Lipid Materials Between Cells

However, it should also be remembered that the intercellular space of the epithelial cells of the oral cavity contains lipidic material, deposited from the membrane coating granules. Lipidic moieties (depending, as always, on their physicochemical properties) may be able to permeate through this lipidic environment between the cells, thereby being absorbed via the paracellular route. 

Current electron-microscopic evidence shows that intercellular regions in SC are filled with lipid-rich amorphous material (43, 45, 46). In the dry membrane the intercellular diffusion volume may be as large as 5% and at least 1% in the fully hydrated tissue. . . this intercellular volume is at least an order of magnitude larger than that estimated for the intra-appendageal pathway and allows the possibility of significant intercellular diffusion. Diffusion between cells cannot be ruled out, but various data show that diffusion cannot be primarily intercellular. 

If a rapidly growing photosynthetic cell such as an alga cell is exposed to C 02 for 1 to 2 minutes and then killed, as much as 30% of the radioactive compounds formed behave as lipidlike substances when partitioned between aqueous and organic solvents. A considerable portion of the chloroplast structure consists of lipid materials, and rapid lipid synthesis is required for chloroplast growth and division. 

The currently accepted structure of B. is the fluid mosaic model. Lipid molecules and membrane proteins are free to diffuse laterally and to spin within the bilayer in which they are located. However, a flip-flop motion from the inner to the outer surface, or vice versa, is energetically unfavorable, because it would require movement of hydrophilic substituents through the hydrophobic phase. Hence this type of motion is almost never displayed by proteins, and it occurs much less readily than translational motion in the case of lipids. Since there is little movement of material between the inner and outer layers of the bilayer, the two faces of the B. can have different compositions. For membrane proteins, this asymmetry is absolute, and, at least in the plasma membrane, different proportions of lipid classes exist in the two monolayers. Attached carbohydrate residues appear to be located only on the noncytosolic surface. Carbohydrate groups extending from the B. participate in cell recognition, cell adhesion, possibly in intercellular communication, and they also contribute to the distinct immunological character of the cell. 

It can be seen from Table 1.1 that the main component of the DM of pasture grass is carbohydrate, and this is true of all plants and many seeds. The oilseeds, such as groundnuts, are exceptional in containing large amounts of protein and lipid material. In contrast, the carbohydrate content of the animal body is very low. One of the main reasons for the difference between plants and animals is that, whereas the cell walls of plants consist of carbohydrate material, mainly cellulose, the walls of animal cells are composed almost entirely of lipid and protein. Furthermore, plants store energy largely in the form of carbohydrates such as starch and fructans, whereas an animal s main energy store is in the form of lipid. 

Animal cells are separated from each other by lipid membranes. During signal transduction this barrier has to be passed, which can be realized by permanently or temporarily opened channels or by an indirect mechanism without material flux between the extra- and intracellular lumen (Fig. 1). 

Quite generally, the interphase between an organism and its environment encompasses the elements outlined in Figure 1 of Chapter 1. The scheme shows that the cell membrane, with its hydrophobic lipid bilayer core, has the most prominent function in separating the external aqueous medium from the interior of the cell. The limited and selective permeabilities of the cell membrane towards components of the medium - nutrients as well as toxic species - play a governing role in the transport of material from the medium towards the surface of the organism. 

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