Cell structure: The electron microscope 3.2
Give four advantages of the optical microscope.
- Cheap. - Easy to use. - Portable. - Can be used to study whole live specimen.
How is a sample prepared for the TEM?
- Chemically-fixed and so is: - Dehydrated, sample isn't live. - Stained with metal salts.
How is the sample prepared for an SEM?
- Fixed, not live. - Coated with a film of metal. - In a vacuum.
What type of magnification do microscopes use?
- Linear. - Object's ratio of width:height is preserved.
How the SEM work?
- The SEM passes a beam of electrons back and forth across a portion of the specimen, in a regular pattern. - The specimen scatters the electrons according to the contours of the specimen surface. - The scattered electrons bounce off the specimen and hit a plate.
What gives the light microscope such poor resolution?
- Uses visible light. - Wavelength of visible light is 400-700nm. - Anything less than 200nm would be seen as 1 blob.
Give the maximum magnification of a TEM.
- x 2 million. - Although new TEMs have x 50 million
What is the resolution of the best modern electron microscopes?
0.1nm.
In basic terms, how does the TEM work?
1) An electron gun produces a beam which is focused onto the specimen by a condenser electromagnet. 2) The beam passes through a thin section of the specimen. 3) The parts of the specimen that absorb electrons appear dark and the parts that allow electrons to pass through appear bright. 4) An image can be produced on a screen which can be photographed to produce a photomicrograph.
Give two reasons as to why the TEM's resolving power of 0.1nm cannot always be achieved?
1) Difficulties preparing the specimen, limit the achievable resolution. 2) A higher energy electron beam is required which may destory the specimen.
Give the two main advantages of electron microscopes over light microscopes?
1) Electron beam has a very strong wavelength and so has a high resolving power. 2) The beam can be focused using electromagnets as the electrons are negatively charged.
Give two advantages of the SEM over the TEM?
1) The specimen doesn't need to be thin as electrons do not penetrate. 2) A 3D image is produced.
What are the two types of electron microscopes?
1) Transmission electron microscopes, TEM. 2) Scanning electron microscopes, SEM.
What are the four main limitation of the TEM?
1) Whole system must be in a vacuum and so living specimens cannot be observed. 2) The complex 'staining' process doens't produce a colour image. 3) Specimen must be very thin. 4) The image may contain artefacts which can appear on the finished electron micrograph, distorting our perception of what really exists.
When was the electron microscope developed?
1930s.
What is the resolution of the SEM?
20nm.
What are the components of the nose piece?
3/4 objective lenses.
How can we produce a 3D image?
A computer analyses the pattern of the scattered electrons, producing a 3D image.
What image is produced from the TEM?
A flat, 2D image.
What is the image produced called?
An electron micrograph.
Why does a near-vacuum have to be created within the chamber of an electron microscope?
As electrons can be deflected or absorbed by the molecules in the air.
How can the problem of the 2D image be overcome?
By taking a series of sections through a specimen to build a 3D image by looking at the series of photomicrographs produced.
Why is it advantageous that the optical microscop is portable?
Can be used in field work.
Define magnification.
How much bigger an image appears compared with the original object.
What is the problem with making the image 3D?
It is a slow and complicated process.
What is special about the light microscope?
It was the first microscope used.
What is the course adjustment nob used for?
Moving the stage closer to the nose piece, until almost touching.
What is meant by studying whole live specimen?
Organisms move and respire under the microscope.
Give a disadvantage of using a light microscope.
Poor resolution.
Give an example of an organelle that couldn't be seen with a light microscope.
Ribosomes as they have a diameter of 20nm.
Define resolution.
The ability of an optical instrument to see or produce an image that shows fine detail clearly.
Give another name for the light microscope.
The optical microscope.
What is meant by 'artefacts'?
Things that result from the way the specimen is prepared but aren't part of the natural specimen.
How is the total magnification calculated?
eyepiece magnification X objective magnification.
Calculate the total magnification of an obejct when a x10 eyepiece and x40 objective lens was used.
eyepiece magnification X objective magnification. x10 x x40 = 400
Give the maximum magninifcation of the optical microscope.
x1500
