Phospholipids

Phospholipids, also called phosphatides, are present in every cell of the human body and belong to the membrane lipid family. They form the main component of the lipid bilayer of a biomembrane, such as the cell membrane. In the myelin membrane of Schwann’s cells, which surround the axons of nerve cells, the phospholipid content is particularly high. It amounts to about 80 %. Phospholipids are amphipolar lipids, that is, they are composed of a hydrophilic head and two hydrophobic hydrocarbon tails. Phosphatides consist of fatty acids and phosphoric acids, which are esterified on the one hand with the alcohols glycerol or sphingosine and on the other hand with the nitrogen-containing active groups choline, ethanolamine, serine or inositol. The phosphoglycerides or glycerophospholipids, which have the trivalent alcohol glycerol as a building block, are the most widespread in nature.The most common phospholipids occurring in a cell membrane include:

  • Phosphatidyl choline – lecithin, PC.
  • Phosphatidyl Serine (PS)
  • Phosphatidylethanolamine (PE)
  • Sphingomyeline (SM)

Phosphatidyl-serine is found exclusively in the inner layer of the cell membrane – cytoplasmic side – while sphingomyelin is found mostly in the outer layer of the biomembrane – exoplasmic side.Phosphatidyl-choline and phosphatidylethanolamine are enriched in both membrane layers, but in different concentrations. PC is predominantly a component of the exoplasmic side, while PE belongs mainly to the cytoplasmic side of the cell membrane. The requirement of phospholipids is produced by the body itself or taken in through food and supplied to the body cells after self-synthesis. The content of phosphatides in an organism – including plants – is distributed differently. High phospholipid concentrations are found mainly in bone marrow (6.3 to 10.8%), brain (3.7 to 6.0%), liver (1.0 to 4.9%), and heart (1.2 to 3.4%).

Functions

Phospholipids exhibit different properties. This is partly due to their oppositely charged head groups – phospholipids in the outer membrane are positively charged, while those in the inner membrane are negatively charged or neutral – and partly due to the fatty acids.The amount and relative composition of fatty acids in phospholipids, which is dependent on dietary intake, is quite crucial. For example, a high proportion of polyunsaturated fatty acids, such as arachidonic acid (AA) and eicosapentaenoic acid (EPA), is important because AA and EPA give rise to important lipid mediators – prostaglandins PG2, PG3 – which are generated from phospholipases. Prostaglandins influence blood pressure, blood coagulation, lipoprotein metabolism, allergic and inflammatory processes, among others. Phospholipids confer certain general properties to cell membranes.Phospholipids, together with other membrane components, such as cholesterol, proteins and carbohydrates in the form of glycolipids and glycoproteins, are in permanent motion, resulting in biomembranes in a “liquid-crystalline” state. With more or less intensive movement of the membrane components, the degree of fluidity (flowability) varies. One of the determining factors is the lipid composition of the membrane. The more unsaturated fatty acids in the membrane, the more permeable it is to water. This increases the fluidity. This effect is due to the cis-double bonds of the unsaturated fatty acids, which cause the fatty acid tails to “kink” and thus disrupt the ordered “crystal structure” of the membrane. The phospholipid bilayer of the plasma membrane exerts a barrier function. This barrier is essential to prevent the cell’s constituents from mixing with the extracellular medium in an undirected manner. Consequently, the existence of the plasma membrane is important in preventing a breakdown of cellular metabolism and consequent cell death.