Structure and functions of biological membranes

Dynamic Clustering Regulates Activity of Mechanosensitive Membrane Channels

Experiments have suggested that bacterial mechanosensitive channels separate into 2D clusters, the role of which is unclear. By developing a coarse-grained computer model we find that clustering promotes the channel closure, which is highly dependent on the channel concentration and membrane stress. This behaviour yields a tightly regulated gating system, whereby at high tensions channels gate individually, and at lower tensions the channels spontaneously aggregate and inactivate. We implement this positive feedback into the model for cell volume regulation, and find that the channel clustering protects the cell against excessive loss of cytoplasmic content.

The effect of POPC acyl chains packing by aromatic amino acid methyl esters investigated by ATR-FTIR combined with QM calculations

​The packing of POPC acyl chains can be influenced by aromatic amino acid methyl esters significantly, thus the HCCH motif is packed closed to the other one of an adjacent acyl chain with enhancement by dispersion interactions.

Mitochondrial phospholipid composition and microviscosity in myocardial ischaemia

Normothermic ischaemic arrest of the isolated perfused rat heart causes profound changes in mitochondrial ultrastructure. Since the mitochondrial membranes contain a high percentage of phospholipids, an evaluation of the effect of different periods of ischaemia on mitochondrial phospholipid content and fatty acid composition was made. The results showed that ischaemia had no effect on the content of the different phospholipid classes and no correlation was observed between ultrastructural changes and mitochondrial phospholipid content. However, the phospholipid fatty acid composition of several phospholipids showed marked changes. For example, with lysophosphatidylcholine a progressive increase in the percentage saturated fatty acids was observed with increasing periods of ischaemia, while a reduction occurred in lysophosphatidylethanolamine. To determine whether the ischaemia-induced changes in mitochondrial phospholipid fatty acid composition had an effect on the physical properties of the membrane, the microviscosity of mitochondrial preparations was studied, using the lipophilic probe, 1,6-diphenyl-1,3,5-hexatrine. Mitochondria isolated from ischaemic hearts showed a progressive increase in fluorescence polarization with longer periods of ischaemia, indicating an overall increase in microviscosity. This phenomenon may be responsible for the increased mitochondrial fragility which is characteristic of ischaemic damage.

Lipids in Viruses

This chapter discusses lipids in viruses. Lipid forms an integral part of many viruses and exists either in the form of a continuous envelope or in lipoprotein complexes that surround a nucleoprotein core or helix. In general, the envelope can be described as a molecular container for the genetic material of the virus. Viruses are obligate intracellular parasites and are not known to carry genetic coding for enzymes involved in lipid synthesis. Hence, they generally contain the same classes of lipid as are found in the host cell or their membrane of assembly. Lipids make up 20–35% by weight of most viruses; however, there are exceptions such as vaccinia virus, which has only 5% lipid despite having a complex multimembrane envelope structure. Naked herpesvirus capsids closely resemble non-lipid-containing viruses such as adenovirus or polyoma virus, which are also assembled in the nucleus but show full infectivity without any envelope. Both naked and enveloped herpesvirus particles are found in infected cells; however, only enveloped particles are found in extracellular fluids.

Symposium on the fine structure and replication of bacteria and their parts. II. Bacterial cytoplasm

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CELL MEMBRANE CONSTITUENTS CONCERNED WITH TRANSPORT MECHANISMS

The biological membrane is a multiphasic, polyionic, regionally differentiated structure, the constituents of which are closely linked to the physiological and metabolic processes of the entire cell. Knowledge of the types of molecules, their orientation, and the relative importance of them for transport processes is still very fragmentary. The information at present available on the composition of the protoplast membrane, the red cell membrane, and the neuronal membrane is brought together and discussed in terms of the possible role in transport processes. A labile phosphate attached to protein or specific phosphatides or shared between them as a lipo-phosphoprotein complex is suggested as the intermediate in the active transport of sodium. The rapid phosphorylation of these constituents by ATP through the activity of membrane ATPase and their subsequent dephosphorylation could lead to rhythmic transitions in the configuration of membrane proteins and control active cation transport.