18.1: Endocytosis
Eukaryotic cells acquire nutrients for growth and proliferation. Nutrients and other molecules that require degradation are internalized from the extracellular space by a process called endocytosis. The term ‘endocytosis' was first coined by Christian de Duve in 1963.
Endocytosis always begins with the plasma membrane enclosing an incoming molecule to form a transport vesicle which, in some cases, can be coated with a protein called ‘clathrin.' Endocytosed material is either sorted through vesicles called early endosomes or can be utilized by the cell as in case of certain nutrients or is degraded upon fusion with a lysosome. All vesicles containing the material endocytosed for the purpose of degradation eventually fuse with a lysosome. Three main types of endocytosis include phagocytosis, pinocytosis, and receptor-mediated endocytosis.
In phagocytosis, specialized immune cells called phagocytes engulf pathogenic microorganisms, including parasites, playing a vital role in providing immunity to the host organism. The internalized pathogens are contained in a vesicle called ‘phagosome,' which later fuses with a lysosome. Additionally, cellular waste material such as cell debris of other dying cells is also engulfed by phagocytes and degraded upon fusion with a lysosome. However, some pathogenic organisms take advantage of endocytosis to enter a cell and infect the host organism.
Pinocytosis is another mode of endocytosis, where a cell can engulf dissolved substances from the extracellular space. The third type of endocytosis, called receptor-mediated endocytosis, is a very specific process wherein an incoming molecule is only internalized after it binds to its associated receptor. Both pinocytosis and receptor-mediated endocytosis are followed by a process where a coated vesicle containing the endocytosed fluid or molecule fuses with an early endosome.
