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Name: Sonia S.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: Wednesday, April 02, 2003


Question:
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 !


Replies:
Sonia,

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.

Regards,
ProfHoff 631



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