Question

I did a simple experiment regarding osmosis and the rate of diffusion along a concentration gradient. I practiced changes of weight of dialysis bags used as cellular models. My last measurement of Bag A, which contained 10 ml of 1% sucrose in a beaker of 10%sucrose and water, ended up having a total weight of 28.1 grams after 60 minutes. This was a -0.5 change in weight from the 45 minute interval where I calculated the weight. I am just wondering why I got a negative number. Could it simply be human error or was there no change in the movement of water across the membrane?

Answer 1

Bag A contained 10 ml of 1% sucrose solution. It is kept in a beaker containing a solution of 10% sucrose and water. So the beaker is having more concentration of sucrose than the Bag A. More solute in a solution , it will have less water potential. Water will move from where water potential is more to where water potential is less. Beaker has sucrose in the solution , so it has less water potential.

The Bag A has more water potential as it has less solute. So the water has moved out of the bag because of osmosis. Water moves from where water is more to where water is less through a semipermeable membrane , which is called osmosis. The water has come out of the bag due to osmosis. That is why you have got negative number. It is not human error. Dialysis bags are semipermeable. They allow water to move. But does not allow movement of sucrose.