Biology
Topic 1 Cells and tissues
Improving practical work
1 Overview of practical work in this topic
The study of cells, by its very nature, involves the use of microscopes to make them visible, or
some other means of viewing magnifying structures. Much of this section will therefore relate to
students becoming familiar with the use of microscopes and understanding the concept of
magnification and cell size. A very good introduction to the use of the microscope,
magnification and analytical biology is the investigation of the pollen grains that can be
found in certain types of honey. The suggested activity ‘Pollen forensics’ is a very good
introduction to the use of the microscope, magnification and analytical biology. It involves the
investigation of the pollen grains that can be found in certain types of honey.
Practical work can also be used to emphasise how small cells are, and to view their structure to
some extent, and also cell division. It is worth noting that many cell components can be difficult to
visualise with school laboratory microscopes, and so the use of video clips or images of cells will
be helpful.
One important concept that can be emphasised with careful observation of prepared tissues is the
idea that everything is composed of cells. The list below gives a range of biological materials that
can be used to demonstrate this.
 Onion skin cells
This is a traditional favourite and an easy and quick method for obtaining plant cells. A fresh onion
is taken and cut to expose the inner ‘leaves’. These should then be cut into 1 cm squares and the
inner epidermis carefully peeled off with fine forceps. More dextrous students can do this for
themselves, but it may be a good idea to have some prepared in advance of the lesson. It should
be noted that these are atypical plant cells, in that they do not contain chloroplasts. The cells can
be stained with any suitable stain (such as iodine) to help visualise the nucleus and cell wall.
 Human cheek cells
Another good source of cells is the inner cheek. Students can use a sterile cotton bud to gently
scrape the inside of their mouth. This is then gently rubbed onto a clean glass slide and a very
small drop of methylene blue stain added. A cover slip is placed on top to prevent it from drying
up. These cells are very small and initially they may be difficult to spot. Using a microscope fitted
with a camera to project images is a good way of helping students to identify the stained cheek
cells.
 Cladophora sp.
This is a filamentous pond alga that can be found in many ponds or streams. A small quantity can
be placed on a slide with a drop of water and then covered with a cover slip. The chloroplasts are
very distinctive and students may also be able to see the cell wall, nucleus and cytoplasm with
careful observation.
 Commelina communis
This is a common garden plant that has thin papery leaves. By tearing a leaf, the epidermis comes
away as a thin strip. This can then be used to observe the epidermal cells, guard cells and
stomata. Many plants will produce a good epidermal strip in this way, and it may be a case of
experimenting with whatever is growing in the school grounds.
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Biology • Topic 1 • Cells and tissues
 Pond water
Pond water will often contain single-celled organisms (protoctista) and these can be an impressive
sight. They can also be quite elusive, however, and it is a good idea to have some prepared slides
of these organisms available as an alternative.
 Epithelial cells
It is easy to obtain epithelial cells from a wide range of sources, such as liver or kidney obtained
from a butcher. Simply scrape the outer surface of the tissue or organ and carefully transfer this to
a microscope slide. Cells should be visible with a stain of methylene blue.
Alternatively, students can take a small strip of sticky tape and apply it to their skin (the inner wrist
works well). When the tape is removed and stuck onto a microscope slide, epithelial skin cells can
be seen (although not stained).
 Diffusion
Students can investigate the rate of diffusion into and out of cells by making up a series
of dialysis tubing ‘sausages’ filled with sugar solution. These are weighed and then left in
water and reweighed at regular intervals to determine the rate of water uptake. The effect
of sugar concentration, surface area, or water temperature can all be investigated.
2 Practical work in everyday teaching
 Observation skills
One of the most important aspects of teaching about cells and cell structures is developing the
students’ ability to make observations and record these accurately. Students should be given as
much experience as possible of using their observational skills. This can be achieved through use
of hand lenses or microscopes to view and then record observations. This could include simple
drawings or sketches of cells, insect tissues or even parts of a feather.
 Magnification
The idea that cells need to be magnified in order to be seen is an important one that helps to
emphasise the nature of cells and their small size. Simple calculations of scale (for example,
drawing an object such as a leaf half of its original size and then measuring and calculating the
magnification) can be used throughout the topic.
 Microscope use
Microscopes can be used to visualise a wide range of objects. Students should be given the
opportunity to explore the possibilities and limitations by looking at familiar objects such as parts of
a flower, roots from germinating seeds, leaves, hair, and a variety of natural and synthetic textiles
such as cotton wool or woven nylon. This will allow students to familiarise themselves with the
microscope but also demonstrate its limitations – when looking at opaque objects, for example.
 Research work
It is important for students to realise that all living things are made up of cells. One idea is for
students to research the type of cells that make up an item of food (such as eggs, meat or fruit).
As many of these cells are impossible to visualise using school equipment, students should
research other practical techniques used to see cells and cell components.
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Biology • Topic 1 • Cells and tissues
The diffusion of materials into and out of cells can be demonstrated by dropping a few
crystals of potassium permanganate into a large beaker of warm water at the start of the
lesson and observing its movement at regular intervals.
3 Activity: Pollen forensics
The structure of pollen grains varies between different species of plant. This difference in size and
shape allows pollen grains to be used for a variety of investigative activities.
Pollen grains are usually found in raw honey, and by microscopically examining these it is possible
to establish the flower source used by the bees during the honey’s production. This technique may
be used commercially to validate the origin of certain types of honey and can be used to help
combat ‘honey laundering’.
In this investigation, students will attempt to identify pollen grains that match those found in honey,
and therefore identify the species of flowers used by the bees to produce the honey. The
investigation will familiarise students with the use of the microscope, allow them to make
observations and then record what they see. It will also emphasise the importance and ubiquitous
nature of cells, and links well with the Reproduction topic. In addition, the role of flowers and bees
in honey production can be explained, and the link between pollen allergens and hay fever
discussed.
There are other applications of this technique, such as using pollen extracted from insect guts to
identify food plants, archaeological investigation of fossil pollen, and forensic investigation of
pollen found at crime scenes.
 Guidance
In Topic 1 Improving Practical Work you will find Video: Pollen forensics, a video of the
techniques used in this practical. See also the Technician notes below.
The actual pollen used for the investigation will depend on the flowers available and the time of
year, although slides of pollen can be pre-prepared at the most suitable time of the year.
Students will need to use the highest power objective in order to see any detail of the pollen
surface sculpture, but even at quite low magnification the differences in pollen shape and size are
apparent with careful observation.
Hand out to students the Activity: Pollen forensics worksheet.
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Biology • Topic 1 • Cells and tissues
Tips on the use of the microscope

The prepared slide should be placed on the microscope stage and centred with the light
source before looking through the eyepiece. This will make locating the specimen much
easier.

The low-power objective lens should be brought as close as possible to the specimen, and
again this can be done viewing the stage from one side rather than looking through the
eyepiece. Focusing can then be done by very slowly moving the objective lens away from the
specimen. This reduces the risk of pushing the objective lens through the slide and damaging
both the specimen and the lens.

Once focused, it should be possible to switch to a higher magnification without needing to
refocus to any great extent.

If the microscope has a diaphragm or condenser it is often worth altering this to reduce light
levels. This will produce greater contrast and make the specimen easier to visualise.

Because a microscope has a very small depth of field, it is often worth very carefully focusing
slightly above and slightly below the specimen to catch all of its surface detail and build up a
complete definition of the object.
 Answers
1 Possible contamination of the sample
2 Magnification = drawing size (in mm) ÷ 0.1
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Biology • Topic 1 • Cells and tissues
 Hazard assessment

If using a microscope with mirrors, precautions should be taken to avoid students using the
Sun as a light source.

Be aware that pollen is an allergen.

Care should be taken when handling the glass slides and, in particular, the fine glass cover
slips. Students should not be expected to remove cover slips from used slides.
 Technician notes
In this investigation students make temporary slides of a selection of pollen types from flowers
brought in to the lab. The students examine each of the pollen samples and make sketches of the
shape and size of the pollen grains. They then compare these with a pre-prepared slide of pollen
in a ‘honey sample’.
Pollen from three to six flowers should be obtained and examined the day before the investigation.
Pollen which is particularly distinctive can then be selected as the ‘target’ pollen in the ‘honey
sample’ that students are attempting to identify. Any flower that has intact pollen-producing
anthers will be suitable – including shop-bought flowers during the winter.
A dry-mount of the pollen can be made by simply taping pollen grains from the flower’s anther onto
a clean dry slide. A cover slip can then be placed on top and, if required, sealed in place with a
small amount of nail varnish or suitable slide mountant. An alternative is to tap the pollen onto a
drop of glycerol on a slide and cover with a cover slip as before. Label as ‘HONEY SAMPLE’.
Depending on the distinctiveness of the pollen grains, two types of flower pollen could be
combined to make up the ‘honey sample’.
Prepare enough pollen slides so that all students have easy access to make comparisons.
Students need access to clean dry microscope slides, cover slips, a mounted needle and a
selection of flowers. A sheet of black and a sheet of white paper under the slides are useful to help
students see the pollen grains.
A large plastic ‘waste’ beaker should also be made available for students to drop their slides into
after use.
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