Answer:
The protein is too large to simply diffuse across the phospholipid bilayer of the cell, so it uses phagocytosis to diffuse across, where it is contained in a vesicle and pinched into the cell? Credits to yahoo answer. Hope I could help sweets.
Cell membranes acts as a barrier for the large,charged and polar molecules due to their hydrophobic nature.
These molecules are transported with the help of membrane proteins such as carrier proteins across the plasma membrane by a process known as facilitated diffusion. These membrane proteins diffuse larger molecules across the cell membrane that cannot be transported by simple diffusion.