Final answer:
Fructose, being smaller, diffused through the semi-permeable dialysis tubing, while lactose did not, demonstrating the selective permeability of the membrane related to the size of the molecules.
Step-by-step explanation:
In the described experiment, the dialysis tubing serves as a semi-permeable membrane, which allows certain molecules to pass through while blocking others. The detection of fructose in the distilled water outside of the dialysis bag but not lactose is an indication of the selective permeability of the membrane in regards to molecule size. Fructose, a smaller monosaccharide, was able to diffuse through the membrane, while lactose, a larger disaccharide, could not due to its larger size.
Understanding the concept of osmosis is essential in explaining this process. Osmosis is the movement of water across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration. In this experiment, the water moved into the dialysis bag containing lactose and fructose, and the smaller fructose molecules diffused out into the water, equilibrating the concentration on both sides of the membrane.