Receptor-Mediated Endocytosis

In order to sustain normal metabolic processes, the cell must recruit from its external environment an array of ions, molecules, and macromolecules. Small molecules, such as ions, water, and monosaccharides, can gain entry into the cytosol of the cell through water-filled channels. However, molecules larger than approximately 1000 Da cannot pass through these channels. Therefore, if diffusion through plasma membrane pores or channels were the only method of entry into the cell cytoplasm, larger molecules such as polypeptides, polysaccharides, and other nutrients could not be utilized by the cell for normal metabolic processes. In addition, these larger molecules are often present in the extracellular environment of the cell at much lower concentrations than ions or monosaccharides. Therefore, in order to overcome problems associated with internalizing large and scarce molecules, the cell has developed the process of receptor-mediated endocytosis. The extracellular macromolecules bind to specific, high-affinity receptors on the cell surface, thereby trapping them and concentrating these macromolecules at the plasma membrane.