What Is It, How It Occurs, and More
Editors:Alyssa Haag,Ian Mannarion, MD, MBA,Kelsey LaFayette, DNP, RN
Illustrator:Jessica Reynolds, MS
Copyeditor:David G. Walker
What is 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 is a cellular process that requires substantial energy in the form of adenosine triphosphate (ATP) and is a form of endocytosis, a general process by which substances are brought into a cell. Other forms of endocytosis include phagocytosis, which allows the cell to engulf and digest bacteria, dead tissue cells, and protozoa; as well as receptor-mediated endocytosis (RME), which uses specific cell surface receptor proteins to help carry large particles across the cell membrane.
What is the difference between phagocytosis and pinocytosis?
Pinocytosis primarily refers to the uptake of extracellular fluids and small molecules by a cell, whereas phagocytosis is a process by which the cell membrane invaginates around large macromolecular structures (e.g., proteins and viruses) that are otherwise unable to diffuse into the cell. The purpose of phagocytosis is to break down these large molecules, whereas pinocytosis takes up molecules that have already been dissolved in fluid. While both phagocytosis and pinocytosis can play a role in immune system function, pinocytosis is not as important as phagocytosis in regard to cellular breakdown and elimination. In fact, phagocytosis is primarily used by cells of the immune system (i.e., macrophages, neutrophils, monocytes, and dendritic cells) to destroy an invading virus and degrade damaged or apoptotic cells. Lastly, invaginated pinocytosis vesicles are much smaller (i.e., typically 100-200 nanometers in diameter) than those generated by phagocytosis (i.e., typically larger than 500 nanometers in diameter).
How does pinocytosis occur?
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.As the pouch is pinched off, the internalized vesicle buds off and drifts into the cytoplasm of the cell. The contents in the pinocytotic vesicles are subject to intracellular digestion by fusing with a lysosome, which is an intracellular organelle that contains digestive enzymes and is responsible for breaking down cellular debris, bacteria, and viruses. Pinocytosis is nonspecific for the substances it accumulates, in contrast to another form of endocytosis known as receptor-mediated endocytosis (RME). During RME, specific metabolites, hormones, proteins, and viruses bind to designated receptors on the cell membrane and follow a similar process of invagination where the cell engulfs the contents.
Why does pinocytosis occur?
The main function of pinocytosis is to absorb extracellular fluids with various solutes. In adults, pinocytosis occurs in the villi lining the small intestine in order to absorb dissolved fats (e.g., low-density lipoprotein [LDL]) and vitamins from our diet. It also takes place in the cells of the kidney to remove waste products during the formation of urine. The immune system can sample the extracellular fluid via pinocytosis to assess for antigens, such as toxins or foreign substances, and activate the body’s immune system if small amounts of antigens are detected. However, some viruses and bacterial toxins have adapted to use certain methods of pinocytosis to gain entry to cells. Lastly, pinocytosis plays a role in the maternal transfer of immunoglobulin G (IgG) to the fetus. This process passively protects the fetus from infections while still in the womb and remains in the baby’s blood stream for a period of time after birth.
What are the most important facts to know about pinocytosis?
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.
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Resources for research and reference
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Parsons, D. S. (1963). Quantitative aspects of pinocytosis in relation to intestinal absorption. Nature, 199: 1192–1193. DOI: 10.1038/1991192b0
Stillwell, W. (2016). An introduction to biological membranes: Composition, structure and function (2nd ed.). Amsterdam: Elsevier Science.
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