Regulatory Functions Membrane Structure

The basic structure of biological membranes consists essentially of phospho-lipids, glycolipids, and cholesterol. Triglycerides are not found inside membranes.

Partially dependent on nutrition (see p. 112) and hormonal status, membrane lipid fatty acids vary in chain-length and number of double-bonds. The lipid composition varies between tissues, within each cell, and within individual cell organelles. For instance, the plasma membrane of nerve cells is rich in glycolipids, which are rarely found in erythro- and hepatocyte membranes. Mitochondrial membranes contain more phosphatidyl-ethanolamine and -choline than the plasma membrane. There are additional differences between inner and outer mitochondrial membranes. Apparently, the membrane-specific lipid composition is essential for differential membrane function.

The fluid-mosaic model of the membrane (A) best represents the functional condition of cell membranes. In the watery medium provided by cytoplasm and the interstitium, amphipathic lipids form a lipid bilayer -5 nm thick. The polar, hydrophilic component, consisting of glycerol, phosphate group, R-groups and/or sugars point to the "water side," while the nonpolar fatty acids, the lipophilic components, cling to each other. This way, a bilayer forms, which may be called "fluid." The lipid molecules can "float" within their respective layers, whereas their ability to switch between the two layers is limited. This behavior, termed "membrane fluidity," is strongly influenced by the fatty acid composition of the membrane lipids. Due to the character of the bond with glycerol, rotation is possible between the hydrophilic "head" and the lipophilic "tail." At the same time, the fatty acids are able to "dangle" from the glycerol. This possible movement is limited in long fatty acids, whereas shorter or unsaturated fatty acids with cisdouble-bonds increase fluidity. The cholesterol and protein components of the membrane further influence membrane fluidity.

Embedded in the bilayer are specific membrane proteins, each with a particular task. Integral proteins are the globulins, each of which may traverse the bilayer one or more times. They are asymmetrically distributed throughout the cell membrane. Peripheral proteins "float" on the membrane and are equipped with a lipid anchor or just loosely associated with a membrane component. Most receptor- and channel proteins are integral proteins. Glyco-proteins involved in cell-cell recognition tend to be peripheral proteins. It is not fully understood at this time how membrane lipids influence the function of membrane proteins. It is imaginable that a change in the fluidity of the phospholipid bilayer might affect the conformation of the embedded proteins and thereby alter their function.

Regulatory Functions I 109

- A. Membrane Fluidity a

Extracellular space a

Extracellular space

Peripheral Proteins

Peripheral membrane proteins

Integral membrane proteins

Cholesterol

Peripheral membrane proteins

Integral membrane proteins

Cholesterol

Cytoplasm

Rotation

Nitrogen atom Phosphorus atom cis-Double bond

Rotation

Nitrogen atom Phosphorus atom cis-Double bond

Membrane Proteins Functions

Dangling

Polar = Hydrophilic

Nonpolar=Lipophilic

Dangling b

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