Analysis of cell components Flashcards
- define a microscope
microscopes are instruments that produce a magnified image of an object
- what can act as a magnifying glass
a simple convex glass lens
but such lenses work more efficiently if they are used in pairs in a compound light microscope
- what does the relative long wavelength of light rays mean
the relative long wavelength of light rays means that a light microscope can only distinguish between two objects if they are 0.2 micrometers, or further, apart
- how do we overcome the limitation of light microscopes
we can use beams of electrons rather than beams of light.
with their shorter wavelengths, the beam of electrons in the electron microscope can distinguish between two objects only 0.1 nanometers apart
- formula for magnification
magnification = size of image / size of real object
- define resolution
the resolution, or resolving power, of a microscope is the minimum distance apart that two objects can be in order for them to appear as seperate items
- what does the resolution depend on
the resolving power depends on the wavelength or form of radiation used.
- what does greater resolution mean
greater clarity
- Explain the limit of resolution
increasing the magnification will reveal more detail but beyond this point increasing the magnification will not do this.
the object, while appearing larger will just be more blurred
- why do light microscopes have poor resolution
because of the relatively long wavelength of light
- what are the two main advantages of an electron microscope
shorter wavelength means a higher resolving power
as electrons are negatively charged the beam can be focused using electromagnets
- how do we make an electron microscope work efficiently
Because electrons are absorbed or deflected by the molecules in air, a near-vaccuum has to be created within the chamber of an electron microscope in order for it to work effectively
- what are the two main types of electron microscopes
the transmission electron microscope
the scanning electron microscope
- explain how we get an image from the transmission electron microscope
the TEM consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet.
in a TEM the beam passes through a thin section of the specimen.
parts of this specimen allow the electrons to pass through and so appear bright.
An image is produced on a screen and this can be photographed to give a photomicrograph
- why is the resolving power of 0.1 nm with a TEM not always achieved
difficulties preparing the specimen limit the resolution that can be achieved
a higher energy electron beam is required and this may destroy the specimen
- what are the main limitation of a TEM
the whole system must be in a vacuum and therefore living specimens cannot be observed
a complec staining process is required and even then the image not in colour
the specimen must be extremely thin
the image may contain artefacts
- what are artefacts
things that result from the way the specimen is perpared.
artefacts may appear on the finished photomicrograph but are not part of the natural specimen
it is therefore not always easy to be sure that what we see on a photomicrograph really exists in that form
- explain the result of an image using the TEM
in the TEM the specimens must be extremely thin to allow electrons to penertrate.
the result is therefore a flat 2-d image
- how do we get over a 2d image
we can partly get over this by taking a series of sections through a specimen
we can then build up a 3-d image of the specimen by looking at the series of photomicrographs produced
- what limitations apply to the SEM
all the limitations of the TEM also apply to the SEM, except that specimens need not be extremely thin as electrons do not penertrate
- explain how an image is produced using the scanning electron microscope
the SEM directs a beam of electrons on to the surface of the specimen from above, rather than penertrating it from below.
the beam is them passed back and forth across a portion of the specimen in a regular pattern
the electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface
a 3d image is created by computer analysis of the pattern of scattered electrons and secondary electrons produced
- what do we use to measure the sice of objects
an eyepiece graticule
- describe the eyepiece graticule
the graticule is a glass disc that is placed in the eye [iece of a microscope.
a scale is etched on the glass disc.
this scale is typically 10mm long and is divided into 100 sub-divisions
the scale is visible when looking down the eyepiece of the microscope
- explain how to use the eyepiece graticule
the cale on the eyepiece graticule cannot be used directly to measure the size of objects under a microscopes objective lens because each objective lens will magnify to a different degree
the graticule must first be calibrated for a particular objective lens.
once calibrated in this way, the graticule can remain in position for future use, provided the same objective lens is used
- explain how to calibrate the eyepiece graticule
you need to use a stage micrometer.
this slide also has a scale etched onto it.
usually, the scale is 2nm long and its smallest sub-divisions are 0.01 mm
