Final answer:
Fructose diffused through the dialysis membrane due to its smaller size, unlike lactose. The experiment demonstrates pore specificity, and similarly, glucose would move according to its concentration gradient across cell membranes with the help of transport proteins.
Step-by-step explanation:
During the experiment, fructose, a monosaccharide, was detected outside of the dialysis-tube membrane, suggesting that it diffused through the membrane. Since lactose, a disaccharide, was not detected outside the dialysis membrane, this indicates that the membrane was permeable to the smaller fructose molecules but not to the larger lactose molecules. This can be attributed to the pore specificity of the semipermeable membrane which allows only certain molecules to pass through based on size.
Additionally, in cases where glucose transport proteins or GLUTs are present in a cell membrane, glucose would move according to its concentration gradient. However, if the concentration of glucose is equal on both sides of the membrane, there would not be a net flow of glucose across the membrane unless facilitated by carriers or active transport mechanisms. In the kidneys, glucose is filtered and reabsorbed, and the rate of glucose reabsorption depends on the availability of specific carrier proteins which can be overwhelmed, leading to glucose in the urine, as seen in diabetics.