Osmosis and Potato
Name: Sonia S.
Date: Wednesday, April 02, 2003
re: osmosis and potato
One slice of potato in water with green (or red) food dye and another
slice of potato in salt (very) water with green (or red) food dye. Of
course the potato in salt water goes soft, but it is also the one that
absorbs the dye. The slice in the water without salt barely picks up the
coloring. I had expected the opposite; that the potato absorbing the
water would also absorb the dye. Does this mean that the dye molecule is
to large to penetrate the potato cell walls when these are full of water
(healthy ?), but in the salt water the cell walls degrade and allow the
dye to penetrate them ? I cannot do the experiment with the kids until I
have a good explanation !
You confuse the experiment a little -- and complicate my attempt to answer
your question -- by specifying, "One slice of potato in water with green
(or red) food dye and another slice of potato in salt (very) water with
green (or red) food dye." Red and green food dyes do not have the same
molecular structure. Also, you did not specify how much dye was added to
each solution. Did you use the same number of drops of each dye? Thus, it
is difficult to assess the extent to which the dye structures are involved
in the process you observed. I suspect that red and green performed
differently in those slices that did accept the dye.
As you know, the salt solution dehydrates the potato slice by osmosis --
water inside the potato cells diffuses out in an attempt to dilute the
salt solution. Osmotic flow always occurs in the direction that favors
dilution. Rather than the salt entering the cells, water inside flows out
into the salt solution.
Dehydration, damages the cell membrane as it lowers the concentration of
water inside thereby rendering it more permeable to the dye. The dye in
the unsalted solution must diffuse through cell membranes that are intact
and undamaged by dehydration. Also, in so doing, it must flow into a cell
that already contains dissolved materials -- in other word, the dye must
diffuse "uphill" against an already-established concentration barrier.
Try this: Place a dyed slice in pure (distilled) water and observe the
rate at which dye diffuses out of the cells back into the water.
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