Hint: Although most of the transport of substances takes place by diffusion in cells but few large molecules need to be exported or imported by two important phenomena – exocytosis and endocytosis.
This answer consists of three parts explained as follows-
Exocytosis & endocytosis-
The endocytosis is a kind of transport mechanism which takes up large molecules inside a cell by the invagination of cell membrane thereby forming a pocket around a target substance.
Whereas exocytosis is opposite to endocytosis and herein large molecules or waste materials are expelled out of the cell to the external environment.
Differences between endocytosis and exocytosis-
• Endocytosis refers to uptake of large molecules inside the cell whereas exocytosis refers to expulsion of substances out of cell to extracellular fluid.
• Endocytosis has three o types (phagocytosis, pinocytosis, receptor-mediated endocytosis) whereas exocytosis is either regulatory or constitutive exocytosis.
• In case of endocytosis substances are taken up by formation of small vesicles whereas in exocytosis substances are expelled by membrane fusion.
Similarities between endocytosis and exocytosis-
• Plasma membrane is involved in case both endocytosis as well as endocytosis
• Both are involved in transporting large molecules across the cell/plasma membrane.
• Both processes require energy in actively transporting substances across membranes.
Note: In endocytosis the plasma membranes forms an invagination around a food particle or large molecule and engulf it in the forming a pocket like structure which later pinches off small vesicle and the substance is carried inside the cell (within the vesicle). In exocytosis, a waste material or large molecule is surrounded or taken up in a membrane bound vesicle which later fuses with the interior of the plasma membrane and is expelled to the external environment.