Final answer:
Starch did not diffuse out of the dialysis tube membrane because it is a semipermeable membrane that only allows small molecules and ions to pass. The experiment showed fructose could diffuse through, but lactose and starch, being larger particles, could not.
Step-by-step explanation:
We know that starch did not diffuse out of the dialysis tube membrane because dialysis tubing is a type of semipermeable membrane that separates substances based on their size. Starch molecules are too large to pass through the pores of the dialysis membrane. Dialysis tubing allows the passage of water, small molecules, and ions but not colloidal particles like protein and starch. This property is highlighted in Figure 9.11.1, which illustrates that substances like starch are separated from water and smaller molecules during dialysis.
In the experiment mentioned, we can conclude that fructose, a monosaccharide, was small enough to pass through the membrane, while lactose, a larger disaccharide, could not. The detection of fructose in the distilled water outside the dialysis tubing after some time indicates that it diffused through the semipermeable membrane. However, the absence of lactose outside the dialysis tubing suggests that its molecular size was too large to pass through the pores of the membrane.
Experiments with dialysis tubing that have different pore sizes can be conducted to determine the size threshold that allows a molecule like glucose to pass through. By observing which membranes allow glucose to diffuse, while larger molecules like proteins and starch do not, researchers can estimate the pore specificity of the membrane and thus the approximate size of glucose molecules.