Final answer:
The factor that prevented the protein molecules from moving out of the cell is molecule size. Only small and nonpolar materials can easily diffuse through the cell membrane's lipid bilayer.
Step-by-step explanation:
The factor that prevented the protein molecules from moving out of the cell is molecule size. Only materials that are relatively small and nonpolar can easily diffuse through the lipid bilayer of the cell membrane. Large particles cannot fit in between the individual phospholipids and polar molecules are repelled by the hydrophobic/nonpolar lipids. Therefore, the size of the protein molecules would prevent them from moving out of the cell.