Transport systems membrane dynamics

Some transport proteins merely provide a path for the transported species, whereas others couple an enzymatic reaction with the transport event. In all cases, transport behavior depends on the interactions of the transport protein not only with solvent water but with the lipid milieu of the membrane as well. The dynamic and asymmetric nature of the membrane and its components (Chapter 9) plays an important part in the function of these transport systems. 

Lehninger, A. L. Metabolite carriers in mitochondrial membranes. In The Ca++ transport system in the dynamic structure of cell membranes, pp. 119. Wallach, D. F. H., Fisher, H. (eds.). New York, Heidelberg, Berlin Springer 1971... 

Another aspect to take into account is that all microorganisms need to maintain adequate fluidity in their membranes (Rodriguez et al. 2007). Excessive rigidity can prevent cellular transport systems from functioning correctly (Los and Murata 2004).In contrast, excessive fluidity can alter the organization and the dynamic properties of phospholipidic bilayer (Laroche et al. 2001). 

Membranes are dynamic structures in which proteins float in a sea of lipids. The lipid components of the membrane form the permeability barrier, and protein components act as a transport system of pumps and channels that endow the membrane with selective permeability. 

The other chapters then lead from the simple to the more complex molecular assemblies. Syntheses of simple synkinons are described at first. Micelles made of 10-100 molecules follow in chapter three. It is attempted to show how structurally ill-defined assemblies can be most useful to isolate single and pairs of molecules and that micelles may produce very dynamic reaction systems. A short introduction to covalent micelles, which actually are out of the scope of this book, as well as the discussion of rigid amphiphiles indicate where molecular assembly chemistry should aim at, namely the synkinesis of solid spherical assemblies. Chapter four dealing with vesicles concentrates on asymmetric monolayer membranes and the perforation of membranes with pores and transport systems. The regioselective dissolution of porphyrins and steroids, and some polymerization and photo reactions within vesicle membranes are also described in order to characterize dynamic assemblies. 

Fig. 2 The red blood cell has played a special role in the development of mathematical models of metabolism given its relative simplicity and the detailed knowledge about its molecular components. The model comprises 44 enzymatic reactions and membrane transport systems and 34 metabolites and ions. The model includes glycolysis, the Rapaport-Leubering shunt, the pentose phosphate pathway, nucleotide metabolism reactions, the sodium/potassium pump, and other membrane transport processes. Analysis of the dynamic model using phase planes, temporal decomposition, and statistical analysis shows that hRBC metabolism is characterized by the formation of pseudoequilibrium concentration states pools or aggregates of concentration variables. (From Ref...

As mentioned above, membrane transport is a dynamic, nonequilibrium process. The transported compound has to dissolve in the organic, hydro-phobic membrane phase and diffuse through it to enter the aqueous stripping phase. The mass transfer in this system takes place due to the difference in the chemical potential across the membrane as a driving force. The variation of chemical potential of component i can be expressed as... 

The outer membrane of the cell is an organelle, richly endowed with receptors which recognise and react to signalling molecules from the environment, endowed with enzymes for the degradation and synthesis of nutrients within the cell and external to it, and bearing transport systems which control the entrance and egress of specific metabolites. Subsequent chapters of this series will deal with all aspects of this dynamic commerce of the cell membrane as mediated by these membrane proteins. The present chapter confines itself, however, to those properties of the cell membrane which arise from the lipid baekbone or substructure into which these more dynamic proteins are embedded. For a nutrient or any foreign molecule which finds no specific membrane component with which to interact, the lipid bilayer of the membrane provides the barrier which determines whether the molecule in question can cross the membrane. How then does this cell membrane matrix discriminate between possible permeants This question is the theme of the present chapter. 

The area of membrane transport has always been an interdisciplinary field. Physiologists, biochemists, biophysicists, cell biologists and pharmacologists have all made their contributions to the development of our knowledge in this field, often in collaborative studies. The appearance of this book in the series New Comprehensive Biochemistry is justified perhaps more by the future contributions to be expected from fundamental biochemistry than by the contributions made by biochemistry so far. Our biochemical understanding of the molecular structure and dynamics of the various transport systems is still in a primitive state compared to that for biomolecules like nucleic acids and water-soluble proteins. The editors hope that the publication of this volume may arouse the interest of many biochemists, especially the younger ones, for this field of biochemistry and thus contribute to its development. 

Carrier-mediated transport across membranes adds additional complexity to the system and, thus, to the model. For even the simplest transporter, the concentration of the transporter and its affinity for the substrate must be known before it can be modeled. Also, active transport is inherently a saturable process. Thus, to analyze the dynamics of tracer-labeled substrate, the model must account for both labeled and unlabeled substrate as the transport dynamics will depend on total substrate concentration. 

Studies of Na+ in cellular systems have been performed on cells such as superfused isolated rat cardiomyocytes, Methanobacterium thermoauto-trophicum, porcine vascular endothelial cells, the halotolerant bacterium Brevibacterium sp., Escherichia coli, murine TM3 Ley dig and TM4 Sertoli cell lines, and mouse 3T3 fibroblasts. These experiments have been employed to determine the NMR visibility of Na% its intracellular concentration, membrane transport properties and dynamics and its ionic mobility inside the cells, NMR studies have contributed to the study of Na/K/ ATPase. 

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