A cell posses hydrophobic layer called plasma membrane which contains transmembrane proteins to transport the molecules in and out of the cells. These trans-membrane proteins allow only the passage of small molecules like ions, glucose.
But the macro-molecules having high molecular weight cannot pass the plasma membrane so they can enter the cell by a complex process called endocytosis.
The endocytosis helps engulf the large molecule by surrounding the particle and then invagination of the plasma membrane inside the cell-mediated by clathrin-coated proteins.
