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Pinocytosis is the process by which the cell takes up fluid through pinocytic vesicles and occurs during endocytosis[1].

In pinocytosis, small amounts of extracellular fluid and solute molecules are taken up by the pinocytic vesicles, or pinosomes, which then pinch off the plasma membrane along the endosome pathway. Pinocytosis differs from Phagocytosis, as pinosomes take up small amounts of fluid and phagosomes take up large particles. However, both are classed as endosomes, as they are both formed by endocytosis.

In eukaryotic microorganisms, there are different types of pinocytosis. In fluid-phase endocytosis, a small amount of extracellular fluid is pinched off non-selectively[2]. There is no mechanism that takes up or excludes particular molecules and the ingested material is not concentrated, so the concentration of the ingested material is the same as the concentration of the extracellular environment. Fluid-phase endocytosis continues at a constant rate in most eukaryotic cells. It is a way to control the cells volume and surface area, to prevent shrinking or swelling. This is because it is removing the same amount of material that has been added by exocytosis [3].

Another type of pinocytosis is receptor-mediated endocytosis. In this form of pinocytosis, vesicles are coated with clathrin, which then pinches off the plasma membrane. The process starts in coated pits in the plasma membrane, which allow specific macromolecules to bind, e.g hormones, and therefore allow a control over molecules that need to be degraded[4]. An example of receptor-mediated endocytosis is the uptake of cholesterol. This process prevents accumulation of cholesterol which can contribute to strokes and heart attacks[5]. During this form of endocytosis, the clathrin-coated pit take up cholesterol in the form of LDL, and then form clatharin coated vesicles. The vesicle is uncoated and then fuses with the early endosome, and then the LDL ends up in lysosomes which contain hydrolytic enzymes that breaks down LDL into free cholesterol[6].

However, not all pinocytic vesicles are coated with clathrin. Pinocytic vesicles can form through caveolae, which are seen under the electron microscope as flask-shaped invaginations of the plasma membrane. Unlike other pinocytic vesicles, they do not fuse with endosomes or lysosomes, therefore they are not degraded[7].


  1. Alberts, B et al. Molecular Biology of the Cell, (2008) 5th Edition,US: Garland Science. Page 787
  2. Harley, J et al. Microbiology,(2005) 6th edition, New York: McGraw-Hill Page 79
  3. Becker, W et al. The World of The Cell,(2009) 7th Edition, San Francisco: Pearson Benjamin Cummings, Page 348
  4. Harley, J et al. Microbiology (2005), 6th Edition. New York: McGraw-Hill Page 80
  5. Alberts, B et al. Molecular Biology of the Cell (2008), 5th Edition,US: Garland Science Page 791
  6. Alberts, B et al. Molecular Biology of the Cell, (2008) 5th Edition,US: Garland Science. Page 793
  7. Harley, J et al. Microbiology (2005), Sixth Edition, New York: McGraw-Hill Page 80
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