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AQA alevel biology - Cells (unit 2) AS
These are my notes for the second unit of alevel biology - Cells! I got an A in my alevel AQA biology exam using these notes. Happy revising :)
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Cells,Microscopes
• MICROSCOPE = an instrument that produces a magnified image fro
an object
• Light microscopes have a long wavelength and can distinguish
between two objects that are 0.2um apart or more
• Electron microscopes have a short wavelength and can distinguish
between two objects that are 0.1nm apart or more
,Magnification
• MAGNIFICATION = how many time bigger the image is compared t
the object
• The material put under microscope is called the object
• The appearance of the object under the microscope is called the
image
,Resolution
• RESOLUTION = the minimum distance apart for two objects to be seen as
separate items
• Resolution depends on wavelength
• Light microscopes = 0.2um
• 0.2 um or more = two objects
• Less than 0.2um = one object
• Electron microscopes = 0.1nm
• 0.1nm or more = two objects
• Less than 0.1nm = one object
• Resolution is better in electron microscopes
,LIGHT MICROSCOPE ELECTRON MICROSCOPE
cheap Expensive
Easy to operate Hard to operate
Small and needs little room Large and needs a special room
Simple preparation Complex preparation
Easy staining process and image Complex staining process and e
produced is in colour then image is not in colour
Specimen doesn’t have to be in Specimen must be in vacuum- a
vacuum- and therefore can be alive therefore must be dead
or dead
,Electron microscopes
1. TEM
• Electron gun produces a ray of electrons that is concentrated onto
specimen using electromagnets
• The electrons pass through the specimen and is absorbed forming
dark areas or passed through forming light areas
• This forms an image that is photographed forming a photomicrogra
,Disadvantages
1. Must be in a vacuum
2. Artefacts appear on the final photomicrograph but not on the
object
3. Complex staining process and even then the final photomicrogra
is not in colour
4. Specimen must be thin so electrons can penetrate through
5. The image produced is 2D but a 3D image can be produced by
looking at different 2D photomicrographs
6. Better resolving power than SEM
,Electron microscopes
2. SEM
• Electron gun produces a ray of electrons that pass through specime
and move back and forth in a regular pattern
• The pattern in which the electrons are scattered depends on the
colour of the specimen the electrons hit
• A computer can analyse the pattern of scattered electrons and
produce a 3D image
,Disadvantages
1. Lower resolving power than TEM
2. Artefacts appear on the final photomicrograph but not on the
object
3. Complex staining process and even then the final photomicrogra
is not in colour
4. Needs to be in a vacuum
5. Specimen doesn’t have to be thin as electrons don’t penetrate
through
, How did the scientific community
distinguish between artefacts and
cell organelles ?
• By repeatedly preparing specimens in different ways
• If one object was seen with one preparation technique and not
another, its likely to be an artefact
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