In-depth notes on Module 2 in order of specification. Everything is included and expected to be learned in terms of the OCR A Biology A-level specification. It included diagrams to explain each concept.
2.1.1 Cell Structure
(a) the use of microscopy to observe and investigate different types of cell
and cell structure in a range of eukaryotic organisms.
To include an appreciation of the images produced by a range of microscopes:
light microscope, transmission electron microscope, scanning electron
microscope and laser scanning confocal microscope
Microscopes can be used to analyse cell components and observe
organelles
Optical (light) microscopes
Use of visible light (400-700nm) to from an image
Relatively cheap
Easy to use
Portable
Used to observe whole living specimens
Limited resolution : 200nm
Maximum useful magnification: x1500
Cannot be used to observe ribosomes or smaller organelles
Transmission electron microscopes (TEMs)
Uses electromagnets to focus a beam of electrons
Transmitted through the specimen
Denser = absorb more electrons = darker
TEM advantages:
They give high-resolution images (more detail)
allows the internal structures within cells
TEM disadvantages:
Only use thin specimens
Cannot use live specimens since there is a vacuum inside TEM
removing water and can kill the specimen whilst observing
Lengthy treatment to observe the specimen
Not a colour image
Scanning electron microscopes (SEMs)
, Scan a beam of electrons across the specimen
Beam bounces off the surface of the specimen and the electrons are
detected, forming an image
Form 3D images that show the surface of specimens
SEM advantages:
Used on thick 3D specimens
Allow the external 3D structure of specimens to be observed
SEM disadvantages:
Lower resolution images
Cannot observe live specimens
Not coloured
Laser scanning confocal microscopes
Specimens/cells must be stained with fluorescent dyes
Specimen scanned with a laser beam
Laser beam is reflected by the fluorescent dyes
Multiple depth of the specimen are scanned to produce an image
Advantages:
Used on thick or 3D specimens
External 3D structure specimens to be observed
Very clear images
High resolution
Disadvantages:
Slow process
Long
Laser can cause photodamage to the cells
(b) the preparation and examination of microscope slides for use in light
microscopy
Including the use of an eyepiece graticule and stage micrometre
How optical microscopes work
Light is directed through the thin layer of biological material
, Supported on a glass slide
Light is focused through several lenses so that an image is visible
through the eyepiece
The magnifying power of the microscope can be increased by
rotating the higher power objective lens into place
Preparing a slide using a liquid specimen:
Add a few drops of the sample to the slide using a pipette
Cover the liquid/smear with a coverslip
Gently press down to remove air bubbles
Wear gloves to ensure there is no cross-contamination of foreign
cells
Methods of preparing a microscope slide using a solid specimen:
Use scissors to cut a small sample of the tissue
Cut a very thin layer of cells from the tissue sample to be placed on
the slide (using a scalpel or forceps)
The tissue needs to be thin so that the light from the microscope
can pass through
Apply a stain
Gently place a coverslip on top and press down to remove any air
bubbles
Graticule: Small disc that has an engraved ruler
It can be placed into the eyepiece of a microscope
Act as a ruler in the field of view
As a graticule has no fixed units it must be calibrated for the objective
lens that is in use.
This is done by using a stage micrometre
Stage micrometre: Scale engraved on a microscope slide
By using the two scales together the number of micrometres each
graticule unit is worth can be worked out
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