Detailed, colourful, nicely displayed revision notes with images on AQA AS/ A-Level Biology on section 2, topic 3, Cell structure. According to the AQA AS/ A-Level specification, these notes are on section 3.2 including the subsections 3.2.1.1 to 3.2.1.3. My notes are collected from many sources to...
Cell structure
Methods of studying cells
Microscopes are instruments that produce a magnified image of an object.
Magnification is how much bigger the image is than the actual size
Light Microscopes
Light microscopes use a pair of convex glass lenses that can resolve images
that are 0.2um apart. The reason for this is that the wavelength of light is short
and therefore restricts the resolution. This is compared to electron
microscopes which can distinguish between items 0.1nm apart.
The magnification of an image as seen through a microscope can be calculated
using the following equation:
Magnification = size of image/size of real object
Resolution is defined as the minimum distance apart that two objects can be
distinguished as separate objects in an image. The greater the resolution the
clearer the image will be.
Living specimens can be observed.
Electron Microscopes
The limitation of light microscopes only resolving to a resolution of 0.2um
means that electron microscopes can be used to look at objects that are closer
than 0.2um apart. The electron beam has a very short wavelength, so the
microscope has a high resolving power of 0.1nm. There are two main types of
electron microscope, these are transmission electron microscopes (TEM) and
scanning electron microscopes (SEM). Electron microscopes work in a similar
way to light microscopes, but instead use a beam of electrons that are focused
by electromagnets (because electrons are negatively charged) inside a vacuum
environment. The vacuum environment is needed so that particles in the air do
not deflect the electrons out of the beam alignment.
The following details how each type of electron microscope works:
1. Transmission Electron Microscope – a beam of electrons penetrates a
thin section of a specimen. Areas that absorb the electrons appear
darker on the electron micrograph that is produced while areas that
allow the electrons to pass through appear bright.
Advantages:
Has the highest resolving power of 0.1nm but this can’t always be
achieved because there may difficulties in preparing the
specimen, and a higher electron beam energy is needed which
may destroy the specimen.
Has a larger magnification
, Disadvantages:
Whole system must be in a vacuum so living specimens can’t be
observed
A complex ‘staining’ process is required and even then, the image
is not in colour
The specimen must be extremely thin
The image may contain artefacts due to the way the specimen was
prepared. They may appear on the finished photomicrograph but
are not part of the natural specimen, so you can’t always be sure
that what you see in the photomicrograph exists in real life.
The specimen must be very thin, this results in a 2D image, but
this can be overcome in a slow and complicated process by taking
micrographs of a series of sections of the specimen and build up a
3D image of the specimen.
2. Scanning Electron Microscope –in a scanning electron microscope a
beam of electrons passes across the surface and scatter. The pattern of
scattering builds up a 3D image depending on the contours of the
specimen.
Advantages:
The specimen doesn’t need to be extremely thin as the electrons
do not penetrate through it.
Produces a 3D image
Higher resolving power and magnification than light microscope
Disadvantages:
Whole system must be in a vacuum so living specimens can’t be
observed
A complex ‘staining’ process is required and even then, the image
is not in colour
The image may contain artefacts due to the way the specimen
was prepared. They may appear on the finished photomicrograph
but are not part of the natural specimen, so you can’t always be
sure that what you see in the photomicrograph exists in real life.
The resolving power is 20nm so lower than the TEM.
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