asymmetrical in structure, with an outside part that is water- loving and attracts water and an inside part that is water-hat- ing and repels water.


Cellular membranes are con- structed as a fluid matrix of two layers or a bilayer of phospho- lipids along with intercalated proteins and glycoproteins with the water-hating portions of their structures inside the membrane and the water-loving por- tions facing outside on each side of the membrane.


attract other cells and extracel- lular matrix that exists between different tissues.


Some of the membrane protein and glycoprotein components are integral to the membrane and fit directly into the lipid bilayer, whereas others are pe- ripheral to the membrane and are attached to one side or the other of the membrane. Some of these peripheral membrane


teins and complexes of these can move rapidly and laterally within membranes, if they are untethered by other structures associated with the membrane or are not in large multi-com- ponent complexes. Fluctuation or motion is the essence of this model. This fluctuation allows maximal responsiveness to the inner and outer (extracellular) environmental changes.


The membrane takes on an ever-changing or dynamic mosaic form, as if made of “floating spars or buoys” some of which are bound together in complexes and others that are not joined, and have gaps between them along the plane of the mem- brane. This mosaic of floating spars is made of proteins and glycoproteins that bob and move laterally in the fluid lipid bilayer. In some cases the lip- ids also get together to form lipid islands that can separate membranes into different flu- idity zones. Carbohydrates, mostly on the outer surface of the membrane, are bound to proteins and certain lipids, and function as types of mark- ers of each cell, and this helps cells distinguish one cell from another. They also function as cell barriers and can repel or


8 Focus August 2012


Now, with new understanding of the precise composition, location and function of the lipids, proteins and sugars that compose the membrane, we can use dietary methods to provide the essential molecules that rejuvenate and repair biological membranes.


components at the inner sur- face of the membrane are also attached to other structures in- side the cell.


Finally, some of the integral membrane proteins and glyco- proteins span the entire lipid bi- layer and have portions of their structures on the inner side of the membrane and portions on the outer side of the mem- brane. These structures, which are often composed of multiple protein and glycoprotein com- ponents are often involved in trans-membrane ion and other molecular transport and also enzymatic functions.


One of the most important dis- coveries in this model is the dynamic change or motion of the components. Lipids, pro-


Another key finding of this model is that the two layers of lipids and the proteins and glycopro- teins have asymmetri- cal properties: one part repels water, the other attracts water. This al- lows the membrane to remain stable yet dy- namic and also allow it


to form an electrical and ionic barrier. Finally, the model pro- poses that sugar molecules embedded in the outer layer glycoproteins and some gly- colipids partially “mark” and “identify” the cell itself.


Some key phospholipids that compose the lipid bilayer in- clude: phosphatidyl choline (the most prevalent), phos- phatidyl serine, phosphatidyl inositol, phosphatidyl etha- nolamine and phosphatidyl glycerol (which is the precur- sor for cardiolipin, a crucial molecule in the inner mito- chondrial membrane). In addi- tion to phospholipids, glyco- lipids and cholesterol are also found in the membrane, and play key roles.


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