Summary AQA A Level Biology - Unit 2 Cells Concise Notes
4 views 0 purchase
Course
Unit 2 - Cells
Institution
AQA
Concise and detailed notes
Specification based
Unit 2 - Cells
Includes diagrams to make it easier to understand
Follows specification order
Recently written hence suitable for new/current specification AQA.
Key:
LM = Light microscope
EM = Electron microscope
Microscopy:
- Microscopes = instruments that produce a magnified image of an object,
- The relatively long wavelength of light rays means that a LM can only distinguish between 2
objects if they’re 0.2 micrometers or further apart.
- This limitation can be overcome by using beams of e- rather than light,
As they have shorter wavelengths.
Beam of e- in EM can distinguish between 2 objects only 0.1 nano meters apart.
Magnification:
- Object = material put under a microscope.
- Image = Appearance of this material when viewed under the microscope.
- Magnification = magnification of an object is how many times bigger the image is when
compared to the object.
Magnification = Size Of Image / Size Of Real Object.
AIM M=I/A
Make both units of object and image the same. (Often smallest).
Resolution:
- Resolution or Resolving Power of a microscope = minimum distance apart that 2 objects can
be in order for them to appear as separate items.
- Resolving power of any microscope depends on wavelength or form of radiation used.
- In a LM it’s about 0.2 micrometers.
This means any 2 objects which are 0.2 micrometers or more further apart will be seen
separately.
But any objects closer than 0.2 micrometers will appear as a single item.
- Therefore, greater resolution means greater clarity, image produced is clearer and more
precise.
- Increasing magnification increases size of image, but doesn’t increase the resolution.
Cell Fractionation:
, - Cell Fractionation = process where cells are broken up and the different organelles they
contain are separated out.
- Before cell fractionation can begin, tissue is placed in a cold, buffered solution of the same
water potential as the tissue.
The Solution Is:
1- Cold – to reduce enzyme activity that might break down the organelles.
2- Same water potential as tissue – to prevent organelles bursting or shrinking as a result of
osmotic gain or loss of water.
3- Buffered – So that pH doesn’t fluctuate. Any change in pH could alter the structure of
the organelles or affect the functioning of enzymes.
Two Stages to cell fractionation:
Homogenation:
- Cells are broken up by a homogeniser (blender).
- Releases organelles from cell.
- Resultant fluid is know. As homogenate.
- It’s then filtered to remove any complete cells/large pieces of debris.
Ultracentrifugation:
- Ultracentrifugation = process by which the fragments in the filtered homogenate are
separated in a machine called a centrifuge.
- Centrifuge spins tubes of homogenate at very high speed in order to create a centrifugal
force.
Process for animal cells:
- Tube of filtrate is placed in the centrifuge and spun at a slow speed.
- Heaviest organelles (nuclei) are forced to the bottom of the tube, where they form a pellet.
- Fluid at the top of the tube (supernatant) is removed, leaving the sediment of nuclei.
- Supernatant is transferred to another tube and spun In centrifuge at a faster speed than
before.
- Next heaviest organelles (mitochondria) are forced to the bottom of tube.
- Process is continued in this way, at each increase In speed, the next heaviest organelle is
sedimented and separated out.
3.2 – THE ELECTRON MICROSCOPE:
,- LM have poor resolution due to long wavelength of light.
- EM uses a beam of electrons instead of light.
Main 2 advantages of an EM:
1- Electron beam —> short wavelength —> microscope can therefore resolve objects very
well – high resolving power.
2- As electrons are negatively charged, the beam can be focused using electromagnets.
- Modern EM can resolve objects that are just 0.1 nano meters apart.
2000x better than a LM.
- Because, electrons are absorbed or deflected by molecules in air
A near-vacuum must be created within the chamber of an EM in order for it to work
effectively.
There’s 2 types of electron microscope:
1- Transmission electron microscope (TEM).
2- Scanning electron microscope (SEM).
The Transmission Electron Microscope:
- TEM consists of an e- gun that produces a beam of electrons that is focused onto the
specimen by a condenser electromagnet.
- In a TEM, beam passes through a thin section of the specimen.
- Parts of this specimen absorb e- and therefore appear dark.
- Other parts of the specimen allow e- to pass through and so appear bright.
- An image is produced on a screen
This can be photographed to give a photomicrograph.
- Resolving power of TEM is 0.1 nanometers.
, This cannot be always achieved because;
1- Difficulties preparing the specimen, limit the resolution that can be achieved.
2- A higher energy e- beam is required and this may destroy the specimen.
Main limitations of TEM are:
1- Whole system must be in a vacuum, therefore living specimens can’t be observed.
2- A complex ‘staining’ process is required, even then the image isn’t in colour.
3- Specimen must be extremely thin.
4- Image may contain artefacts.
Artefacts = things that result from the way the specimen is prepared.
Artefacts may appear on the finished photomicrograph but aren’t part of the natural
specimen.
- In TEM specimens must be extremely thin to allow electrons to penetrate.
The result is therefore a flat 2D image.
We can build a 3D image of the specimen by looking at the series of photomicrographs
produced.
However it’s a slow and complicated process.
The Scanning Electron Microscope:
- All limitations of TEM also apply to SEM, except that specimens don’t need to be extremely
thin as electrons don’t penetrate.
- SEM directs a beam of electrons on to the surface of the specimen from above, rather than
penetrating it from below.
- Beam is then passed back and forth across a portion of the specimen and the pattern of this
scattering depends on the contours of the specimen surface.
- We can build a 3D image by computer analysis of the pattern of scattered electrons and
secondary electrons produced.
- SEM has a lower resolving power than a TEM.
Around 20 nano meters.
But it’s still 10x better than a LM.
3.3 – MICROSCOPE MEASUEREMENTS AND CALCULATIONS:
Measuring Cells:
- When using a LM we can measure the size of objects using an eyepiece graticule.
- Graticule = glass disc that’s placed in the eyepiece of a miscroscope.
- A scale is etched on the glass disc.
- This scale is typically 10mm long and is divided into 100 subdivisions.
- Scale is visible when looking down the eyepiece of the microscope.
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller 22harmans. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $8.36. You're not tied to anything after your purchase.
