The bulb of an onion is
formed from modified leaves. While photosynthesis takes place in the leaves of
an onion containing chloroplast, the little glucose that is produced from this
process is converted in to starch (starch granules) and stored in the bulb.
Unlike animal cells (such as cheek cells) the cell wall of an onion and other plants are made up of cellulose, which protects the cell and maintains its shape.
How to Obtain a Thin Layer of Onion Cells
An onion is made up of
layers that are separated by a thin membrane. For this experiment, the thin
membrane will be used to observe the onion cells. It can easily be obtained by
peeling it from any layer of the onion using tweezers.
**Note- In microscopy, wet
mount refers to a glass slide holding a specimen suspended on a drop of liquid
Moreover, to avoid breaking
the slide and damage to the microscope objective lenses during observation, it's important that the optical tube be lowered to the point that the objective
lens is as close to the slide as possible.
This should be done starting with
low power while looking from the side of the microscope rather than through the
eye piece. From this point, it becomes easier to focus for clarity without any
Add a drop of water at the center of the microscopic slide
Having pulled of a thin membrane from the onion layer, lay it at
the center of the microscopic slide (the drop of water will help flatten the membrane)
Add a drop of iodine
solution on the onion membrane (or methylene blue)
Gently lay a microscopic cover slip on the membrane and press it
down gently using a needle to remove air bubbles.
Touch a blotting paper on one side of the slide to drain excess
Place the slide on the microscope stage under low power to
Adjust focus for clarity to observe.
Students can make another
slide without adding the stain to see the difference between a stained slide
and a non-stained slide.
Large, rectangular interlocking cells,
Clearly visible distinct cell walls surrounding the cells,
Small granules may be observed inside the cells (within the
The layers of an onion contain simple
sugars (carbohydrates) some of which are stored as starch (starch granules).
Given that iodine tends to bind to starch, it stains the starch granules when
the two come in to contact making them visible.
Although onions may not have as
much starch as potato and other plants, the stain (iodine) allows for the
little starch molecules to be visible under the microscope. Although onions are
plants, students will not see any chloroplasts in their slides.
This is because
of the fact that the chloroplast necessary for photosynthesis is largely
present in the leafy part of the onion, which is exposed to the sun and absent
in the bulb which is below ground and away from sunlight.
Unlike animal cells, students will also notice that the
plant cells have a more regular shape. This is because they have a cell wall
made up of cellulose which maintains its shape.
This is an easy and fun activity that will allow the student
to learn about the cell structure. Students have the opportunity to experiment
and observe the onion membrane with different stains (methylene blue and
iodine) and without the stain and observe the differences.
Having observed the
onion cell under the microscope, students will be able to learn the differences
between animal and plant cells in addition to the function of the different
parts of the cell.
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