Concepts of Membrane Structure by Ronald Aloia download in iPad, pdf, ePub
Depending on the temperature, cholesterol has distinct effects on membrane fluidity. These dense lines are separated by the lightly stained interior portion of the membrane, which contains the hydrophobic fatty acid chains. Role as osmotic barrier The cytoplasmic membrane is the osmotic barrier of the cell, owing to its ability to restrict the passage of salts and polar organic compounds. These junctions not only seal the space between cells but also serve as barriers to the movement of membrane lipids and proteins.
In most bacteria, the cell does not change size owing to the presence of its rigid cell wall. Integral membrane proteins span across the bilayer, but can diffuse within the plane of the bilayer and even associate into large complexes.
These interactions frequently involve ionic bonds, which are disrupted by extreme pH or high salt. Human and mouse cells were fused to produce hybrid cells. At least part of the mechanism by which this occurs involves the formation of tight junctions which are discussed later in this chapter between adjacent cells of the epithelium.
The fracture plane often extends along or through the weakly connected central section of a membrane bilayer. Portions of these integral membrane proteins are inserted into the lipid bilayer, so they can be dissociated only by reagents that disrupt hydrophobic interactions. By interfering with interactions between fatty acid chains, cholesterol prevents membranes from freezing and maintains membrane fluidity.
In contrast, the function of the other major transmembrane protein of red blood cells is well understood. This osmotic flow of water occurs in response to the natural forces that seek to eliminate gradients or differences in the concentration of water on the two sides of the membrane. In contrast to blood cells, epithelial cells are polarized when they are organized into tissues, with different parts of the cell responsible for performing distinct functions. The apical surface of the cell contains microvilli and is specialized for absorption of nutrients from the intestinal lumen.
Bacterial and archaeal cells typically lack intracellular membrane organelles and contain only the single cytoplasmic membrane, perhaps surrounded by an outer membrane. Role as cell boundary The cytoplasmic membrane is the boundary between the cell and its surroundings and thus must regulate the passage of nutrients and metabolic products. The membrane lipids from archaea are quite different from those in bacteria and eukarya. Likewise, the carbohydrate portions of glycolipids are exposed on the outer face of the plasma membrane. The side chains of polar amino acids line the pore, whereas side chains of hydrophobic amino acids interact with the interior of the membrane.
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