In addition to fats and oils, phospholipids are also prevalent and important lipids that abundant in biological membrane systems. They make up half of cell and organelle membrane. They’re categorized into glycerophospholipid or phosphoglyceride and sphingomyelin.
Structure of Phospholipids
Glycerophospholipid, phosphoglycerides
The glycerophospholipids synthesized in smooth endoplasmic reticulum is a major building block in cell membranes. Their molecule contains a glycerol backbone, fatty acids, phosphate and a hydrophilic organic group. Although they’re similar to fat molecules, there’re some differences. In fat, the fatty acids esterify all three hydroxyl groups in glycerol. Whereas in phospholipids, the hydroxyl in third carbon is esterified by phosphate where various hydrophilic molecules attach to form different phospholipids including phosphatidylcholine (PC) and phosphatidylethanolamine (PE).
Their fatty acids have an even carbon number that ranges from 16 to 18 usually. Sometimes 14 or 20 carbon atoms are also available. The saturated fatty acid is located in first carbon of glycerol, and the second carbon is attached by an unsaturated fatty acid. The cis-configuration with an 30° bend is most common in unsaturated fatty acids. Occasionally, a few trans fatty acids will be found after consuming artificial fats. The straight hydrophobic tails make them similar to saturated fat, so cell membrane fluidity and permeability is reduced significantly.
Sphingomyelin
Sphingomyelins are another type of phospholipid that are rich in neurons and myelin sheaths. Their synthesis is done by smooth endoplasmic reticulum and Golgi apparatus. Sphingosine backbone, fatty acids, phosphate and hydrophilic organic group make up their molecules. Phosphate esterifies hydroxyl while amino group is acylated by fatty acids. One hydrophobic tail comes from fatty acid and the other with a double-bond is part of sphingosine backbone. Fatty acids with14-26 carbons are longer than those found in glycerophospholipids. They can be saturated or unsaturated. The first carbon is linked by phosphate group that choline attaches to.
Physical, Chemical Properties
They dissolve in organic solvents such as ethanol, benzene, ether, and chloroform but insoluble in methyl acetate and acetone. Pure phospholipids appear as white soft, waxy solids or powders at room temperature. Sometimes the impurities give them a pale yellow or light brown color. Being lighter than water causes them to float on solution surface. The unstable double-bond in glycerophospholipids or sphingomyelins make them prone to oxidation when exposed to sunlight or air. High extraction temperatures also degrade phospholipids and darken their color. To make them more stable, hydrogen gas is used to convert them into saturated fatty acids with the help of catalyst. Sometimes, sulfonation is applied to increase their polarity and water solubility.
Glycerophospholipids have three types of ester bonds situated in fatty acid, phosphate, and organic base. These bonds can be hydrolyzed by acids, bases or phospholipases (A1, A2, B, C, D). Each enzyme can only cleave specific ester bonds. Lipases also hydrolyze phospholipids where they cleave fatty acid chains. Meanwhile, it’s similar in sphingomyelin.
Biological Functions
The hydrophilic and hydrophobic properties exhibit unique behaviors in aqueous environments. The flat monolayers, flat bilayers, single-layered spheres, and double-layered spheres are all the structures formed by phospholipid in water.
When phospholipids are gently added into an aqueous medium, they form a monolayer that spreads over surface. Hydrophilic heads orient towards water, while hydrophobic tails extend away from water surface. If there’re supports in the aqueous solution, they will gather in the supports gaps to form phospholipid bilayer planes.
If you stir water, they spontaneously form stable bilayer vesicles. Hydrophilic heads are located in the outer and inner surfaces of bilayer. Hydrocarbon tails are positioned inside the bilayer and away from water. Monolayer vesicles are rare in water because phospholipids have two hydrophobic tails. If one tail is hydrolyzed by enzyme, the monolayer vesicles appear.
Cell membranes are composed of phospholipid bilayers where the glycerophospholipids are majority. The remainder are glycolipids and sphingolipids. These bilayers separate intracellular environment from extracellular space and regulate substances passage. One unsaturated fatty acid in the tail creates a kink that prevents them from being tightly bound, so the phospholipid bilayer has excellent fluidity. Their head size affects membrane curvature: phosphatidylcholine (PC) is located in flat regions and phosphatidylethanolamine (PE) is found in curved areas.
The longer fatty acids give sphingomyelin stronger intermolecular forces, so sphingomyelin rich cell membranes are more stable and more hydrophobic, especially in nerve cell and skin cutin. Sphingomyelin is also abundant near membrane proteins where they’re arranged tightly and orderly like rafts.