Lipid bilayer of biologic membranes

The lipid bilayer of biological membranes, as discussed in Chapter 12. is intrinsically impermeable to ions and polar molecules. Permeability is conferred by two classes of membrane xoXems, pumps and channels. Pumps use a source of free energy such as ATP or light to drive the thermodynamically uphill transport of ions or molecules. Pump action is an example of active transport. Channels, in contrast, enable ions to flow rapidly through membranes in a downhill direction. Channel action illustrates passive transport, or facilitated diffusion. 

Lipophilicity is a measure of a chemical s affinity for the lipid bilayer of biological membranes. The logarithm of the partition coefficient between water and 1-octanol (log Kow) is used as an indicator of a chemical s lipophilicity. The parabolic relationship between log P and effect can be used as evidence that there are limits to absorption for super-lipophilic compounds , and why these limits exist (20). Chemicals that have log P values greater than 6 tend to dissolve in the non-polar interior of a membrane inhibiting transport. 

What experimental observation shows that proteins diffuse within the lipid bilayers of biological membranes ... 

Although for the moment this model is only partially supported by experimental data it offers the opportunity to design new experiments which will help to understand the mechanisms of pardaxin insertion and pore formation in lipid bilayers and biological membranes which at a molecular level are the events leading to shark repellency and toxicity of this marine toxin. 

It has been known for some years that gramicidin forms transmembrane ion channels in lipid bilayers and biological membranes and that these channels are assembled from two molecules of the polypeptide 213). The channels are permeable specifically to small monovalent cations [such as H+, Na+, K+, Rb+, Cs+, Tl+, NH4+, CHjNHj, but not (CH3)2NH2+J and small neutral molecules (such as water, but not urea). They do not allow passage of anions or multivalent cations 21 n. 

Table 4. Major building blocks of lipid bilayers in biological membranes and their speciation and acidity constants...

As stated, biological membranes are normally arranged as bilayers. It has, however, been observed that some lipid components of biological membranes spontaneously form non-lamellar phases, including the inverted hexagonal form (Figure 1.9) and cubic phases [101]. The tendency to form such non-lamellar phases is influenced by the type of phospholipid as well as by inserted proteins and peptides. An example of this is the formation of non-lamellar inverted phases by the polypeptide antibiotic Nisin in unsaturated phosphatidylethanolamines [102]. Non-lamellar inverted phase formation can affect the stability of membranes, pore formation, and fusion processes. So-called lipid polymorphism and protein-lipid interactions have been discussed in detail by Epand [103]. 

The characteristics of the water associated with the polar lipid bilayer surfaces are of particular interest because they become very important in processes of physiological significance such as membrane fusion, and they may play a role in the mechanisms of association of proteins and small molecules with lipid bilayers and biological membranes. A very small fraction of the lipid bilayer-associated water molecules are actually immobilized, and a larger fraction (about 30% of the total bilayer-associated water in multilamellar PC bilayers in the L phase) has a probability distribution function that is more or less coincident with the probability distribution function for the polar head group of the lipids. Nevertheless, the pressure, P, which must be exerted to remove the bilayer-associated water, is large and varies (from 0.5-500 N cm ) with the thickness of the inter-bilayer water space as ... 

What is the structure of lipid bilayers A biological membrane consists of a lipid part and a protein part. The lipid part is a bilayer, with the polar head groups in contact with the aqueous interior and exterior of the cell, and the nonpolar portions of the lipid in the interior of the membrane. 

In the recent past, there have been a number of reports on self-assemblies of molecules as advanced materials or smart materials . Without question, the inspiration for this exciting work comes from the biological world, where, e.g., the lipid bilayer of cell membranes plays a pivotal role. In this cormection it should be stated that many other researchers have also described self-assembling systems such as the liposome. Liposomes are modeled after biomembranes, which have been extensively investigated since the late 1960s (see Table 1 for references). 

---