Biological membranes composition

This chapter discusses the composition, structure, and dynamic processes of biological membranes. 

Native biological membranes also display characteristic phase transitions, but these are broad and strongly dependent on the lipid and protein composition of the membrane. 

Improved ISEs. In 1980, Ammann et al.154 reported on clinical and biological applications of liquid membrane electrodes based on an ion-selective component and a suitable plasticizer in a PVC matrix for the determination of Na +, K, Ca2, Cl and H + in blood serum or whole blood and Na+ and K4 in urine. They gave extensive information on membrane compositions and selectivity... 

Myelin in situ has a water content of about 40%. The dry mass of both CNS and PNS myelin is characterized by a high proportion of lipid (70-85%) and, consequently, a low proportion of protein (15-30%). By comparison, most biological membranes have a higher ratio of proteins to lipids. The currently accepted view of membrane structure is that of a lipid bilayer with integral membrane proteins embedded in the bilayer and other extrinsic proteins attached to one surface or the other by weaker linkages. Proteins and lipids are asymmetrically distributed in this bilayer, with only partial asymmetry of the lipids. The proposed molecular architecture of the layered membranes of compact myelin fits such a concept (Fig. 4-11). Models of compact myelin are based on data from electron microscopy, immunostaining, X-ray diffraction, surface probes studies, structural abnormalities in mutant mice, correlations between structure and composition in various species, and predictions of protein structure from sequencing information [4]. 

It is necessary to elaborate on yet another essential aspect of biological membranes, i.e. their complexity . This keyword points to the large number of different molecules that are usually found in the biological membrane. First of all, there is a large variety of lipid molecules. The lipid composition of the biological membranes varies from one species to another, and is adapted to meet the needs of organs, cells, organelles, etc. The variations in the head-tail... 

In many biological systems the biological membrane is a type of surface on which hydrophilic molecules can be attached. Then a microenvironment is created in which the ionic composition can be tuned in a controlled way. Such a fluffy polymer layer is sometimes called a slimy layer. Here we report on the first attempt to generate a realistic slimy layer around the bilayer. This is done by grafting a polyelectrolyte chain on the end of a PC lipid molecule. When doing so, it was found that the density in which one can pack such a polyelectrolyte layer depends on the size of the hydrophobic anchor. For this reason, we used stearoyl Ci8 tails. The results of such a calculation are given in Figure 26. 

From the information given above it is obvious that cell surfaces display an enormous complexity. A perfect model to study the interaction of a peptide with a biological membrane would require knowledge about the cell membrane composition in that particular tissue. Even if such information were available it will most probably not be possible to fully mimic the biological environment. However, some important aspects may still be studied with the available models. Whenever possible, one should try to relate the information derived from such a model to information gained from biological data taken on real cells (cell-lines) such as binding affinities etc. in order to prove the validity of the model for the study of a particular aspect. 

V. M. Mazhul, Y. S. Ermolaev, and C. V. Konev, Tryptophan phosphorescence at room temperature New method for the study of the structural composition of biological membranes and proteins in cells, Zh. Prikl. Spectrosk. 32, 903-907 (1980). 

Biological membranes consist of lipids, proteins, and carbohydrates (see p. 214). These components occur in varying proportions (left). Proteins usually account for the largest proportion, at around half. By contrast, carbohydrates, which are only found on the side facing away from the cytoplasm, make up only a few percent. An extreme composition is seen in myelin, the insulating material in nerve cells, three-quarters of which consists of lipids. By contrast, the inner mitochondrial membrane is characterized by a very low proportion of lipids and a particularly high proportion of proteins. 

B. Biologic membranes regulate the composition and the contents within the spaces they enclose. 

As was the case for composites, there is little new in the way of fundamental concepts for mass transport in biologies that has not already been presented. However, it is possible to briefly describe extensions of some previously introduced topics that are of particular importance to biological materials—namely, diffusion of nonspherical molecnles in solution, diffusion throngh biological membranes, and convective mass transfer in biological systems. 

---