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Summary OCR a level biology - module 2 A* notes
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OCR a level biology module 2 A* concise notes
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Microscopes and imagesThere is a range of microscopes for creating images of cells and ultrastructure
Each microscope has different limits of magnification and resolution.
Magnification is the ratio of image size to object size (size of image/size of object). Resolution is
the ability to distinguish between two objects that are close together — the ability to provide
detail in the image.
Light microscopes have a magnification of 1000-2000x, and a resolution of 50-200 nm. They are
used for viewing cells and tissues.
Light microscopes are cheap and easy to use and allow us to see living things, but their
resolution is limited.
Scanning electron microscopes have a magnification of 50,000-500,000x, a resolution of 0.4-20
nm, and are used for viewing the surface of cells and organelles, and providing depth in three
dimensional images.
Transmission electron microscopes have a magnification of 300,000-1,000,000x, a resolution of
0.05-1.0 nm, and are used for detailing organelle ultrastructure.
Scanning electron and transmission electron microscopes have better resolution than a light
microscope, which means it is worth magnifying the image more as the image will show more
detail. Scanning electron microscopes also:
• give 3D images with depth of field
• are good for viewing surfaces
Electron microscopes are large and very expensive. They require trained operatives
The sample must be dried out and is therefore dead. This may affect the shape of the features
seen (called an artefact).
The image is in black and white, but colours may be added later by computer graphics. These
are called false colour electron micrographs.
Laser scanning confocal microscopes have a magnification of 1000-2000x, a resolution of 50-
200 nm, and are used to produce three dimensional images with good depth selection.
Laser scanning confocal microscope can also see living things and have the advantage that
they can focus at a specific depth so the image is not confused by other components that are
not in focus. It relies on a computer to piece together all the information from the dots of light
created by the lasers. This means that the image is an interpretation rather than a real-life
image.
, Staining
Most cell components are colourless and hard to see. Staining is the application of coloured
stains to the tissue or cells. Staining:
1 makes objects visible in light microscopes
2 increases contrast so that the object can be seen more clearly
3 is often specific to certain tissues or organelles. For example, acetic orcein stains
chromosomes dark red, eosin stains cytoplasm, Sudan red stains lipids, and iodine in potassium
iodide solution stains the cellulose in plant cell walls yellow and starch granules blue/black
In an electron microscope, the stains are actually heavy metals or similar atoms that reflect or
absorb the electrons.
Cells
Cells are the basic unit of living organisms. All eukaryotic cells share a similar basic structure
containing membrane-bound organelles. Each organelle, whether membrane-bound or not, has
its own function within the cell.
You should be able to prepare cells and tissues to view under an light microscope. This involves
creating a smear or cutting a very thin section. These cells can then be stained appropriately
and covered by a cover slip. Viewing these cells, interpreting what you can see and drawing
what
you see
are
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