ABSTRACT
Proteins destined to be secreted are synthesized on ribosomes bound to the rough endoplasmic reticulum membrane and are transported in membrane-bound vesicles to the Golgi apparatus where they are sorted and packaged up into secretory vesicles. Endocytosis is the uptake of extracellular macromolecules across the plasma membrane into the cell. Endocytosis can be divided into three distinct types depending on the size of the ingested macromolecule and whether specific cell surface receptors are involved. These three processes are: phagocytosis, pinocytosis and receptor-mediated endocytosis. In protozoa, phagocytosis is a form of feeding, where the ingested material is broken down in the lysosomes and utilized as food. Receptor-mediated endocytosis provides a way of selectively concentrating particular macromolecules that are at low concentrations in the extracellular fluid, thereby increasing the efficiency of their uptake without having to take in large quantities of extracellular fluid.
