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This Week's Experiment - #319 Potato Water
This week's experiment is a variation of several that we have tried in the
past. It deals with something called osmosis. To find out more about this
important part of the process of life, you will need:
a potato
a knife
salt
a plate
Select a large potato and carefully cut it in half. Place both halves of
the potato on a plate, cut side down. Use the knife to carefully dig a hole
in the top of each potato. Each hole should be at least big enough to stick
the end of your finger into. Leave one just as it is. Into the other
hole, put about 1/4 a teaspoon of salt. Set the plate aside and wait a
while. How long is "a while"? That is a scientific term for the amount of
time that it takes to eat a bowl of ice cream, with sliced bananas and chocolate sauce.
When you are done with your snack, look at the potatoes. The plain potato
probably has not changed much. The potato with the salt has changed quite a
bit. The hole probably has water in it, and the potato has probably begun
to turn brown. Let the potato sit for a few hours and the difference will
be even more striking. The salted potato will have lots of water and will
be quite brown.
What caused the change? A wonderful process called osmosis. This is one
of the driving forces of the cells in all living things. Each cell is
surrounded by a thin skin called a cell membrane. This membrane lets water
flow in and out of the cell. The flow of the water is controlled by the
chemicals dissolved in the water. The water will flow towards the highest
concentration of dissolved chemicals. If the concentration of dissolved
chemicals inside the cell is higher, then water flows into the cell. If the
dissolved chemicals are more concentrated on the outside of the cell, then
water flows outwards. The salt you placed into the hole in the potato
dissolved in the small amount of juice that was released when you cut the
hole. This made a very concentrated salt solution, which pulled more water
from the cells. The water pulled from the cells dissolved more salt,
letting the salt solution reach more cells, to take more water from the
potato.
That explains the water. What about the brown? Pulling that much water
from the cells damages them. That causes them to release an enzyme called
catechol oxidase, which combines with oxygen in the air to produce the brown
color. With time, the plain potato will turn brown too, from the damage
caused by cutting, but the salt causes much more cell damage and more
browning.
This browning happens with many other fruits and vegetables. In fact, if
you have any sliced banana left, it has probably started to turn brown by
now. I can't be sure, as I ate all of mine.
Have a good week.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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