Explain how exocytosis and endocytosis occur across the plasma membrane.

Exocytosis and endocytosis are processes of bulk transport of materials out or into cells, respectively. The materials involved are those that are too large to be absorbed by diffusion or active transport.

During endocytosis, the cell membrane invaginates to form a cup-shaped depression in which the material is enclosed. The depression is then pinched off to form a vacuole in the cytoplasm of the cell. If the materials taken up are solid this is called phagocytosis and pinocytosis for liquids.

During exocytosis, a vacuole containing material to be excreted fuses with the cell membrane from within. The membranes unite and open up the vacuole releasing the contents to outside.