2.1 Microscopy Flashcards
What does the cell theory state?
- Plant and animal tissues are composed of cells
- Cells are basic units of life
- Cells develop from existing cells
What are the benefits of light microscopes?
- Easily available
- Cheap
- Can be used outside in the field
- Can be used to observe both living organisms and dead (prepared) specimens
How does a compound light microscope work?
Why is a compound light microscope better than a simple light microscope?
Two lenses:
- Objective lens - placed near specimen
- Eyepiece lens - through which specimen is viewed
How it works:
Objective lens produces a magnified image which is magnified again by the eyepiece lens
- Illumination is provided by light underneath the sample
- Illumination provided from the top if the specimen is non-transparent
Why it’s better:
The combination of eyepiece lens + objective lens produces images with higher magnification and reduced blurring when compared to a simple light microscope
What are the 4 methods of preparing a sample?
- Dry Mount
- Wet Mount
- Squash Slides
- Smear Slides
How do you prepare a dry mount slide? Give examples.
- Either:
- Solid specimens viewed as whole
- Solid specimens cut into thin slices (known as sectioning) - Specimen is placed onto centre of slide + cover slip is placed over sample
Example:
- Viewing hair, pollen, insect parts or dust as whole
- Muscle/plant tissue as sectioned
How do you prepare a wet mount slide?
- Specimens are suspended in a liquid e.g. water or immersion oil
- A cover slip is placed at an angle:
Example: Aquatic sample
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How do you prepare a squash slide?
- Wet mount first prepared
- Lens tissue is used to press down cover slip
{damage to cover slip can be prevented by squashing the sample between 2 slides}
- Good for soft samples
E.g. Root tip squashes to look at cell division
How do you prepare a smear slide?
- Edge of a slide used to smear sample, creating thin, even coating on another slide.
- A cover slip is placed over the sample
Example: A sample of blood is a smear slide
Why are images at low contrast with a basic light microscope?
Most cells don’t absorb a lot of light
What limits resolution in light microscopy?
- Wavelength of light
- Diffraction of light
What is diffraction?
Bending of light as it passes close to the edge of an object
- Why do stains increase contrast of a cell’s components? 2. Why is this useful?
- Increase contrast as different components of cells absorb stains to different degrees
- Increases contrast which allows components to become visible so they can be identified
How do you prepare a sample for staining?
- Sample is placed on a slide + allowed to air dry
- Sample then heat fixed by passing through a flame
- The specimen will adhere to the slide and absorb the stains
How do positive dyes work (in terms of staining)?
Positive dyes (i.e. Crystal violet/methylene blue, positively charged) get attracted to negatively charged materials in cytoplasm, causing staining of cell’s components
How do negative dyes work (in terms of staining)?
Negative dyes (i.e. Nigrosin or Congo Red, negatively charged) get repelled by negatively charged cytosol.
These dyes stay outside of cells, leaving the cells unstained, which then stand out against the stained background (Called negative stain technique)
