endocytosis


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endocytosis

(ĕn'dōsītō`səs), in biology, process by which substances are taken into the cellcell,
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
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. When the cell membranemembrane,
structure composed mostly of lipid and protein that forms the external boundary of cells and of major structures within cells. Membrane organization is based on a sheet two molecules thick—a double layer of lipids aligned with their long hydrocarbon tails tucked
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 comes into contact with a suitable food, a portion of the cell cytoplasm surges forward to meet and surround the material and a depression forms within the cell wall. The depression deepens and the movement of the cytoplasm continues until the food is completely engulfed in a pocket called a vessicle. The vessicle then drifts further into the body of the cell where it meets and fuses with a lysosome, a vessicle normally found in the cell that contains digestive enzymes known as acid hydrolases. The food is then broken down into molecules and ions that are suitable for the cell's use. There are two types of endocytosis: pinocytosis, the engulfing and digestion of dissolved substances, and phagocytosis, the engulfing and digestion of microscopically visible particles. Phagocytosis is the process by which many protozoansprotozoan
, informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues.
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 obtain most of their food supply. It is also the process through which specialized cells in animals eliminate foreign matter, such as infecting microorganisms, as part of the body's defense system (see bloodblood,
fluid pumped by the heart that circulates throughout the body via the arteries, veins, and capillaries (see circulatory system; heart). An adult male of average size normally has about 6 quarts (5.6 liters) of blood.
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; immunityimmunity,
ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
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). The various phagocytic cells in higher animals are derived from relatively unspecialized cells called stem cells that are either fixed within a network of supporting (reticular) cells and fibers of the spleen, thymus, and bone marrow, or that wander freely throughout body tissues. Many phagocytic cells respond chemically to substances produced by foreign bodies or by degenerating tissue by moving toward the substances, a mechanism known as chemotaxis. When a particle of the proper charge or chemical composition adheres to the cell surface, the cell cytoplasm moves so that it finally surrounds the particle and traps it within a cytoplasmic vacuole. Various enzymes are then secreted into the vacuole to digest the foreign substance. In higher animals each phagocyte can ingest about 5 to 25 invading bacterial cells. Phagocytosis often precedes production of antibodiesantibody,
protein produced by the immune system (see immunity) in response to the presence in the body of antigens: foreign proteins or polysaccharides such as bacteria, bacterial toxins, viruses, or other cells or proteins.
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 by the body, but some species of bacteria cannot be phagocytized unless specific antibody is already present. Although phagocytosis is an effective response to infection, some organisms, such as the bacteria causing brucellosis and tuberculosis, can survive for years within the descendant cells of the phagocytes that ingested them. The process of phagocytosis was first described in the late 19th cent. by the Russian zoologist Élie Metchnikoff.

Endocytosis

The process by which animal cells internalize particulate material (such as cellular debris and microorganisms), macromolecules (such as proteins and complex sugars), and low-molecular-weight molecules (such as vitamins and simple sugars). Cells engage in at least three different types of endocytosis: phagocytosis where cells engulf particulate material, receptor-mediated-endocytosis of macromolecules, and potocytosis of small molecules.

Some of the essential nutrients that a cell needs are scarce in the environment. The cells overcome this problem by expressing high-affinity receptors, or binding sites, on the membrane surface. Each type of receptor is specific for either macromolecules or molecules. These endocytic receptors are capable of concentrating their ligand at the cell surface before carrying it into the cell, thus increasing the efficiency of uptake.

In all three endocytic pathways the internalization step begins with the invagination of plasma membrane and the conversion of this membrane into a closed vesicle called an endosome. Each of the pathways has its own set of molecules that control internalization. These molecules assemble at the cell surface and physically deform the membrane into the shape of a vesicle. The vesicle, the endosome, then detaches and migrates to other locations within the cell. The same cell-surface assemblage of molecules also attracts endocytic receptors that are moving around on the cell surface, causing them to cluster over the site of internalization. Receptor clustering, which is essential for efficient uptake, is sometimes stimulated by ligand binding.

Endosomes that are generated by the phagocytic and receptor-mediated endocytic pathways often fuse with lysosomes that contain many different hydrolytic enzymes. Small molecules, by contrast, do not need further processing, so during potocytosis they are delivered directly to the cytoplasm. See Cell membranes, Lysosome

Phagocytosis

Phagocytosis is a receptor-mediated process where the receptors function as adhesive elements that bond the plasma membrane to the particle. The adhesive interaction of the phagocytic receptors with the membrane stimulates invagination. A critical molecule in this activity is actin, the same protein that provides power for muscle contraction. Surface membranes contain actin-binding proteins that link the phagocytic receptor to the actin cytoskeleton of the cell. Thus, when a particle binds to its endocytic receptor, a signal cascade is initiated that stimulates the recruitment of actin filaments to the site of phagocytosis. See Cytoskeleton, Phagocytosis, Signal transduction

Receptor-mediated endocytosis

The clathrin-coated pit is a segment of cell membrane that is specialized for receptor-mediated endocytosis. Each pit can be recognized by the presence of a polygonal lattice on the cytoplasmic surface of the membrane. This lattice shapes the plasma membrane into a coated vesicle that immediately uncoats and fuses with endosomes. The endosome functions as a switching area that directs membrane and content molecules to specific locations within the cell.

Potocytosis

Potocytosis uses membrane proteins that are anchored by lipid rather than protein as endocytic receptors. The lipid anchor causes the attached proteins to migrate in the plane of the membrane and cluster in a membrane specialization called a caveola. Clustering ensures that any ligand bound to these receptors will be concentrated in this location. When caveolae close, they create a tiny compartment of uniform size that is sealed off from the extracellular space. When the ligand dissociates from its receptor, it reaches such a high concentration that it naturally flows through water-filled membrane channels into the cell.

The closed caveolar compartment appears to be a unique space for the cell. It is transient, does not merge with other organelles, and can selectively concentrate extracellular molecules or ions and deliver them to the cytoplasm. In addition to importing molecules, cells can also use this space to store and process incoming or outgoing messengers that affect cell behavior. See Cell (biology), Cell permeability

endocytosis

[¦en·do·sī′tō·səs]
(cell and molecular biology)
An active process in which extracellular materials are introduced into the cytoplasm of cells by either phagocytosis or pinocytosis.
The process by which animal cells internalize large molecules and large collections of fluid.
References in periodicals archive ?
Cd-MT causes endocytosis of brush-border transporters in rat renal proximal tubules.
In addition, different substances such as nutrients and growth factors are apparently guided to their destination by different set of genes and endocytosis is controlled by various signalling pathways.
One of the most consistent and intriguing findings associated with both dominant and recessive forms of Parkinson's, including those due to parkin mutations, have been defects in synaptic transmission, possibly related to altered synaptic vesicle endocytosis, recycling or release," Dr.
1) During endocytosis, material outside the cell is progressively enclosed by a portion of the plasma membrane, which then is taken up into the cell to form a bubble-like structure called an endosome, containing the ingested material.
Secreted proprotein convertase subtilisin/kexin-type 9 reduces low-density lipoprotein receptor through receptor-mediated endocytosis.
Recent thinking in GPCR endocytosis has been summarized (37), the dynamics of which, upon agonist stimulation, has recently been imaged in real time in vivo for an opioid GPCR (38).
Inhalation exposures may pose potential risks given that QDs have been shown to be incorporated via endocytosis by a variety of cell types and may reside in cells for weeks to months.
In these proceedings from the July 2004 symposium, contributors describe their research in the endocytosis and exocytosis of proteins and lipids, especially on the functions of lipids and rafts and clathrin-coated vesicles.
Several cytosolic proteins compete for integrin binding to regulate activation, endocytosis and recycling.
The toxin then attaches itself to a surface receptor on a cell's plasma membrane and enters the cell through a process called endocytosis.
The research team used specific inhibitors, and discovered that alpha-synuclein is transmitted via endocytosis, the normal process by which cells absorb proteins from the extracellular media by engulfing them within their cell membrane.