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It will work well for all of these specimens to stain the starch granules with io-
dine. To do this, you can either place the specimen directly into iodine solution
or you can subsequently exchange the water under the cover slip for iodine
solution (see "Exchanging Solutions" on page 16).
Potato starch: In order to get potato starch onto the slide, simply scrape with
the razor blade over the cut surface of a halved potato and dab it on a slide.
After adding water or iodine solution and covering it with a cover slip, you
should have a look at the starch granules at different levels of magnification.
It works exactly the same with the other starch-containing vegetables.
Starch in different types of flour: It is simplest to stir a pinch of flour into a
beaker with a little water and place a drop of the cloudy liquid onto the slide.
Starch from rice, beans, peas or other foods: For beans and peas, it's best
to cut the seeds in half, pinch a little off from the inside using the tweezers or
the dissecting needle, and rub it in a water drop or in a drop of iodine solution
on the slide.
The Starch Leaves and Sweetness Follows
Perhaps you've seen a potato that has been waiting too long to be cooked? Pale
yellow sprouts grow out of the tuber from which entire potato plants would grow
if the potato were ever to make it out of the dark potato drawer and into the
light of day. The energy and the "construction material" for growth come from
the stored starch. You can test this out quite easily by allowing a (small) potato
that has begun to sprout to grow in a bowl with moist paper towel covered by a
box that does not allow any light in. Monitor the growth for a few days and check
periodically whether there is enough moisture. As long as the sprouts continue to
grow, the potato still has enough starch reserves, but over time it clearly shrivels
up. In the dark, the sprouts remain yellow, and without light and chlorophyll, the
plant cannot perform photosynthesis. Once the starch reserves are used up, then
the plant urgently needs light or else it will die.
You can monitor the breaking-down of the starch in another way, too. Have you
ever asked yourself why green bananas are tart while ripe bananas are so sweet?
At the beginning, when you first buy them, they are still almost green and any-
thing but sweet. After a few days, their peels become yellower and yellower and
the fruit sweeter and sweeter. Starch is being broken down in them too.
Metamorphosed Bananas
You can observe the breaking-down of the starch in bananas using your micro-
scope. To do this, you will need:
• a slide and a cover slip
• the pipette and water
• a knife
• a razor blade (see page 11)
• the tweezers or the dissecting needle
• iodine solution (see tip on page 10)
• a green banana
Get a banana that is as green as you can find. Using the knife, cut a thick piece
off of it and drip a bit of iodine solution onto the cut surface. Using the twee-
zers or the dissecting needle, you can remove several pieces from the stained
area and wipe them in a drop of water on the slide. Work your way step by
step to the higher magnifications. The starch granules are most visible under
the highest magnification (400x).
Repeat the experiment after a few days. In the meantime, the rest of the
banana has ripened. Examine the ripened banana as described above and
compare the results.
Starch granules in the cells of banana fruit flesh
In the unripe banana (bottom), the iodine
solution turns blue-black, whereas it remains
unchanged in the ripe one (top).
Did You Know?
Bananas and other exotic types of
fruit are harvested in the produc-
ing countries in a completely unripe
state? But naturally only if they are
going to be transported far away.
They ripen only on the ship — or in
your kitchen. Consequently, fruits
usually have a much more intense
taste in the countries in which they
are grown, since they are allowed to
ripen on the tree or vine.
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