Question

Conclusion: Starch has a larger molecule size than glucose. So you can conclude that larger molecules are
likely than smaller molecules to diffuse across a semipermeable membrane.

Answer 1

That conclusion is backwards.

Correct conclusion: smaller molecules are more likely to diffuse across a semipermeable membrane than larger molecules.

Why:
- Diffusion through a membrane with pores depends on molecular size relative to the pore size. Large polymers (starch) cannot pass through pores that admit small sugars (glucose).
- Diffusion rate also decreases as molecule size increases (qualitatively described by the Stokes–Einstein relation).
- Experimental evidence: in the classic dialysis-tube experiment, iodine (small) moves into a starch solution and gives a color change, while starch remains inside the tubing; glucose can pass through many dialysis membranes and be detected outside (e.g., with Benedict’s test), whereas starch cannot.

Caveats:
- Permeability also depends on polarity/charge and the type of membrane. A lipid bilayer passes small nonpolar molecules more readily, and membranes with channels or transporters can allow large or polar molecules to cross (facilitated diffusion or active transport).
- So the general rule is: all else equal, smaller molecules diffuse more readily than larger ones through a semipermeable membrane.