Microscopy

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Why couldn't we see cells until mid 19th century?
didn't have access to microscopes with high enough magnification
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What is cell theory?
both plant and animal tissue composed of cells, cells basic unit of all life, cells only develop from existing cells
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Define magnification.
the number of times larger the image is compared to the size of the actual object
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Why does magnification not increase the amount of detail?
As resolution does this
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Define resolution.
the ability to see individual objects as separate entities and the ability to see detail
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What is resolution limited by?
the diffraction of light as it passes through samples
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What is diffraction?
tendency of light waves to spread as they pass close to physical structures such as those present in samples
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How can detail be lost by diffraction?
the light reflected from individual structures can overlap due to diffraction, meaning the structures can no longer be seen as separate entities
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Compound light microscope?
easily available, relatively cheap, used out in field, can observe living and dead
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How do light microscopes work?
light is shone underneath the sample and passes through the sample into the lenses
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Objective lens?
produces a magnified image of the specimen
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Eyepiece lens?
magnifies the image produced by the objective lens
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What is the advantage of having two lenses?
allows the compound light microscope to produce a greater magnification
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How can we stop diffraction in samples?
in sample preparation, ensure any materials have a similar refractive index to glass
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Dry mount?
solid specimens viewed as a whole or sectioned, specimen placed in centre of slide and cover slip placed over
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What would a dry mount be used for?
hair, pollen, dust, insects
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Wet mount?
specimens suspended in a liquid such as water or immersion oil with similar refractive index to glass, cover slip at 45 to reduce air bubbles
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What is a wet mount used for?
aquatic samples and other living organisms
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Squash slide?
wet mount prepared, then lens tissue gently press down on cover slip. damage to slip can be avoided by squashing between two microscope slides, good for soft samples
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What is a squash slide used for?
root tip squash
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Smear slide?
edge of slide used to smear sample, creating thin, even coating on another slide, cover slip then placed over. used for blood
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Why is staining used?
most cells aqueous and transparent, do not absorb light, so low contrast, staining inc. contrast so components visible
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How is a sample prepared for staining?
placed on slide and air dried, heat fixed to make it adhere, stain added following method
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Methylene blue (crystal violet)?
pos. charged dye attracted to neg. charged cell components in nucleus and cytoplasm
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Iodine?
stains plant cells as it interacts with the starch stored in plant cells
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Negative staining?
neg. charged stain repelled by neg. charged cell components in cytoplasm. do not enter cell, leaving cells unstained which stand out against background
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What happens in fixing?
secures sample to slide, chemicals like formaldehyde used to preserve specimens in near natural state as possible
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Sectioning?
dehydrated with alcohols to stop metabolic reactions, placed in mould with wax or resin forming hard block, can be sliced very thinly with microtome
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Mounting?
specimen secured to microscope slide and cover slip placed on top
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Why is the resolution better in electron microscopes?
a beam of electrons used instead of light, shorter wavelength so resolution better
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What is the inside of an electron microscope?
vacuum to ensure electron beams travel in straight lines
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Disadvantages of electron microscopy?
controlled environment, harsher preparation, artefacts in samples
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Define artefacts.
visible structural details caused by processing the specimen,which are not a feature of the specimen
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Preparation for electron microscopy?
fixing with them or freezing to prevent decomposition, embedding in resin so surface seen or fractured open for inside, dehydration to prevent vaporisation which would damage, staining w heavy metals to inc contrast
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Transmission electron microscope?
beam of electrons transmitted through the specimen,focused produce an image,similar to light microscopy
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Scanning electron microscope?
beam of electrons sent across surface of specimen. reflected electrons collected and produce image, direction of electrons controlled by magnet
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Laser scanning confocal microscopy?
uses higher light intensities in form of lasers, specimen treated with fluorescent dye, non invasive
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Define fluorescence.
reabsorption and radiation of light
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Other cards in this set

Card 2

Front

What is cell theory?

Back

both plant and animal tissue composed of cells, cells basic unit of all life, cells only develop from existing cells

Card 3

Front

Define magnification.

Back

Preview of the front of card 3

Card 4

Front

Why does magnification not increase the amount of detail?

Back

Preview of the front of card 4

Card 5

Front

Define resolution.

Back

Preview of the front of card 5
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