Engulfing membrane

Endocytosis is an ubiquitous system for engulfing solid/colloidal materials by cells by encapsulating them in membrane-bound vesicles. 

Studies from our laboratories by Pantarotto et al. (2004a, b), Wu et al. (2005) and Kostarelos et al. (2007) using covalently functionalised CNTs (1,3-dipolar cycloaddition reaction chemistry) have reproducibly described that CNTs were uptaken by cells via pathways other than endocytosis. This work has experimentally observed that CNTs were able to interact with plasma membranes and cross into the cytoplasm without the apparent need of engulfment into a cellular compartment... 

When neutrophils encounter bacteria, possibly coated with opsonin proteins of the complement system, the invaders are engulfed by the phagocytes and taken into the cells by endocytosis. A small part of the neutrophil membrane is used to create a phagosome - that is, a vacuole enclosing the bacterial cells. Within a matter of a few... 

Pinocytosis Pinocytosis involves the engulfing of fluids by a cell. The process commences with the infolding of cell membrane around fluids containing the drug. The membrane then fuses and forms a vesicle with the fluid core. In this way, the drug is taken into the cell interior within the vesicle. 

Marguet, D Luciani, M.F., Moynault, A., WiUiamson, P., and Chimini, G., 1999, Engulfment of apoptotic cells involves the redistribution of membrane phosphatidylserine on phagocyte and prey, iVttt. Cell Biol. 1 454-456. 

Binding to the cell surface proceeds at 0°C, but the cells are not infected (Helenius et al., 1980). When the cells are warmed to 37 C the virus is rapidly removed from the cell surface and infection ensues. In general there are two ways to envisage the entry of enveloped viruses into cells—either by penetration directly through the plasma membrane, or by endocytosis (engulfment by a plasma membrane-derived vesicle) (see Lonberg-Holm and Philipson, 1974). In both cases delivery of the nucleocapsid with the RNA would have to involve a fusion reaction between the viral envelope and either the cell surface membrane or the vesicle membrane. Paramyxoviruses are known to fuse their envelopes with the plasma membrane (see Hosaka and Shimizu, 1977). However, whether this process leads to productive infection has not yet been settled. 

After membrane ruffling and formation of the so-called pseudopodia, the material is engulfed by the cell and is further transported to vesicles (phagosomes/macropinosomes) that have the ability to become acidified. These vesicles fuse rapidly with late endosomes and/or lysosomes, exposing their contents to the hydrophilic enzymes. 

These three changes take several honrs, and the whole process is termed capacitation. These changes allow a sperm to reach the plasma membrane of the ovnm. Then the plasma membrane of the sperm fnses with that of the ovum, the head is engulfed within the membrane of the ovum and the head, and only the head, enters the ovum. Within the ovum, the nucleus of the sperm is released and the two nnclei fuse. A diploid zygote is the result. 

Transcytosis (vesicular transport, C). When new vesicles are pinched off, substances dissolved in the extracellular fluid are engulfed, and then ferried through the cytoplasm, vesicles (phagosomes) undergo fusion with lysosomes to form phagolysosomes, and the transported substance is metabolized. Alternatively, the vesicle may fuse with the opposite cell membrane (cytopempsis). 

A. Phagocytosis of large external particles, such as bacteria, occurs by engulfment or surrounding of the particle by the membrane. 

Chemicals may cross the cell membrane via membrane pores. This diffusion depends on the size of the pore and the size and weight of the chemical. The chemical flows through the membrane along with water. Finally, the membrane can actually engulf the chemical, form a small pouch called a vesicle, and transport it across the membrane to the inside of the cell. This process is called pinocytosis. 

A few substances are so large or impermeant that they can enter cells only by endocytosis, the process by which the substance is bound at a cell-surface receptor, engulfed by the cell membrane, and carried into the cell by pinching off of the newly formed vesicle inside the membrane. The substance can then be released inside the cytosol by breakdown of the vesicle membrane. Figure 1-5D. This process is responsible for the transport of vitamin B12, complexed with a binding protein (intrinsic factor) across the wall of the gut into the blood. Similarly, iron is transported into hemoglobin-synthesizing red blood cell precursors in association with the protein transferrin. Specific receptors for the transport proteins must be present for this process to work. 

Two other types of specialized transport mechanisms, pinocytosis and phagocytosis, may also account for the transmembrane movement of some macromolecules (2). In these complex processes, the cell engulfs a droplet of extracellular fluid or a particle of solid material such as a bacterium. The droplet or particle is completely surrounded by a portion of the cell membrane and the resulting vesicle becomes detached and moves into the cell cytoplasm. 

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