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A Level Biology CIE Your notes
1.1 The Microscope in Cell Studies
Contents
1.1.1 T he Microscope in Cell Studies
1.1.2 Magnification Calculations
1.1.3 Eyepiece Graticules & Stage Micrometers
1.1.4 Resolution & Magnification
1.1.5 Calculating Actual Size
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1.1.1 The Microscope in Cell Studies
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Microscope Slide Preparation
In order to observe cellular material in more detail, specimens can be prepared for viewing under
a light microscope
Samples need to be thin enough to allow light to pass through
The type of preparation that is appropriate is dependent on the cellular material that needs to be
viewed
Slide preparation methods table
Samples sometimes need to be stained, as the cytosol and other cell structures may be
transparent or difficult to distinguish
To stain a slide the sample needs to be first air-dried and then heated by passing it through a
Bunsen burner flame – this will allow the sample to be fixed to the slide and to take up the stain
As with the type of preparation required, the type of stain used is dependent on what type of
specimen is being used
Common microscope stains & uses table
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Drawing Cells
To record the observations seen under the microscope (or from photomicrographs taken) a Your notes
labelled biological drawing is often made
Biological drawings are line pictures which show specific features that have been observed
when the specimen was viewed
There are a number of rules/conventions that are followed when making a biological drawing
The conventions are:
The drawing must have a title
The magnification under which the observations shown by the drawing are made must be
recorded
A sharp HB pencil should be used (and a good eraser!)
Drawings should be on plain white paper
Lines should be clear, single lines (no thick shading)
No shading
The drawing should take up as much of the space on the page as possible
Well-defined structures should be drawn
The drawing should be made with proper proportions
Label lines should not cross or have arrowheads and should connect directly to the part of
the drawing being labelled
Label lines should be kept to one side of the drawing (in parallel to the top of the page) and
drawn with a ruler
Drawings of cells are typically made when visualiz ing cells at a higher magnification power,
whereas plan drawings are typically made of tissues viewed under lower magnifications
(individual cells are never drawn in a plan diagram)
Exam Tip
When producing a biological drawing, it is vital that you only ever draw what you see and not what
you think you see.To accurately reflect the siz e and proportions of structures you see under the
microscope, you should get used to using the eyepiece graticule.
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1.1.2 Magnification Calculations
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Magnification Calculations
Magnification is how many times bigger the image of a specimen observed is in comparison to
the actual (real-life) siz e of the specimen
The magnification (M) of an object can be calculated if both the siz e of the image (I), and the
actual siz e of the specimen (A), is known
An equation triangle for calculating magnification
Worked example
An image of an animal cell is 30 mm in siz e and it has been magnified by a factor of X 3000.
What is the actual siz e of the cell?
To find the actual siz e of the cell:
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Your notes
The siz e of cells is typically measured using the micrometre (μm) scale, with cellular structures
measured in either micrometers (μm) or nanometers (nm)
When doing calculations all measurements must be in the same units. It is best to use the
smallest unit of measurement shown in the question
To convert units, multiply or divide depending if the units are increasing or decreasing
Magnification does not have units
Converting units of measurement
There are 1000 nanometers (nm) in a micrometre (µm)
There are 1000 micrometres (µm) in a millimetre (mm)
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There are 1000 millimetres (mm) in a metre (m)
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Worked example
Step 1: Check that units in magnification questions are the same
Remember that 1mm = 1000µm
2000 / 1000 = 2, so the actual thickness of the leaf is 2 mm and the drawing thickness is 50 mm
Step 2: Calculate Magnification
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Magnification = image siz e / actual siz e = 50 / 2 = 25
So the magnification is x 25 Your notes
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1.1.3 Eyepiece Graticules & Stage Micrometers
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Eyepiece Graticules & Stage Micrometers
An eyepiece graticule and stage micrometer are used to measure the siz e of the object when
viewed under a microscope
The type of microscope and magnification used can vary signficantly so the eyepiece graticule
needs to be calibrated each time when measuring objects
The calibration is done using a stage micrometer, this is a slide with a very accurate known scale in
micrometres (µm)
The eyepiece graticule is a disc placed in the eyepiece with 100 divisions, this has no scale
To know what the graticule divisions equal at each magnification the eyepiece graticule is
calibrated to the stage micrometer at each magnification
Using stage micrometer & eyepiece graticule
A stage micrometer alongside an eyepiece graticule.
In the diagram, the stage micrometer has three lines each 100 µm (0.1 mm) apart
Each 100 µm division has 40 eyepiece graticule divisions
40 graticule divisions = 100 µm
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1 graticule division = number of micrometres ÷ number of graticule division
1 graticule division = 100 ÷ 40 = 2.5 µm this is the magnification factor Your notes
The calibrated eyepiece graticule can be used to measure the length of the object
The number of graticule divisions can then be multiplied by the magnification factor:
graticule divisions x magnification factor = measurement (µm)
Exam Tip
The calculations involving stage micrometers and eyepiece graticules are often seen in exam
questions, so make sure that you are comfortable with how to calibrate the graticule and
calculate the length of an object on the slide.
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1.1.4 Resolution & Magnification
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Resolution & Magnification
Magnification
Magnification is how many times bigger the image of a specimen observed is in compared to the
actual (real-life) siz e of the specimen
A light microscope has two types of lens:
An eyepiece lens, which often has a magnification of x10
A series of (usually 3) objective lenses, each with a different magnification
To calculate the total magnification the magnification of the eyepiece lens and the objective
lens are multiplied together:
eyepiece lens magnification x objective lens magnification = total magnification
Resolution
Resolution is the ability to distinguish between two separate points
If two separate points cannot be resolved, they will be observed as one point
The resolution of a light microscope is limited by the wavelength of light
As light passes through the specimen, it will be diffracted
The longer the wavelength of light, the more it is diffracted and the more that this diffraction
will overlap as the points get closer together
Electron microscopes have a much higher resolution and magnification than a light microscope
as electrons have a much smaller wavelength than visible light
This means that they can be much closer before the diffracted beams overlap
The concept of resolution is why the phospholipid bilayer structure of the cell membrane cannot
be observed under a light microscope
The width of the phospholipid bilayer is about 10nm
The maximum resolution of a light microscope is 200nm (half the smallest wavelength of
visible light, 400nm)
Any points that are separated by a distance less than 200nm (such as the 10nm phospholipid
bilayer) cannot be resolved by a light microscope and therefore will not be distinguishable as
“separate”
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Your notes
The resolving power of an electron microscope is much greater than that of the light microscope, as
structures much smaller than the wavelength of light will interfere with a beam of electrons
Comparison of the electron microscope & light microscope
Light microscopes are used for specimens above 200 nm
Light microscopes shine light through the specimen, this light is then passed through an
objective lens (which can be changed) and an eyepiece lens (x10) which magnify the
specimen to give an image that can be seen by the naked eye
The specimens can be living (and therefore can be moving), or dead
Light microscopes are useful for looking at whole cells, small plant and animal organisms,
tissues within organs such as in leaves or skin
Electron microscopes, both scanning and transmission, are used for specimens above 0.5 nm
Electron microscopes fire a beam of electrons at the specimen either a broad static beam
(transmission) or a small beam that moves across the specimen (scanning)
The electrons are picked up by an electromagnetic lens which then shows the image
Due to the higher frequency of electron waves (a much shorter wavelength) compared to
visible light, the magnification and resolution of an electron microscope is much better than a
light microscope
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Electron microscopes are useful for looking at organelles, viruses and DNA as well as
looking at whole cells in more detail
Electron microscopy requires the specimen to be dead however this can provide a snapshot Your notes
in time of what is occurring in a cell eg. DNA can be seen replicating and chromosome
position within the stages of mitosis are visible
Light v Electron Microscope Table
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1.1.5 Calculating Actual Size
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Calculating Actual Size
When investigating the siz e of organisms and biological structures you will use a microscope of a
specific magnification to produce an image
Photomicrographs are images obtained from a light microscope, these are used for specimens
above 200 nm (a bacteria cell is about 1000 nm)
Electron micrographs are images obtained from electron microscopes, both scanning and
transmission, these are used for specimens above 0.5 nm
Electron microscopes are useful for looking at organelles and biological molecules, eg. DNA
can be seen replicating
To better understand the images we produce using microscopes we need to know the actual
siz e of the specimen
Worked example: Calculating the actual size of a specimen
A scientist looks at a sample of red blood cells under a light microscope.
The eyepiece lens of the microscope has a magnification of x10 and an objective lens of x40
was used to view the blood cells. The scientist takes a photomicrograph of the blood cells, in
which the average siz e of each cell is 3 mm.
What is the average siz e of the red blood cells in the sample? Give your answer in micrometres.
Known values:
Eyepiece lens magnification: x10
Objective lens magnification: x40
Image siz e: 3 mm
Step 1: Calculate the total magnification of the specimen
eyepiece lens magnification x objective lens magnification
= total magnification
x10 x x40 = x400
Step 2: Calculate the image siz e in the units asked for (micrometres)
1 mm = 1000 μm
3 mm = 3000 μm
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Step 3: Calculate the actual siz e of the red blood cell
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Therefore, the average siz e of a red blood cell in this sample is 7.5 micrometres
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