Skeletonized vesicle

The formation of skeletonized vesicles was also reported for vesicles composed of IS and DPPC, where the mole fraction of DPPC varied from 5 to 25 mole percent. Takeoka et aL analyzed the release of entrapped water soluble molecules in order to assess the size of the pores formed in the vesicle wall [49]. The ease of release of saccharides, primarily dextrans, of various molecular... 

Polymerized lipids do not occur in natural cell membranes. Nature tends to support fragile membrane structures with polymeric skeletons, i.e. protein cytoskeletons, polysaccharide cell walls etc. Analogous synthetic polymeric nets are simply constructed from polymerizable counterions. Negatively charged dihexadecyl phosphate vesicles can be neutralized with choline methacrylate polymerization of the latter produces a polycationic vesicle coat which is not inserted into the membrane (Figure 4.30). A cytoskeleton at the... 

Cl , as well as other oxidized chemical species that were hardly available before the emergence of oxygen, such as bound Zn " and then Cu /( u , but low concentrations of K", Fe", and Mg +. In addition, the rejection of Ca to the environment and then to internal compartments allowed the formation of, for example, organized calcium carbonate to be used in external skeletons (as in coccoliths). The export of Ca and Zn enzymes enabled acid-base hydrolytic digestion not only outside cells, but more generally in vesicles. 

Fig. 23 Strategy for preparation of porous vesicles bearing hydroxyl groups from skeletonized vesicles via hydrolysis. Reprinted with permission from [114]. Copyright 2003, American Chemical Society...

Although cells sometimes are spherical, they more commonly have more elaborate shapes due to their internal skeletons and external attachments. Three types of protein filaments, organized into networks and bundles, form the cytoskeleton within animal cells (Figure 1-15). The cytoskeleton prevents the plasma membrane of animal cells from relaxing into a sphere (Chapter 5) it also functions in cell locomotion and the intracellular transport of vesicles, chromosomes, and macromolecules (Chapters 19 and 20). The cytoskeleton can be linked through the cell surface to the extracellular matrix or to the cytoskeleton of other cells, thus helping to form tissues (Chapter 6). 

Despite the complexity of internal cell structure, which comprises the containing membrane, gelatinous interior cytoplasm, internal granular bodies, fibrous skeleton, and nuclei, each cell may be treated approximately as a spherical viscoelastic shell containing a viscous fluid. The model of an outer shell with an inner fluid describes very well the deformation of red blood cells and also of artificial vesicles made by sonicating phospholipids in water. The shell dictates the equilibrium while the fluid contents dictate the rate of approach to equilibrium, A red cell needs three numbers to describe its response time of 0.1 s an area compressibility k— 10 mNm , a shear modulus 10 Nm"", and a viscosity 10 Pa s. ... 

Pearse and Bretscher (1981) have discussed the role of coated vesicles in membrane synthesis and function. Eukaryotic cells are able to specifically take up macromolecules by absorptive endocytosis. The macromolecules are usually transferred to lyso-somes where they may be degraded. The first stage of the process involves the binding of macromolecules to receptors which are localized in coated pits. The latter are indented sites on the plasma membrane and the coated pit buds into the cytoplasm to form a coated vesicle in which lie the endocytosed macromolecules. The coated vesicle sheds its coat rapidly and the endocytic vesicles fuse with each other. This allows receptors to be returned to the plasma membrane while the contents are transferred to the lyso-somes. In order to explain how lysosomal and plasma membranes remain different, it was suggested that the coated pits are able to accept certain macromolecules while excluding others. The accepted proteins enter the coated pit and were presumed to bind directly or indirectly to clathrin. Clathrin, a 180000-dalton protein on the cytoplasmic face of coated pits, provides the polyhedron skeleton for the coated vesicles. Examples of the use of coated vesicles for mediated endocytosis are in the uptake of low-density lipoprotein from the blood and in humans for the transport of immunoglobulins from the mother to the child. For other mammals such as the rat the antibodies are selectively absorbed from the mother s milk by the intestinal epithelium. Coated vesicles also provide a mechanism for virus transport into cells. 

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