AS Biology Unit 1.3 Cells and Movement in and out of them Part 3

valakopa's version from 2017-06-21 18:44

Section 1

Question Answer
What are microscopes? What can a simple convex glass lens do?Microscopes are instruments that magnify the image of an object. A simple convex glass lens can act as a magnifying glass but such lenses work more effectively if they are in a compound light microscope.
What does the relatively long wavelength of light rays mean? How can this limitation be overcome?The relatively long wavelength of light rays means that a light microscope can only distinguish between 2 objects if they are 0.2um, or further, apart. This limitation can be overcome by using beams of electrons rather than beams of light. With their shorter wavelengths, the beam of electrons in the electron microscope can distinguish 2 objects as close together as 0.1nm
What is the image and object?The material that is put under a microscope is referred to as the object. The appearance of this material when viewed under the microscope is referred to as the image.
What does magnification equal?size of image/size of object
Size of object =?size of image/magnification
What is resolution? What does it depend on?Resolution/Resolving power of a microscope is the minimum distance apart that 2 objects can be in order for them to appear as separate items. Resolution depends on the wavelength or form of radiation used.
What is the resolution of a light microscope? What does this mean? What does greater resolution mean?Light microscope - about 2um. This means that any 2 objects which are 0.2um or more apart will be seen separately, but any objects closer than 0.2um will appear as a single item. Greater resolution means greater clarity - the image produced is clearer and more precise.
What does increasing the magnification do? What about resolution?Increasing the magnification will increase the size of an image, but does not always increase the resolution - every microscope has a limit. Up to this point increasing the magnification will reveal more detail but beyond this point increasing the magnification will not do this - object will appear larger but be blurred.
Why do light microscopes have poor resolution? What happened in the 1930s?Light microscopes have poor resolution as a result of the relatively long wavelength of light. 1930s, microscope developed that used a beam of electrons instead of light - electron microscope.
Advantages of an electron microscope over a light microscopeElectron beam has a very short wavelength and the microscope can therefore resolve objects well - it has a high resolving power. As electrons are negatively charged the beam can be focused using electromagnets. cherry bomb -NCT 127
What can the best electron microscopes resolve to? What needs to be created in order for it to work effectively and why?Best modern electron microscopes can resolve objects that are just 0.1nm apart - 2000 times better than a light microscope. Because electrons are absorbed by the molecules in air, a near-vacuum has to be created within the chamber of an electron microscope in order for it to work effectively.

Section 2

Question Answer
What two types of electron microscopes are there?The transmission electron microscope (TEM). The scanning electron microscope (SEM)
What does a transmission electron microscope consist of? Why do parts of the specimen appear dark?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 absorb electrons and therefore appear dark. Other parts of the specimen allow the electrons to pass through and so appear bright.
Where is an image produced on a TEM? What is the resolving power of a TEM?An image is produced on a screen and this can be photographed to give a photomicrograph. Resolving power of a TEM is 0.1nm, although problems with specimen preparation mean that this cannot always be achieved.
What are 4of the main limitations of the TEM?The whole system must be in a vacuum and therefore living speicmens cannot be observed. A complex 'staining' process is required and even then the image is only in black and white. The specimen must be extremely thin. 4.The image may contain artefacts. Artefacts are things that result from the way the specimen is prepared. 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.
Why does the TEM produce a 2D image?In the TEM the specimens must be extremely thin to allow electrons to penetrate. The result is therefore a flat, 2D image. We can partly get over this by taking a series of sections through a specimen. We can then build up a 3D image of the specimen by looking at the series of photomicrographs produced. However, this is slow and complicated. Can overcome this by using a SEM.
How does the SEM work?SEM directs a beam of electrons on to the surface of the specimen from above, rather than penetrating it from below. The beam is then 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.
Limitations of the SEM?All the limitations of the TEM apply except that specimens need not be extremely thin as ELECTRONS DO NOT PENETRATE.
How can we build up a 3D image using an SEM? Resolution?We can build up a 3D image by computer analysis of the pattern of scattered electrons and secondary electrons produced. The basic SEM has a lower resolving power than a TEM, around 20nm, but is still ten times better than a light microscope.