STRUCTURE OF CELL MEMBRANES


In 1972, S. Jonathan Singer and Garth Nicolson developed the fluid-mosaic model of membrane structure. According to this model, a membrane is a double layer (bilayer) of proteins and phospholipids, and is fluid rather than solid. The phospholipid bilayer forms a fluid “sea” in which specific proteins float like icebergs. Being fluid, the membrane is in a constant state of flux—shifting and changing, while retaining its uniform structure. The word mosaic refers to the many different kinds of proteins dispersed in the phospholipid bilayer.

The following are important points of the fluid-mosaic
model:
1. The phospholipids have one polar end and one nonpolar
end. The polar ends are oriented on one side toward the outside
of the cell and into the fluid cytoplasm on the other
side, and the nonpolar ends face each other in the middle of
the bilayer. The “tails” of both layers of phospholipid molecules
attract each other and are repelled by water (they are
hydrophobic, “water dreading”). As a result, the polar spherical
“heads” (the phosphate portion) are located over the
cell surfaces (outer and inner) and are “water attracting”
(they are hydrophilic).
2. Cholesterol is present in the plasma membrane and organelle
membranes of eukaryotic cells. The cholesterol molecules
are embedded in the interior of the membrane and
help to make the membrane less permeable to water-soluble
substances. In addition, the relatively rigid structure of the
cholesterol molecules helps to stabilize the membrane.
3. The membrane proteins are individual molecules attached
to the inner or outer membrane surface (peripheral proteins)
or embedded in it (intrinsic proteins). Some
intrinsic proteins are links to sugar-protein markers on the
cell surface. Other intrinsic proteins help to move ions or
molecules across the membrane, and still others attach the
membrane to the cell’s inner scaffolding (the cytoskeleton)
or to various molecules outside the cell.
4. When carbohydrates unite with proteins, they form glycoproteins,
and when they unite with lipids, they form glycolipids
on the surface of a plasma membrane. Surface
carbohydrates and portions of the proteins and lipids make
up the glycocalyx (“cell coat”). The complexly
arranged and distinctively shaped groups of sugar molecules
of the glycocalyx act as a molecular “fingerprint” for each
cell type. The glycocalyx is necessary for cell-to-cell recognition
and the behavior of certain cells, and is a key component
in coordinating cell behavior in animals.

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