Hand-typed summary notes of Cell Structure OCR-A Level Biology Unit 2. Very clear and efficient, providing you with all the key points you need to ace your exam. I achieved an A* in Biology, using these notes!
Begins with details on microscopy and moves onto the ultrastructure of cells, before f...
AS Biology – Cell Structure
Microscopy
Resolution (Resolving power) = the ability to distinguish between 2 points on
an image. The greater the resolution, the greater the detail. It depends on the
wavelength of light.
Magnification = how many times larger the microscope can make the image
appear compared to its real size.
Light microscope types:
Compound light microscope
Laser scanning confocal microscope
Compound Light Microscope
Light rays pass though condenser lens before passing through the specimen if
it’s thin enough. Areas where more light passes through are brighter.
Advantages:
Quick & Easy to use
Can see live specimens
Cheaper
Can see colours
Disadvantages:
Wavelength of light is large (compared to electrons) so the resolution isn’t
good
Images close together may be invisible or blurred
Laser Scanning Confocal Microscope
Laser beams pass through a light source aperture (hole) and then are focused
by an objective lens into a small focal volume within or on the surface of a
specimen.
Advantages:
Can be a live specimen
Can produce a 3D image
Better resolution than a compound light microscope
Staining & Sectioning
We stain specimens to increase the contrast which allows us to differentiate
between organelles. Different stains bind to different chemicals in different
structures. This is called differential staining and makes it easier to see more
detail. Blot sample to remove excess stain.
Iodide > starch & cellulose
, Sectioning = cutting up a specimen into thin slices so light can penetrate.
Electron microscope types:
Scanning Electron Microscope (SEM)
Transmission Electron Microscope (TEM)
Scanning Electron Microscope
Electrons bounce off specimen and areas where more electrons bounce of are
brighter. This produces a 3D image of the specimen and can look incredible.
Advantages:
Can see specimens in 3D
Higher resolution and magnification than all light microscopes
Disadvantages:
Can’t be used to look at living things because samples need to be dried
and coated in metal before visualising. They also must be seen in a
vacuum because electrons can be affected by air. It’s therefore
expensive to run.
The resolution isn’t as high as a TEM.
Artefacts (damage to specimen) can be produced during the preparation
of the specimen.
Only can see surface features.
Transmission Electron Microscope
Electrons pass though specimen. Areas where more electrons pass through are
brighter.
Advantages:
Highest magnification and resolution
Can see internal structures of specimens
Can see the relationship between structures at a high resolution
Disadvantages:
No colour
Expensive to run
Cannot be living specimen
Specimen must be cut very thin because electrons cannot penetrate
material as well as light.
Light SEM TEM
Microscope
Maximum 1,500 200,000 2,000,000
Magnification
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