Explain how the process of endocytosis allows a cell to take up large molecules that cannot cross the plasma membrane by diffusion or transport proteins.

Endocytosis is an active process that allows a cell to take up large molecules that cannot cross the plasma membrane by diffusion or transport proteins. The plasma membrane first forms an indentation or invaginates around the large molecule or particle. The indentation then pinches off, forming a vesicle inside the cell that encloses the molecule. This process requires energy, usually in the form of ATP, and the vesicle is formed without the phospholipid bilayer being broken. Consequently the large molecule is transported into the cell while the bilayer remains intact.