Plant Physiology Lab

Does anyone know the purpose of placing the potato slices in a sucrose solution in the " Osmosis experiment " ?

Osmosis is the movement of water from high concentration to low concentration through a semi-permeable member.

Potatoes do not contain sucrose, instead they contain starches. If there is a high concentration of sucrose in the solution (so more sucrose than water) than there is in the actual potato, it will draw water out of the potato until there is no gradient, that is, until both sides have an equal concentration of water. So basically in this experiment, you are exploring osmosis and its effects on different concentrations of water. Also remember that potatoes don 't contain significant concentration of sucrose. They have mainly insoluble starch and no significant amount of sugar.

the purpose of the experiment was to see the effect of temperature on osmosis, so i would think that the steps where the potato slices were placed in distilled water at different temperatures are the steps where osmosis is taking place under different temperatures... putting the slices in the sucrose solution first (under the same temperature) seems to be as a pre-requisite to the "real" osmosis steps.. though i don't see where its importance lies.

Well yeah, the prerequisite demonstrates osmosis because generally water is passing through the semi permeable of the potato.

but how does it demonstrate the effect of temperature on osmosis ?

It doesn't, but when you add temperature as a variable, it introduces brownian movement, thereby increasing the rate of osmosis.

when the slices were placed in sucrose, did it enter the cells ?

No, the molecule is too large to move through the membrane; it's the movement of water molecules we're worrying about. Remember, ssmosis is a special case of diffusion where water moves through a selectively permeable membrane (a membrane that allows for diffusion of certain solutes and water) from a region of higher water potential to a region of lower water potential. Water potential is the measure of free energy of water in a solution. There is a high concentration of water relative to the concentration of water in the sucrose solution, so it will presumably move out of the potato slice and decrease the overall mass of the potato slice.

ok, so we placed the slices in the sucrose solution, water moved out of the potato cells. Then we placed them in distilled water, water moved into the cells. If we hadn't put the slices in sucrose solution to decrease the water pressure inside the cells, the second step where the slices were placed in distilled water wouldn't have been accurate, because water potential would have been low (due to the already high content of water inside the cells), so we decreased the pressure of water inside the cells by placing them in a hypertonic solution, so the water potential later on after placing the slices in distilled water would be high enough for good observation of osmosis to occur.. is that right ?

I'm guessing yes. Like you said, sucrose solution causes potato slice to lose water. Putting the same slice in distilled water (no solute, just pure solvent) draws water into the potato.