3.14 : 囊泡运输：胞吞作用、转胞吞作用和胞吐作用

Endocytosis is a process where the cell membrane folds inward as vesicles to engulf large-sized drugs.

This process includes pinocytosis and phagocytosis.

Pinocytosis facilitates the uptake of small droplets by the cell. Tiny caveolae at the cell surface detach to form fluid-filled bubbles, moving freely in the cytoplasm.

Phagocytosis allows cells to internalize large macromolecules, bacteria, or cellular debris as phagosomes.

Another mechanism, transcytosis, involves the transfer of endocytic vesicles carrying drugs across a cell that fuse with the cell's outer membrane. This process facilitates the absorption of large protein molecules and certain vaccines, such as the Sabin polio vaccine.

In contrast, exocytosis is a reverse process. It involves intracellular vesicles merging with the cell membrane to release their contents outside. For instance, insulin-producing cells in the pancreas,  use exocytosis to pack and release insulin  into the bloodstream.