AQA AS Biology Unit 1: Cells and Microscopes

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  • Created on: 22-08-13 16:15
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All living organisms (except viruses which do not show all of the characteristics of living
things) are made up of one or more cells. A cell is always surrounded by a cell surface
membrane, and contains a solution of proteins and other substances in water. This solution
is called cytoplasm. Within the cytoplasm there are many structures called organelles.
Atoms are organized into molecules, molecules into organelles, and organelles into cells ....
A cell is the basic unit from which living organisms are made.
Some organisms, such as bacteria and many protoctists, are made up of just one cell, and
these are known as unicellular organisms.
Others, including all plants and animals, are made from many cells, and these are called
multicellular organisms.
Most cells are very small, although some are big enough to be seen with the naked eye.
Bacteria tend to have particularly small cells, humans a little larger and plants particularly
Despite this wide variation in size, all cells have certain features in common.
Animal and plant cells are both surrounded by a cell surface membrane which is made
up of phospholipids and proteins. This membrane serves to keep the contents of the
cell separate from its surroundings, but still allows substances to enter and leave in a
controlled way.
Every cell contains cytoplasm. This is a colloidal solution of many substances, mainly
proteins, in water and in which many metabolic reactions take place.
Every cell contains various organelles within its cytoplasm, but the types and numbers of
these organelles vary from cell to cell.
There are two types of cell: Prokaryotic and Eukaryotic

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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.
The relatively long wavelength of the light rays mean that a light microscope can only
distinguish between two objects if they are 0.2µm, or further, apart.
This limitation can be overcome by using beams of electrons rather than beams of light.…read more

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Light Microscopes
The simplest form of microscope is single lens, but it has very limited powers of
magnification ­ around x1500
In the compound microscope, as shown below, two lenses are used.
They have a resolution of about 0.2µm
Light rays are focused on to a transparent specimen by a condenser lens.
The rays pass through the specimen and are focused again by two more lenses ­ the
objective lens and the eyepiece lens. These two lenses produce a magnified image.…read more

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Electron Microscopes
The limitations of the light microscope led scientists to consider the use of forms of
radiation with shorter wavelengths.
This led to the development of the first electron microscope.
This instrument works on the same principal of the light microscope except that instead of
light rays, a beam of electrons is used.
This means that I theory the electron microscope should be able to magnify objects up to
100 000 000 times.
In practice it magnifies over 500 000.…read more

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The Transmission Electron Microscope
It consists of an electron gun that produces a beam of electrons that is focused onto
the specimen by a condenser electromagnet.
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.
An image is produced on a screen and this can be photographed to give a
The resolving power of a TEM is 0.1nm.…read more

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Scanning Electron Microscope
All the limitations of the TEM also apply to the scanning electron microscope, except that
specimens need not be extremely thin as electrons do not penetrate.
The SEM directs a beam of electrons on to the surface of the specimen in a regular
The electrons are scattered by the specimen and the pattern of this scattering depends on
the contours of the specimen surface.…read more

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Comparisons of advantages and disadvantages of
the light and electron microscope
Light Microscope Electron Microscope
Cheap to purchase (£100 ­ 500) Expensive to buy (over £ 1 000 000).
Cheap to operate. Expensive to produce electron beam.
Small and portable. Large and requires special rooms.
Simple and easy sample preparation. Lengthy and complex sample prep.
Material rarely distorted by preparation. Preparation distorts material.
Vacuum is not required. Vacuum is required.
Natural colour of sample maintained. All images in black and white.…read more


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