochondria) form a pellet at the bottom of the tube.
The supernatant containing the rest of the organelles is drained off and spun in the centrifuge at an even higher speed.

3 - This process is repeated at higher and higher speeds, until all the organelles are separated out. Each time, the pellet at the bottom of the tube is made up of lighter and lighter organelles.

____________________
ESSENTIALLY:

The organelles are separated in order of mass
(from heaviest to lightest)
– this order is usually nuclei, mitochondria, lysosomes, endoplasmic reticulum, then ribosomes.