Module 2- Foundations in biology Flashcards
What is a microscope?
A microscope is an instrument which enables you to magnify an object hundreds, thousands and even hundreds of thousands of times. We can see many large organisms with the naked eye, but microscopes open up a whole world of unicellular organisms. By making visible the individual cells which make up multicellular organisms, microscopes allow us to discover how details of their structures relate to their functions.
What is cell theory?
Both plant and animal tissue is composed of cells.
Cells are the basic unit of all life.
Cells only develop from existing cells.
How does a light microscope work?
A light microscope has two lenses- the objective lens, which is placed near to the specimen, and an eyepiece lens, through which the specimen is viewed.
The objective lens produces a magnified image, which is magnified again by the eyepiece lens. This objective/eyepiece lens configuration allows for much higher magnification and reduced chromatic aberration than that in a simple light microscope.
illumination is usually provided by a light underneath the sample. Opaque specimens can be illuminated from above with some microscopes.
Name the parts of a compound light microscope?
Coarse focusing knob Fine focusing knob Eyepiece lens Objective lens Turret Specimen stage Light from mirror or light bulb.
How is an observation of a dry specimen on a light microscope prepared?
Solid specimens are viewed whole or cut into very thin slices with a sharp blade, this is called sectioning. The specimen is placed on the centre of the slide and a cover slip is placed over the sample. For example hair, pollen, dust and insect parts can be viewed whole in this way, and muscle tissue or plants can be sectioned and viewed in this way.
How is an observation of a smear specimen on a light microscope prepared?
The edge of a slide is used to smear the sample, created a thin, even coating on another slide. A cover slip is then placed over the sample. An example of a smear slide is a sample of blood. This is a good way to view the cells in the blood.
Why must a cover slip be placed onto a wet mount at an angle?
You could potentially damage the specimen if you don’t place it down at an angle as you will squash the specimen.
How can a wet mount be prepared for viewing a specimen under a microscope?
Specimens are suspended in a liquid such as water or an immersion oil. A cover slip is placed on from an angle, as shown. For example, aquatic samples and other living organisms can be viewed this way.
When preparing a wet mount, why should the refractive index(ability to bend light) of the medium should be roughly the same as glass?
Prevent distortion Diffraction between liquid and glass reduce / prevent, diffraction between liquid and glass.
How do stains help in the observation of specimens?
The cytosol(aqueous interior) of cells and other cell structures are often transparent. Stains increase contrast as different components within a cell take up stains to different degrees. The increase in contrast allows components to become visible so they can be identified.
How do you prepare a sample for staining?
To prepare a sample for staining it is first placed on a slide and allowed to air dry. This is then heat-fixed by passing through a flame. The specimen will adhere to the microscope slide and will then take up stains.
Name two positively and two negatively charged dyes used in microscopy.
Positively charged: Crystal violet and Methylene blue are attracted to negatively charged materials in cytoplasm leading to staining of cell components.
Negatively charged: Nigrosin and Congo red are repelled by the negatively charged cytosol.
When is the gram stain technique used?
Gram staining is used to separate bacteria into two groups, gram positive bacteria and gram negative bacteria. Crystal violet is first applied to a bacterial specimen on a slide, then iodine, which fixes the dye. The slide is then washed with alcohol. The gram-positive bacteria retain the crystal violet stain and will appear blue or purple under a microscope.
Gram negative bacteria have thinner cell walls and therefore lose the stain. They are then stained with safranin dye, which is called a counterstain. These bacteria will then appear red. Gram-positive bacteria are susceptible to the antibiotic penicillin, which inhibits the formation of cell walls. Gram-negative bacteria have much thinner cell walls that are not susceptible to penicillin.
What is the acid-fast staining technique in microscopy?
Acid-fast technique is sued to differentiate species of Mycobacterium from other bacteria. A lipid solvent is used to carry carbolfuchsin dye into the cells being studied. The cells are then washed with a dilute acid-alcohol solution. Mycobacterium are not affected by the acid-alcohol and retain the carbolfuchsin stain which is bright red. Other bacteria lose the stain and are exposed to a methylene blue stain, which is blue.
What are the four stages in the preparation of slides to look at in microscopy?
Fixing- Chemicals like formaldehyde are used to preserve specimens in as near-natural a state as possible.
Sectioning- Specimens are dehydrated with alcohols and then placed in a mould with wax or resin to form a hard block. This can be sliced thinly with a knife called a microtome.
Staining- Specimens are often treated with multiple stains to show different structures.
Mounting- The specimens are then secured to a microscope slide and a cover slip placed on top.
