Lipoprotein membrane barrier

The membrane is also responsible for helping maintain the correct internal ion concentration so that such variables as pH, electric potential (the different concentrations and charges on ions inside and outside the cell) and osmotic pressure remain constant, often against wildly different external ionic concentrations that would tend to unbalance the internal environment if it were not for the membrane barrier. Research into membrane properties is today one of the most active areas in biochemistry. However, despite this activity, knowledge of the precise mechanisms of selective permeability is still very poor. It has been known for some time that the membrane is predominantly lipoprotein in nature. We have already seen how lipid molecules, because of their hydrophobic nature, spontaneously organize themselves into a double layer or sandwich when placed in a watery environ-... 

The lipoprotein membranes of the cell act as diffusion barriers they have a high resistance to passive penetration by ions. How then can we explain the rapid active movement of ions across such membranes One explanation is to postulate that the ions undergo reversible binding with some constituent of the membrane (this constituent is usually called the carrier). The ion is then visualised to pass across the thickness of the membrane, not as a free ion but as an ion-carrier complex (Fig. 7.3). This concept has the immediate attraction that it suggests an explanation of selectivity selective absorption would reflect the abundance in the membrane and chemical affinities of the carrier molecules. Ions which compete with one another would be ions capable of combination with the same carrier but the affinities of the carrier for the separate ions of the group... 

Another pathway of some importance occurs in the brain this is the cholesterol 24-hydroxylase pathway. About 25% of the body s cholesterol exists in the plasma membranes of myelin sheaths. Here, the blood-brain barrier prevents cholesterol exchanges with the circulating lipoproteins, which makes it difficult for cholesterol to leave the brain. The cytochrome P-450 enzymes (CYP 46), expressed almost exclusively in the endoplasmic reticula of the brain, allows formation of 24-hydroxycholesterol. 

Gram-negative bacteria have a more complex cell surface. The peptidoglycan layer is also the outer layer with respect to the cytoplasmic membrane. However, besides this, they have another outer polysaccharide membrane. This outer membrane is built out of lipopolysac-charides and lipoproteins, and can be a serious barrier for permeating hydrophilic molecules. 

The cells of Gram-positive bacteria have an outer covering or membrane containing teichoic acids, whereas the walls of Gram-negative bacteria are covered with a smooth, soft lipopolysaccharide which also contains phospholipids, lipoproteins, and proteins. This layer acts a barrier and penicillins have to negotiate a limited number of protein channels in order to reach the cell. 

Neurons rely upon a ready supply of cholesterol for maintaining a broad array of physiological functions such as membrane synthesis, myeUn maintenance, electrical signal transduction, synaptic transmission, and plasticity. Cholesterol metabolism in the CNS is unique compared with the rest of the body. Because of the existence of the blood-brain barrier (BBB), almost all the sterol required for new membranes comes from de novo synthesis within the CNS [33]. In addition, the brain has evolved highly efficient mechanisms to maximize the utihzation of cholesterol. UnUke other membrane lipid components, cholesterol cannot be synthesized at neuronal terminals. Therefore, synaptic function depends largely on cholesterol supplied from either axonal transport from the cell body and or uptake of Upidated ApoE produced by astroglia via neuronal lipoprotein receptors. 

No unique structural model for the plasma membrane can yet be deduced from these data (Korn, 1967) but it is now possible to consider that lipoprotein molecules may be the fundamental structural, as well as functional, components of membranes. Individual protein molecules could easily extend through the 75-100 A width of the plasma membrane. This implies that the carrier molecule and the structural molecule may be identical and that transport across the plasma membrane might be conceived as a conformational change of the membrane substructure. The membrane need no longer be viewed primarily as an inert barrier but rather as a dynamic aggregate of functional polymers (Korn, 1967). 

Lipids, lipoproteins, and proteins makeup cytoplasmic membranes. This membrane is a diffusion barrier for ions, nutrients, transport systems, and most importantly, water. It is composed of a lipid grouping with globular proteins that penetrate the lipid bilayer. Most antibacterial agents that inhibit cytoplasmic membranes do so by influencing the balance of cations, anions, or neutral compounds, thus disrupting membrane operation. Of interest is that fungal... 

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