Year 12 A Level Biology Revision
What are the 4 different types of microscopes?
Light Microscope
Transmission electron microscope
Scanning electron microscopes
Laser Scanning Confocal microscopes
What is Resolution?
The minumum distance between 2 objects in which they can still be viewed at seperate
How is resolution determined in an optical microscope?
It is determined by the wavelength of light
How is resolution determined in an electron microscope?
It is determined by the wavelength of the beam of electrons
What is magnification?
It is defined as how many times larger the image is compared to the object
What are the 4 types of slide preparations?
Dry mount
Wet mount
Squash slide
Smear slide
What is the dry mount slide preparations?
This is when thin slices or a whole specimen is viewed with just the cover slip placed ontop. Can be usedd on plant tissue or a hair
What is wet mount slide preparations?
This is when water is added to the specimen before lowering the coverslip with a mounted needle to prevent air bubbles from forming. Aquatic organisms an be viewed this way
What is squash slide preparation?
Similar to went mounts but you push down on a cover slip to squash the sample to ensure you have a thin layer to allow light to pass through. Used in the root tip squash practicle to view chromosomes in mitosis
What is smear slide preparation?
This is when you use the edge of a slide to smear the sample across another slide to creae a smooth thin, even coated specimen. A cover slip is placed ontop after the smearing. Used when examining blood cells in a blood sample
What is the formula for magification
Magnification = image size
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real size
What is differential staining?
A technique used involving many different chyemical stains being used to stain different parts of the cell in different colours
What is crystal violet and methylene blue used for?
Are two commonly used stains. They are positively charged which means they are attracted to negatively charged materials
What are Nigrosin and Congo red used for?
These are negatively charged stains which means they cannot enter the cells as cytosol repels them. This creates a stained background so that the unstained cell stands out
What is crystal violet used for in gram staining?
Add crystal violet and then iodine to fix the stain, and alcohol is used to wahs away any unbond stain. Gram positive bacteria appear blue/purple as the stain is retained due to thicker peptidoglycan cell wall later absorbing the dye
Why can Gram-negative bacteria not absorb crystal violet?
They can not absorb the crytsal violet stain as their peptidoglycan cell wall is too thin, so they do not retain the stain due to their thinner walls
Why is safranin used on gram negative bacteria?
this stain is able to stain the gram negative bacteria turning them red
Why is differentiating between gram negative and gram positive bacteria so helpful?
Being able to distinguish between the two types of bacteria can assist medics to prescribe an approppiate antiobiotic to someone with a bacterial infection
What are the rules in scientific drawings?
1. Draw in pencil
2. Title the diagram to indicate what the specimen is
3. State the magnification
4. Annotate cell components, cells and sections of tissue visible
5. Do not sketch. Only use solid lines that dont overlap
6. Do not colour in or shade
What are the aim of diagrams that are drawn based of observations from under a microscope?
To show size, location and proportion
Why do electron microscopes have a high resolution?
A beam of electrons have a very short wavelength producing a high resolution so small organelles and internal structures can be visualised
How do eletron microscopes create an image?
Using an electromagnet to focus the beam of negatively charged electrons
Why cant electron microscopes view living specimens?
Electrons are absorbed by air and therefore only non-living specimens can be examined since they can only bre used in a vaccuum. Images are also in black and white as the sample must be stained
When are Transmission electron microscopes used and how do they work?
You would use and extremely thin specimen which is stained and placed in a vacuum. An electron gun produces a beam of electrons which pass through the specimen. Some parts of the specimen absorb the electrons which make them darker. A 2D image is produced as the electrons are passing through not bouncing off. It is useful as it provides a detailed internal structure of the cells
What is a scanning electron microscope and how do they work?
The specimen doesnt need to be thin as electrons are not passing through. Electrons are beamed onto the surface of a specimen and reflect/scatter back in different ways depending on the contours of your specimen which prouce the 3D image