cellular biology Cell biology (also cellular biology or cytology) is a branch of biology that studies the Anatomy, structure, Physiology, function, and behavior of cell (biology), cells. All living organisms are made of cells. A cell is the basic unit of life th ...

, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of

endocytosis Endocytosis is a cellular process in which Chemical substance, substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a Vesicle (biology and chem ...

in which small molecules dissolved in

extracellular fluid In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 50–60% (range 45 to 75%) of total body weight; women and the obese typically ha ...

are brought into the cell through an

invagination Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination of Epithelium, epithelial sheets occurs in many contexts during Animal embryonic development, embryonic developme ...

of the

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

, resulting in their containment within a small vesicle inside the cell. These pinocytotic vesicles then typically fuse with early endosomes to

hydrolyze Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

(break down) the particles. Pinocytosis is variably subdivided into categories depending on the molecular mechanism and the fate of the internalized molecules.

Function

In humans, this process occurs primarily for absorption of fat droplets. In endocytosis the cell plasma membrane extends and folds around desired extracellular material, forming a pouch that pinches off creating an internalized vesicle. The invaginated pinocytosis vesicles are much smaller than those generated by

phagocytosis Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

. The vesicles eventually fuse with the lysosome, whereupon the vesicle contents are digested. Pinocytosis involves a considerable investment of cellular energy in the form of ATP.

Pinocytosis and ATP

Pinocytosis is used primarily for clearing extracellular fluids (ECF) and as part of immune surveillance. In contrast to

, it generates very small amounts of ATP from the wastes of alternative substances such as

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

s (fat). Unlike

receptor-mediated endocytosis Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses – by the inward budding of the plasma membrane (invagination). This ...

, pinocytosis is nonspecific in the substances that it does transport: the cell takes in surrounding fluids, including all solutes present.

Etymology and pronunciation

The word ''pinocytosis'' () uses combining forms of ''pino-'' + '' cyto-'' + ''

-osis OSIS or variant may refer to: OSIS * Open Source Information System * Open Scripture Information Standard * A student council in Indonesia (''Organisasi Siswa Intra Sekolah'') -osis The suffix '' -osis'' is used for forming a number of medical t ...

'', all

Neo-Latin Neo-LatinSidwell, Keith ''Classical Latin-Medieval Latin-Neo Latin'' in ; others, throughout. (also known as New Latin and Modern Latin) is the style of written Latin used in original literary, scholarly, and scientific works, first in Italy d ...

from

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

, reflecting ''píno'', to drink, and

cytosis -Cytosis is a suffix that either refers to certain aspects of cells ie cellular process or phenomenon or sometimes refers to predominance of certain type of cells. It essentially means "of the cell". Sometimes it may be shortened to -osis (necr ...

. The term was proposed by W. H. Lewis in 1931.

Non-specific, adsorptive pinocytosis

Non-specific, adsorptive pinocytosis is a form of

, a process in which small particles are taken in by a

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

by splitting off small

vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle In a neuron, synaptic vesicles (or neurotransmitter vesicles) s ...

from the cell membrane.

Cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

ic proteins bind to the negative cell surface and are taken up via the

clathrin Clathrin is a protein that plays a role in the formation of coated vesicles. Clathrin was first isolated by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When the triskel ...

-mediated system, thus the uptake is intermediate between

and non-specific, non-adsorptive pinocytosis. The clathrin-coated pits occupy about 2% of the surface area of the cell and only last about a minute, with an estimated 2500 leaving the average cell surface each minute. The clathrin coats are lost almost immediately, and the membrane is subsequently recycled to the cell surface.

Macropinocytosis

Macropinocytosis is a clathrin-independent endocytic mechanism that can be activated in practically all animal cells, resulting in uptake. In most cell types, it does not occur continuously but rather is induced for a limited time in response to cell-surface receptor activation by specific cargoes, including

growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for ...

s, ligands of

integrin Integrins are transmembrane receptors that help cell–cell and cell–extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, o ...

s, and apoptotic cell remnants. These ligands activate a complex signaling pathway, resulting in a change in

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

dynamics and the formation of cell-surface protrusions of

filopodia Filopodia (: filopodium) are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. Within the lamellipodium, actin ribs are known as ''microspikes'', and when they extend beyond the lamellipod ...

and lamellopodia, commonly called ''ruffles''. When ruffles collapse back onto the membrane, large fluid-filled endocytic vesicles form called

macropinosome Macropinosomes are a type of cellular compartment that form as a result of macropinocytosis. Formation Macropinosomes have been described to form via a wave-like mechanism or via a tent-pole formation both of which processes require rapid polym ...

s, which can transiently increase the bulk fluid uptake of a cell by up to tenfold. Macropinocytosis is a solely degradative pathway: macropinosomes acidify and then fuse with late endosomes or endolysosomes, without recycling their cargo back to the plasma membrane. Some bacteria and viruses have evolved to induce macropinocytosis as a mechanism for entering host cells. Some of these can stop the degradation processes in order to survive inside the macropinosome, which may transform into smaller and long-lasting

vacuole A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...

s containing the viruses or bacteria (some of which may replicate inside), or simply escape through the wall of the macropinosome when inside. For example, the gut pathogen ''Salmonella'' typhimurium injects toxins into the host cell in order to induce macropinocytosis as a form of uptake, inhibits the degradation of the macropinosome, and forms a salmonella-containing vacuole, or SCV, wherein it can replicate.

Inhibitors

* Virapinib

References

*Campbell, Reece, Mitchell: "Biology", Sixth Edition, Copyright 2002 P. 151 *Marshall, Ben, Incredible Biological Advancements of the 20th Century, Copyright 2001 p. 899 *Alrt, Pablo, Global Society Harvard study, copyright 2003 p. 189 *Brooker, Robert: "Biology", Second Edition, Copyright 2011 p. 116 *Cherrr, Malik, The Only Edition, Copyright 2012, p. 256 *Abbas, Abul, et al. "Basic Immunology: Functions and Disorders of the Immune System." 5th ed. Elsevier, 2016. p. 69 {{Authority control Cellular processes