Analysis of cell components

Question 1

• define a microscope

Answer 1

microscopes are instruments that produce a magnified image of an object

Question 2

• what can act as a magnifying glass

Answer 2

a simple convex glass lens

but such lenses work more efficiently if they are used in pairs in a compound light microscope

Question 3

• what does the relative long wavelength of light rays mean

Answer 3

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

Question 4

• how do we overcome the limitation of light microscopes

Answer 4

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

Question 5

• formula for magnification

Answer 5

magnification = size of image / size of real object

Question 6

• define resolution

Answer 6

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

Question 7

• what does the resolution depend on

Answer 7

the resolving power depends on the wavelength or form of radiation used.

Question 8

• what does greater resolution mean

Answer 8

greater clarity

Question 9

• Explain the limit of resolution

Answer 9

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

Question 10

• why do light microscopes have poor resolution

Answer 10

because of the relatively long wavelength of light

Question 11

• what are the two main advantages of an electron microscope

Answer 11

shorter wavelength means a higher resolving power

as electrons are negatively charged the beam can be focused using electromagnets

Question 12

• how do we make an electron microscope work efficiently

Answer 12

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

Question 13

• what are the two main types of electron microscopes

Answer 13

the transmission electron microscope

the scanning electron microscope

Question 14

• explain how we get an image from the transmission electron microscope

Answer 14

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

Question 15

• why is the resolving power of 0.1 nm with a TEM not always achieved

Answer 15

difficulties preparing the specimen limit the resolution that can be achieved

a higher energy electron beam is required and this may destroy the specimen

Question 16

• what are the main limitation of a TEM

Answer 16

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

Question 17

• what are artefacts

Answer 17

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

Question 18

• explain the result of an image using the TEM

Answer 18

in the TEM the specimens must be extremely thin to allow electrons to penertrate.

the result is therefore a flat 2-d image

Question 19

• how do we get over a 2d image

Answer 19

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

Question 20

• what limitations apply to the SEM

Answer 20

all the limitations of the TEM also apply to the SEM, except that specimens need not be extremely thin as electrons do not penertrate

Question 21

• explain how an image is produced using the scanning electron microscope

Answer 21

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

Question 22

• what do we use to measure the sice of objects

Answer 22

an eyepiece graticule

Question 23

• describe the eyepiece graticule

Answer 23

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

Question 24

• explain how to use the eyepiece graticule

Answer 24

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

Question 25

• explain how to calibrate the eyepiece graticule

Answer 25

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