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Slide 1

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· Unicellular organisms: made up of one cell, known as simple
· Multicellular organisms: made up of more than one cell, known as
complex organisms
· Chromatic aberration: deficiency in clarity of an image produced
by a lens, caused by the inability of the lens to bring all colours of
light to focus on the same point. Also called false colour.
· Sectioned: whole specimens are cut into very thin slices with a
sharp blade
· Staining: coloured chemicals that bind to the specimen
· Diffraction: the tendency of light waves to spread as they pass
close to physical structures.
· Artefacts: a visible structure detail which is caused by processing
the specimen but is NOT a feature of the specimen. They appear
as bubbles trapped under the cover slip. Experience will enable a
scientist to be able to distinguish between an artefact and a real
2.1, 2.2, 2.3…read more

Slide 2

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Use of light microscope
Microscopes are instruments that allow us to magnify an object, so we can
see very small unicellular organisms.
They make visible the individual cells that make up multicellular
organisms which allows us to see how the structure of the cell is linked to
their functions
After the mid 19th century the development of microscopes with higher
magnification and resolution meant that cells could now be observed which
led to the development of cell theory:
· Both plant and animal tissue is composed of cells
· Cells are the basic unit of life
· Cells only develop from existing cells
Use of light microscope:
· Can be used in field work.
· Able to observe both living and dead/prepared specimens and
organisms. 2.1 Microscopy…read more

Slide 3

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How a light microscope works
· There are two lenses: the objective lens and the
eyepiece lens.
· The objective lens is close to the specimen and
magnifies it.
· The eyepiece lens magnifies the already magnified
image produced by the objective lens. This is also the
lens you look through to view the specimen
Objective/eyepiece lens configuration allows for:
· Higher magnification
· Reduced chromatic aberration than what you would
get using a simple light microscope
2.1 Microscopy…read more

Slide 4

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The preparation and examination of microscope slides for use
in light microscopy
· Two factors which affect what method of slide preparation is
used are: the nature of the specimen and the desired
4 sample preparation methods:
Dry Mount ­ used to view solid Squash Slides ­ a wet mound is prepared
specimens. They are sectioned and the then cover slip is put on using a lens tissue.
sample is put in the middle of a slide To avoid damaging cover slip the sample can
and a covered with a slip. be squashed between 2 microscope slides.
Ex specimens: muscle tissue, plants Ex specimen: root tip squashes
Wet Mount ­ the specimen is Smear Slides ­ the edge of a slide is
suspended in a liquid (ex: water, used to smear the sample on to
immersion oil). Then a cover slip is put another slide, which creates a thin even
on top from an angle. coating. Cover slip then placed on top.
Ex specimen: aquatic organisms Ex specimen: blood
2.1 Microscopy…read more

Slide 5

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The use of staining in light microscopy
· When looking through a light microscope the whole sample is
illuminated at once with white light from below this gives the
image viewed through the eyepiece a low contrast as cells don't
absorb much light. Less light absorbed = lower resolution (hard to
distinguish between individual structures of the cell) as resolution
is limited by wavelength and diffraction of light as it passes
through the sample.
· Staining can be used to counteract this.
Increases contrast as different parts of the cell become stained to
different degrees
· Features become more visible and can be identified due to
increased contrast
Allows identification of the different cell types/organelles /parts of
Allows identification of different compounds/molecules
2.1 Microscopy…read more

Slide 6

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The use of staining in light microscopy
Preparing sample for staining:
1. Sample is put on slide and allowed to air dry
2. It is then heat-fixed by passing through a flame
3. The specimen will stick to the slide and stain
Differential staining: ability to distinguish between 2 types of
organisms that would normally be hard to identify. Can also
differentiate between organelles of a sample too.
This can be done using positively charged dyes which attract
negatively charged materials in the cytoplasm leading to them
becoming stained. Then negatively charged dyes can be used to stain
components outside of the cytoplasm as that region is positively
charged. When looking through the microscope the inside structures
of the cytoplasm will appear in a different colour to the structures
outside the cytoplasm.
2.1 Microscopy…read more

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