Expert: John Locke - 3/22/2008

Question
two cells are attached by a common membrane. in cell A, there is a sugar concentration of 0.5% and a starch concentration of 0.4%. in cell B there is a sugar concentration of 0.6% and no starch. sugar can pass through the membrane, but the starch cannot. assume that all other solutes are found in equal concentration within the 2 cells. use a simple diagram and a brief explanation to  show:
a) in what direction the sugar will move?
b) in what direction osmosis will occur?

so that is the question that we have for homework.. i guess i am just stumped on whether or not the 2 attached cells would automatically try to equal out their contents to become isotonic solutions or not...i am really confused and so is everyone else that i have talked to..please please please help me..thanks

Answer
Thanks for using AllExperts, Brittany.

We'll start with the fact that both water and solutes move in such a way so as to minimize their concentrations--this is the principle of diffusion, which you can certainly find explanations of online or in your textbook. Higher concentrations of solute will diffuse to regions with lower concentrations; if solute is unable to move, then water will move to equalize the concentrations. Recognizing that, keep in mind where the starch and sugar concentrations are higher. The starch has a higher concentration in cell A. The sugar has a higher concentration in cell B. Sugar can diffuse across the membrane, but starch cannot; you would rightly expect that sugar will move across the membrane. Because starch is unable to move, you might expect that water would move instead--and you would be correct to think so. We need to use the concept of osmotic pressure to solve this problem; I imagine that you have seen this before. To review you can look here:

http://physioweb.med.uvm.edu/bodyfluids/osmosis.htm

Osmotic pressure is given by P=RT[S], where P is the osmotic pressure, R is the ideal gas constant, T is the absolute temperature, and [S] is the solute concentration in molarity. It's important to note that [S] is the total concentration of all solutes. That fact allows us to solve the problem: a net concentration difference exists such that sugar will diffuse from cell A to cell B (why? there's a higher total particle concentration in cell A). Sugar alone can diffuse, so it will move from A to B; it is unaffected by the fact that there's more sugar to begin with in cell A. Osmosis has the opposite direction of the particle diffusion; water will diffuse from cell B to cell A (why? there's a higher concentration of water in cell B, if you want to think about it that way; alternatively, you can recognize that the way to minimize solute concentrations in the two cells is for water to move from cell B to cell A.

You can find additional discussions at the website below. Good luck.

http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html