Phospholipids polar region

Current theories propose that the development of cell membranes accelerated early life. Enclosing chemicals responsible for replication, protein synthesis and energy generation within a confined space allows these essential life processes to occur far more frequently than they would do otherwise. Biological membranes comprise two phospholipid layers held together by interdigitation of their hydrophobic alkyl tails as shown in Fig. 5.1. They form a lamellar structure with their polar internal and external surfaces separated by a non-polar region. In their extended form each... 

Phospholipids are the major lipid building blocks most membranes and their molecules comprise of a hydrophobic (acyl chain) and a hydrophilic (polar) head group. The relative size of the hydrophobic tails and hydrophilic head of the molecule characterizes the molecular shape and determines the structure of the molecular assemblies in contact with w ater. Molecules with polar and non-polar regions (PC, PS, PI, Sphm) of equal size have a cylindrical shape and form lipid bilayers. Molecules that have a larger non-polar region are cone-shaped (PE, PA, Choi, Car), and form reversed micelles, in contact to water. When the polar region is larger (lysophospholipids) the molecule assembles an inverted cone and form micelles. Fig. (8). 

Macromolecular lipid assemblies arise from the amphiphilic character of phospholipids (Figure 1.89). Broadly speaking, all phospholipids contain a compact polar region, which is... 

The plasma membranes of cells, mentioned above, are corrstructed of phospholipids. Phospholipids all have a structure that closely resembles the structure of the soaps and detergent surfactants discussed above in that the cortstituerrt molecules have an amphiphilic nature. This rrature arises from the presence of both polar and non-polar regions within the same molecttle. The polar region is hydrophilic (lipophobic) and the non-polar region is hydrophobic (lipophilic). 

Getting stractural information on the molecular supports of electropermeabilization was not easy. A key property of biological membranes is their dynamics. At a substructural level, 3IP NMR spectroscopy showed that a tilt of the orientation of the phospholipid polar head region was present in the electropermeabilized state of the membrane [45, 46]. The consequence of the interfacial water organization was proposed to be associated with a decrease of the hydration forces and the observed fusogenic state of electropermeabilized surfaces. At a more collective level, phospholipid flip-flop between the two faces of the plasma membrane was observed in the case of electropermeabilized erythrocytes [47]. 

Figure 4.46 Lipid bilayer model of LDL structure. From Lewis [313]. Lewis describes the model in this way A protein network is envisaged, with icosahedral symmetry it is suggested that there are 60 such units, existing as trimers. Instead of the conventional view of a protein>coated molecule with a lipid core, it was proposed that the lipids are org-aized into a spherical bilayer. The two lipid layers are mirror images, the non-polar regions of their main constituents, phospholipid and cholesteryl ester, oriented towards each other. At the outer surface of the outer layer and the inner surface of the inner layer are situated the polar groups of the phospholipids and the cholesterol side chains these regions of the major constituents are thus adjacent to the protein units on the surface and to a presumptive protein component at the centre of the particle. ...

These amphiphilic molecules readily interact with water and form various semi-enclosed environments. One of the best examples are phospholipids, the predominant constituents of the plasma membrane, which encapsulate and protect the cellular contents from the environment and are an absolute prerequisite for almost all living systems. Phospholipids readily undergo self-assembly in aqueous solution to form distinct structures that include micelles, vesicles and tubules. This is largely a result of the hydrophobic forces that drive the non-polar region of each molecule away from water and toward one another. 

Our laboratory has designed a simple peptide system with those properties [52,53]. We made short peptides of around six to seven amino acids that had the properties of surfactant molecules in that each monomer contained a polar and a non-polar region. For example, a peptide called A6D, the peptide molecule looked like a phospholipid in that it had a polar head group and a non-polar tail. 

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