2.1 Light & Electron Microscopes

7/29/2019 2.1 Light & Electron Microscopes

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Chapter 2 : Structure of cells

and organelles

2.1 Light and Electron

Microscopes


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Light & Electron Microscopes

Microscope is used to produce amagnified image of an object or

specimen. Important tool in the study of cell

structure


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Light & Electron Microscopes

Light microscope(a) Light passes through a specimen (the object) & then

through two sets of lenses, the objective lens & theeyepiece lens.

(b) The lenses refract (bend) light to give a magnified imageof the object. The image may be projected into theviewers eye or onto a photographic film, which is calledphotomicrograph / light micrograph.

(c) The resolving power of the light microscope is limited bythe wavelength of light. Its resolution is 200nm / 0.2 m.Only magnify objects up to about 1500 times withoutlosing clarity.


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Eyepiece

BodyTube

RevolvingNosepieceArm

ObjectiveLens

StageStage

Clips CoarseFocus

FineFocus

Base

Diaphragm

Light


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Electron microscope

(a) Uses a beam of electrons.

(b) Is used for the detailed study of cells.

(c) Electron beams have short wavelength. Hence, has

greater resolving power & produce much higher

effective magnification.

(d) The resolution is 0.5 - 1 nm.

(e) Two main types of electron microscope :

i) transmission electron microscope (TEM)ii) scanning electron microscope (SEM)


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Transmission Electron Microscope


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Scanning Electron Microscope


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(a) is used to study the details ofultrastructure of cells.

(b) Electron beams generated by a hot cathode is acceleratedtowards the specimen.

(c) Electron beam passes through ultra-thindehydrated sectionsofdead specimen.

(d) It is placed in a vacuum to minimise electron scattering due tocollision between electron and molecule in the air.

(e) The sections are treated with heavy metals such as uranyl, leadacetate or osmium tetroxide. Electrons are absorbed by theheavily stained parts but pass through the lightly stained parts.

This provides contrast between different parts of the specimen.(f) Electromagnets are used to bend & focus the electrons

producing an image on fluorescent screen or photographicfilm. A two dimensional view of the specimen is produced. Anelectron micrograph is a photograph taken.

Transmission electron microscope (TEM)


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TEM Sample support in grid


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Image of bacteria (Bacillus subtilis)
http://en.wikipedia.org/wiki/File:Bacillus_subtilis.jpg


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(a) is used to produce a three-dimensional view of

objects.

(b) It is uses an electron beam to scan along the surface

of a dead specimen which has been coated with a thin

layer of heavy metal, for example, gold.(c) A large portion of the electrons are reflected. Some of

the electrons absorbed excite other electrons at the

surface to give off secondary electrons.

(d) A recording of the emission from the specimenprovides a picture of the specimen.

Scanning electron microscope (SEM)


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Image of an ant in SEM
http://en.wikipedia.org/wiki/File:Ant_SEM.jpg


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1 kilometer (km) = 1000 (103) metres

1 metre (m) = 100 (102) centimetres

1 metre (m) = 1000 (103) milimetres

1 millimetre (mm) = 1000 (103) micrometres (m / microns)

1 micrometre (m) = 1000 (103) nanometres (nm)

Magnification

Magnification is the number of times that an image is larger than the

specimen and is usually given by the formula:

magnification = size of image

size of specimen

Measurement

2.1 Light & Electron Microscopes

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