This illustration shows a plasma membrane forming a pocket around fluid in the extracellular fluid. The membrane subsequently engulfs the fluid, which becomes trapped in a vacuole.
In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. (credit: modification of work by Mariana Ruiz Villareal)

OpenStax Biology 2e

A variation of endocytosis is pinocytosis. This literally means “cell drinking”. Discovered by Warren Lewis in 1929, this American embryologist and cell biologist described a process whereby he assumed that the cell was purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome.

A variation of pinocytosis is potocytosis. This process uses a coating protein, caveolin, on the plasma membrane’s cytoplasmic side, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis brings small molecules into the cell and transports them through the cell for their release on the other side, a process we call transcytosis.

– What is a membrane-bound compartment inside a eukaryotic cell, and is an organelle of the endocytic membrane transport pathway originating from the trans Golgi network?


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at:


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