Cells actually do seem to “drink” and “eat” to bring molecules inside their structure (endocytosis)—but in a much different way than humans consume food and drink. The two types of endocytosis are pinocytosis (from the Greek pino, meaning “to drink,” and cytosis) and phagocytosis (from the Greek phago, meaning “to eat,” and cytosis). During pinocytosis, the cell membrane folds inward, forming a small pocket (vesicle) around fluid that is directly outside the cell membrane; the fluids consumed by cells may contain small molecules, such as lipids. For example, the endothelial cells that line the body’s small blood vessels (capillaries) are constantly undergoing the process of pinocytosis, “drinking” from the blood within an organism’s capillaries.

Phagocytosis occurs once a particle (or microorganism) is ingested; it is then wrapped within a package called a vesicle. The vesicle then fuses with a lysosome and from there, the digestive enzymes of the lysosome digest the contents of the vesicles. This process is especially important to two types of cells: amoebas (unicellular protozoa) and mammalian white blood cells (macrophages). For the amoebas, phagocytosis provides food for the protozoan; in mammals, the process plays a critical role in the immune—the mammal’s primary defense—systems by getting rid of microorganisms or damaged cells.