Cell Structure - Methods Of Studying Cells Flashcards
What is the resolving power of light microscopes and why is this?
0.2 micrometers (quite a large distance between two points) because of the relatively long wavelength of light rays
What is the resolving power of electron microscopes and why is this?
0.1 nm (short distance between two points) because electron microscopes use electron beams which have a lot shorter wavelengths than visible light rays.
What is the formula for magnification?
Magnification = image size / actual size (being the size of the object)
What is the definition of magnification?
How much bigger the image is compared to the actual size of the object
What is the definition of resolution?
The smaller distance points can be separated and still be seen as separate items. (The amount of detail an image appears in depends on the microscopes resolving power)
What limits resolution?
The wavelength of light used by the microscope
What does the electron beams wavelength mean for the resolving power of the electron microscope?
The electron beam has a very short wavelength so resolves objects well so has a high resolving power
Why is a near-vacuum created within the chamber of an electron microscope for it to work? What does this mean for objects that are being examined under an electron microscope?
Because electrons would otherwise be absorbed or deflected by molecules in the air and would never reach the specimen. This means that objects being seen under an electron microscope have to be dead
Describe the radiation pathway in an electron microscope
Starting from the electron source, the electron beam,
- Goes through a magnetic condenser which goes through the object
- It then goes through the magnetic objective to produce the immediate image
- The electron beam then goes through a magnetic projector which projects the image on a fluorescent screen so the object can be observed
What are the two types of electron microscope
- The transmission electron microscope
- the scanning electron microscope
What is a photomicrograph?
An image produced by a transmission electron microscope which has been photographed from a screen
How does a transmission electron microscope work?
- An electron gun produces a beam of electrons which is focussed onto the specimen by a condenser electromagnet
- The beam then passes through a thin section of the specimen where parts of the specimen absorb electrons and appear dark while other parts allow electrons to pass through and appear bright
- An image is then produced on a screen which can be photographed to give a photomicrograph
The resolving power of a transmission electron microscope is 0.1nm but can’t always be achieved in practice. Why?
- difficulties in 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 limitations of an electron microscope?
- The whole system MUST be in a vacuum and so living specimens can’t be observed
- a complex staining process with heavy metals is required and even then the image isn’t in colour
- the specimen must be extremely thin
- the image may contain artefacts so it’s not always easy to be sure that what is seen on a photomicrograph really exists in that form
What are artefacts?
In transmission electron microscopes, they are the result of the way the specimen is prepared that may appear on the finished photomicrograph that are not part of the natural specimen.