* Size: Starch molecules are very large, much larger than the typical size of molecules that can pass through the cell membrane. The cell membrane is selectively permeable, meaning it only allows certain substances to pass through.

* Polarity: Starch is a complex carbohydrate and is polar. Cell membranes are primarily made of phospholipids, which have a hydrophobic (water-repelling) tail and a hydrophilic (water-attracting) head. This structure makes the membrane more permeable to small, nonpolar molecules.

What can cross the model cell membrane?

* Small, nonpolar molecules: Oxygen, carbon dioxide, and lipids can easily pass through the membrane.

* Small, polar molecules: Water can pass through the membrane, though it does so more slowly than nonpolar molecules.

* Facilitated diffusion: Some larger molecules or charged ions can cross the membrane with the help of transport proteins.

In summary: Starch is too large and too polar to easily cross the cell membrane.