Pinocytosis

Pinocytosis refers to the uptake of extracellular fluids and dissolved solutes, such as fat droplets, vitamins, and antigens. The term pinocytosis is derived from the Greek word “pino,” meaning “to drink,” and “cyto,” meaning “cell.” Therefore, the process of pinocytosis can be thought of as cellular drinking. Pinocytotic vesicles are very small, usually 100 to 200 nanometers in diameter, and require an electron microscope for visualization. 

Pinocytosis occurs in several steps and typically starts with positively charged molecules, such as a peptide or protein, interacting with the negatively charged cell membrane. Once they come into close contact, the particles bind to the cell membrane, thereby changing the cell membrane’s shape to create a pouch that surrounds the particle. As the cell plasma membrane curls around on itself, or invaginates, it brings extracellular fluid along with the particle(s) into the cell. Since the solutes are being moved against their concentration gradient, energy in the form of ATP is required for this process. 

Pinocytosis is an active, energy consuming process where extracellular fluid and solutes are taken up into a cell via small vesicles. It is a  type of endocytosis, which refers to the uptake of substances by a cell. Phagocytosis and receptor-mediated endocytosis are other types of endocytosis. The process of pinocytosis differs from RME in that the molecules taken up are non-specific and the process does not require receptor binding. Pinocytosis occurs in a series of steps, including attraction of positively charged molecules to a negatively charged cell membrane, molecular binding, cell membrane shape-change, invagination, and eventual degradation of vesicular contents by a lysosome. Pinocytosis is important for the processes of nutrient and fat absorption, placental-fetal antibody transfer, immune system function, and renal waste excretion. It differs from phagocytosis in terms of the size of molecules that it captures.