Cell Structure Flashcards
Define magnification
The degree to which the image size is larger than the object itself.
Define resolution
The ability to distinguish between two separate points/objects that are close together, in detail.
What are microscopes, and what can they do?
Microscopes are instruments which enlarge an object to a chosen scale. As a result, scientists have been able to discover the sub-cellular structures of cells and view the functions of the structures, improving scientific understanding.
Cell theory was named after the ability to see individual cells. What does this theory state?
1) Both plant and animal tissue are composed of cells
2) Cells are the basic unit of all life.
3) Cells only develop from existing cells.
What the advantages of using a light microscope?
Easily available, cheap, can be used in the field, and can observe living, as well as dead specimens.
What are the disadvantages of using a light microscope?
Low magnification and resolution. Less detail can be seen as a result. Can only show 2D images. Images tend to be low contrast, as most cells do not absorb a lot of the light.
What types of lenses do compound microscopes have?
1) Objective lens - placed near the specimen.
2) Eyepiece lens - specimen viewed through.
How does a light microscope work?
The objective lens produces a magnified image, which is further magnified by the eyepiece lens. The objective/eyepiece configuration allows for greater magnification and reduced chromatic aberration. Illumination is provided by a light underneath the sample, (which is on a slide, on the stage) . Some microscopes allow illumination from above with an opaque specimen.
Describe a dry mount sample preparation. Give examples of specimen that can be viewed.
Solid specimens are viewed whole or cut into very thin slices with a sharp blade. This is called sectioning. The specimen is placed in the centre of the slide and cover slip placed over the sample. e.g. hair, pollen, dust or insect parts, as well as muscle tissue and plants.
Describe a wet mount sample preparation. Give examples of specimen that can be viewed.
Specimens are suspended in a liquid, like water or an immersion oil. A cover slip is placed on from an angle. e.g. aquatic sample and other living organisms.
Describe a squash slide sample preparation. Give examples of specimen that can be viewed.
A wet mount is first prepared. A lens tissue is used to press down the cover slip. Potential damage to the cover slip can be avoided by squashing the sample between two microscope slides. This is a good technique for soft samples. e.g. root tip squashes are used to view cell division.
Describe a smear slide sample preparation. Give examples of specimen that can be viewed.
The edge of a slide is used to smear the sample, creating a thin, even coating on anther slide. A cover slip is then placed over the sample. e.g. a smear slide for a sample of blood. This helps view the cells easily.
How is resolution limited?
Limited by the diffraction of light as it passes through samples. The structures present in the specimens are very close together and the light reflected from individual structures can overlap due to diffraction. This means the structures are no longer seen as separate entities and detail is lost. Also determined by the wavelength of light as it passes through the sample. Wavelength of light is approx. 500nm.
What is diffraction?
The bending of light as it passes close to the edge of an object.
What happens to an image if magnification is increased, but resolution is not?
As magnification is increased, the limited resolving power gives a blurred image.
What are the benefits of using stains?
The cytosol of cells and other structures are often transparent. Stains increase the contrast as different components within a cell absorb the stain to differing degrees. The greater the contrast, the more visible the cells, and the easier the identification.
How do you prepare a slide for staining?
Place the sample/specimen on the slide and allow to air dry. This is heat - fixed by passing through a flame. The specimen adheres to the slide and will take up the stains.
What can be used for positive stain technique?
Crystal violet or methylene blue are positively charges dyes, which are attracted to negatively charged materials in the cytoplasm, leading to the staining of cell components.
What can be used for negative stain technique?
Nigrosin and Congo red are negatively charged and are repelled by the negatively charged cytosol. The dyes stay outside the cells, leaving the cells unstained, which stand out from the stained background
What is differential staining?
Helps distinguish between two types of organisms that would otherwise be hard to identify. It can also differentiate between different organelles of a single organism with a different tissue sample.
Describe gram stain technique
Used to separate bacteria into two groups, Gram-positive bacteria and Gram-negative bacteria. Crystal violet is applied to a specimen on a slide, then iodine, which fixes the dye. The slide is washed with alcohol. The Gram-positive retain the crystal violet stain and will appear blue or purple under a microscope.
Gram negative bacteria have thinner walls and lose the stain. They are stained with safranin dye, which is called a counterstain. The bacteria then appear red.
What effect does penicillin have on Gram-positive and Gram-negative bacteria?
Gram-positive are susceptible to penicillin, which inhibits the formation of cell walls. Gram-negative have much thinner cell walls, so are not susceptible.
Describe acid-fast technique
Used 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 washed with dilute acid-alcohol solution. Mycobacterium are not affected by the solution and retain the carbolfuchsin stain, which is bright red. Other bacteria lose the stain and are exposed to methylene blue.
How are pre-prepared slides produced?
1) Fixing - chemicals like formaldehyde are used to preserve specimens in as near-natural state as possible.
2) Sectioning - specimens are dehydrated with alcohols then placed in a mould with wax or resin to form a hard block. This can be sliced thinly with a microtome.
3) Staining - specimens treated with multiple stains to show different structures.
4) Mounting - specimens secured to a microscope slide and a cover slip placed on top.
Name some rules for scientific drawings
Include a title, state magnification, use a sharp pencil, use white, unlined paper, use 50% of the page, draw a smooth, continuous line, do not shade, annotate, label lines should not cross or have arrow heads, etc.
What are the units are measurements and how are they converted between?
Meter, centimetre, millimetre, micrometre, nanometre and picometre. Getting smaller x1000. Getting bigger /1000
How is magnification calculated?
magnification = size of image / actual size of the object
Why does an eyepiece graticule need to be used?
The true magnification of the different lenses can vary slightly from the magnification stated, so every microscope and lens had to be calibrated using an eyepiece graticule and a slide micrometer.
What is an eyepiece graticule?
A glass disc marked with a fine scale of 1 to 100. The scale has no units and remains unchanged whichever objective lens is in place. The relative size of the divisions increases with each increase in magnification.
What is a stage micrometer?
Used to to calibrate the scale on the graticule at each magnification. A microscope slide with a very accurate scale in micrometres engraved on it. The scale marked on the micrometre slide is usually 100 divisions = 1mm, so 1 division = 10µm. You calibrate the eyepiece graticule scale for each objective lens separately. Once all 3 lenses are calibrated, if you measure the same cell using the 3 different lenses you should get the same actual measurement each time.
