membrane. Cells have evolved to retain their water and not dry out under normal
conditions. As you saw in the experiment with the radish, however, sometimes
conditions make it impossible for a cell to retain its water. Cells don’t have any
quick defenses against concentrated solutions of salts or even sugar and other sol-
ids. Their water is taken from them because it is drawn to where the most highly
concentrated solution is.
The Cell Wall
Animal cells that are treated with a concentrated salt or sugar solution shrivel up.
In extreme cases, this can even mean the death of the cell.
So how do plant cells manage to retain their shape even when they lose water? It’s
because the actual plant cell enclosed by the cell membrane is also enclosed by the
so-called cell wall. You can imagine it as being like a water-filled balloon in a shoe
box. The water is the liquid inside the cell, called cytoplasm or cell sap, the balloon
is the cell membrane, and the shoe box is the cell wall.
The nice thing about this analogy is that the cardboard in the shoe box (as well as
paper and pulp) is in fact made from the cell wall material of plant cells, namely
cellulose.
If the water is now taken away, then the water balloon inside the box shrinks. A
hollow space is thus created where the balloon and the shoe box had been touch-
ing previously.
Exactly the same thing happens in the radish experiment — only that the result-
ing space is not filled by air but by salt solution which penetrates through the cell
walls. The whole thing can also be reversed if water is fed to the cell.
Staining Technique — Exchanging Solutions
For the microscopic experiment on this page, the liquid in which your object is
lying needs to be exchanged for another liquid. To do this, however, you don’t
need to lift the cover slip each time. There’s a more elegant way: The method
for drawing excess water out from under the cover slip was explained in the
introduction. You can apply the same technique in order to suck another liquid
under the cover slip. Simply dab a drop of the liquid (e.g. the stain) onto the
slide next to the cover slip and suck it up under the cover slip from the other
side using blotter paper or a tissue. After leaving it for a moment so that the
stain takes effect, it is recommendable (especially when using dark stains) to
rinse the slide one or two times (i.e. to repeat the procedure with water) in
order to get a clear image under the microscope.
When Cells Shrink
In principle, any plant will work for this microscopic experiment. The process
can be observed especially well with some red-colored plant parts. You’ll find
out below why not all colored plant parts are equally suited to this.
You will need:
• a slide and a cover slip
• water and the pipette
• a razor blade (see page 11)
• the tweezers
• a quarter of a red cooking onion
• a beaker (or glass) with a salt or sugar solution
• a piece of blotter paper, paper towel, or tissue
First prepare the salt (or sugar) solution: In a beaker, stir a level tablespoon-
ful of salt or sugar into three tablespoonfuls of water. Drip a drop of the salt
or sugar solution onto the slide. Prepare a little piece of onion skin (as on
page 14) and place it in the drop on the slide. Place a cover slip on it and now
quickly look at it under the microscope! Observe it for a while under medium
magnification: After a while, you can see how the red-colored cell contents
Red blood cells in living state (left) and dried
state (right)
This is how you can imagine a living plant cell
that is full of cytoplasm.
This is what a plant cell looks like when it has
given off some of its water.
Using a piece of blotter paper, it’s very easy to
draw liquids under the cover slip.
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Summary of Contents for TK2 Scope
Page 1: ...Experiment Manual ...
Page 52: ......
