Bilayer sheet

Atomic force microscopy (AFM) can be used to obtain high-resolution imagery of molecular orientation and ordering for materials adsorbed onto substrates. Early AFM studies on gluconamides were hampered by the tendency of the fibers to unravel on substrates forming bilayer sheets.41 These layers showed the head-to-tail packing of a monolayer which is similar to the crystal structure reported for anhydrous gluconamides.38 A procedure to retain the fiber networks on surfaces with the addition of a small fraction of... 

Fig. 21 A projection of the structure of [(Hdmbg)ZnCl3] onto the (10 0) plane showing the 2-D bilayer sheet construction (atom identification as for Fig. 1 plus zinc large light grey circles chlorine large dark grey circles) [41]...

The bounding membrane of the neuron is typical of all cells. It is a continuous lipid bilayer sheet of thickness about 60-80 angstroms. Embedded in it, or passing through it, are numerous proteins and glycoproteins, many of which are found only in nerve cells. These have many functions. Some provide structural support to the membrane, but most form ion channels and receptor sites that are essential to nerve function. 

Figure 1. (A) Schematic illustration of a phospholipid molecule with hydrophilic head and hydrophobic tail. Cross-sectional view of (B) the micelle structure and (C) the bilayer sheet structure built through the self-assembly of the lipid molecules (A).

Cross-sectional views of liposome, micelle, and bilayer sheet. 

Fragments of endoplasmic reticulum are transformed from lipid bilayer sheets, with attached ribosomes, into spherical vesicles. This is a result of the homogenization used in preparing the samples and also the tendency of lipid bilayers (Fig. 1-4) to spontaneously reseal. 

The bilayer sheet, also shown in Figure 1,12, is like a sandwich. The "bread" is composed of water-soluble groups and the "filling" of alkane tails. The alkane groups associate with one another but have little contact with the water above and below. 

The third structure in Figure 1,12 is a vesicle. Vesicles are larger than micelles arid contain water in the inferior. The membrane, or lipid-confaining portion, is a bilayer sheet that Is curved to form a spheroid. 

The ceils of mammals and other animals contain the following structures. A plasma membrane (FM), which is the outer border of the cell, has a structure similar to the bilayer sheet shown in Figure 1.12. The PM contains phospholipids and many membrane-bound (embedded) proteins used to facilitate the transport of nutrients and minerals into and out of the cell, The outside of the FM of some cells is coated with polysaccharides for protection. The outside of the PM of other cells bears proteins that control which cells are chosen neighbors. Generally, the material bound to the outside of the PM is synthesized by the cel itself rather than derived from other cells. The cytoplasm is the fluid contained and bounded by the plasma membrane. This fluid has a gel-like consistency because it contains a high concentration of proteins. Most of the biochemical reactions that occur within the cell take place in the cytoplasm. The remainder take place within various organelles. 

Betaire-homocysteine methyltransferase, 502 BHT (bulyiated hydrtmytoLuene), 628 Bicarbonate, 79,80, S2, 717 Bilayer sheet, 25 Bile, 58,331... 

Lipids, 311-312 see also Fats Fatty acids Oils absorphon, 9S-103 acetylcholine, 319 bilayer sheet 25 bile salts and, 96-100 cardiovascular disease and, 357-361 diet and, 361—368 drugs and, 368—370 risk factors, 370-371 cholestentl, 312 326-331,339-343 choline 311,316,317-318... 

Self-assembly of phospholipids, such as phosphatidylcholine, a nutritional supplement, in the presence of calcium creates 50-500 nm large, continuous, solid, lipid bilayer sheets rolled into a spiral structure termed nanocochleates.f These can be made to envelope a lipid-soluble drug, such as amphotericin B, to exclude water and protect against pH, oxidation, light, enzymatic attack, hydrolysis, and extremes in temperature. The nanocochleates will convert to liposomes at a pH greater than 6.5 in a calcium deficient environment as is found in the cell (typically less than 1-2... 

FIGURE 1.12 Phospholipid structures. Most biological membranes take the form of a bilayer sheet. In addition to containing phosopholipids, biological membranes contain proteins and cholesterol. Vesicles are used in biology for transporting and delivering biochemicals from the interior of the cell to the membrane, or to the extracellular fluids. (Reprinted by permission from Darnell et ah, 1990.)... 

The plasma membrane of the cell is a lipid bilayer sheet in which membrane-bound proteins are embedded. Steps 4B-6B of Figure 1.21 illustrate some events in the production of a membrane-bound protein. After synthesis of the protein, the ribosome on which it was formed dissociates from the membrane but the protein remains bound to the membrane (Step 4B). This binding is mediated by a short stretch of lipophilic amino acids that may occur near the C terminus, as shown in Figure 1.21, or near the N terminus in the case of other proteins. Subsequently, part of the ER membrane forms a bud that breaks off (Step 5B) to form a secretory vesicle (Step 6B). The continued association of the entire membrane-bound protein during the budding process and during subsequent events is maintained by the special lipophilic sequence. Eventually, the secretory vesicle fuses with the plasma membrane in a process that resembles a reversal of Steps 4B-6B. After completion of the insertion of the membrane-bound protein into the plasma membrane, its N terminus is in contact with the extracellular fluid and its C terminus is in contact with the cytoplasm, at least for the protein depicted in Figure 1.21. 

When a substance such as sodium chloride is dissolved in water, the ions that form become completely surrounded by water molecules, which form structures called hydration spheres. When the sodium salt of a fatty acid is mixed with water, the caiboxylate group of the molecule becomes hydrated but the hydrophobic hydrocarbon portion of the molecule is poorly hydrated, if at all. The hydrocarbon chains from numerous fatty acids tend to clump together in spherical structures called micelles or, if large numbers are present, into bilayer sheets. Using a circle to represent the carboxylate group and an attached squiggly line to represent the hydrocarbon chain of a fatty acid, draw a picture of a micelle and a bilayer. 

Often these bilayer sheets can wrap around in such a way as to form spherical structures, called vesicles or liposomes (depending on their size). Several new anticancer treatments are based upon the packaging of chemo-... 

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