Phosphatidylethanolamine (cephalin, sometimes abbreviated PE) is a lipid

Lipid

Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

 found in biological membranes. It is synthesized by the addition of CDP

Cytidine diphosphate

Cytidine diphosphate, abbreviated CDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside cytidine. CDP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase cytosine....

-ethanolamine

Ethanolamine

Ethanolamine, also called 2-aminoethanol or monoethanolamine , is an organic chemical compound that is both a primary amine and a primary alcohol . Like other amines, monoethanolamine acts as a weak base...

 to diglyceride

Diglyceride

A diglyceride, or a diacylglycerol , is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages....

, releasing CMP

Cytidine monophosphate

Cytidine monophosphate, also known as 5'-cytidylic acid or simply cytidylate, and abbreviated CMP, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid with the nucleoside cytidine...

. S-adenosyl methionine

S-Adenosyl methionine

S-Adenosyl methionine is a common cosubstrate involved in methyl group transfers. SAM was first discovered in Italy by G. L. Cantoni in 1952. It is made from adenosine triphosphate and methionine by methionine adenosyltransferase . Transmethylation, transsulfuration, and aminopropylation are the...

 can subsequently methylate the amine of phosphatidyl ethanolamine to yield phosphatidyl choline.

Cephalin is a phospholipid

Phospholipid

Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...

, which is a lipid derivative. It is not to be confused with the molecule of the same name that is an alkaloid

Alkaloid

Alkaloids are a group of naturally occurring chemical compounds that contain mostly basic nitrogen atoms. This group also includes some related compounds with neutral and even weakly acidic properties. Also some synthetic compounds of similar structure are attributed to alkaloids...

 constituent of Ipecac.

Function

Cephalin is found in all living cells, although in human physiology it is found particularly in nervous tissue such as the white matter

White matter

White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...

 of brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

, nerves, neural tissue, and in spinal cord

Spinal cord

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

. Whereas lecithin

Lecithin

Lecithin is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids .The word lecithin was originally coined in 1847 by...

 is the principal phospholipid in animals, cephalin is the principal one in bacteria.

As a polar head group, phosphatidylethanolamine (PE) creates a more viscous lipid membrane compared to phosphatidylcholine (PC). For example, the melting temperature of di-oleoyl-PE is -16C while the melting temperature of di-oleoyl-PC is -20C. If the lipids had two palmitoyl chains, PE would melt at 63C while PC would melt already at 41C (See references in Wan et al. Biochemistry 47 2008). Lower melting temperatures correspond, in a simplistic view, to more fluid membranes.

Chemistry

In the chemical sense, cephalin is phosphatidylethanolamine. Like lecithin, it consists of a combination of glycerol

Glycerol

Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

 esterified with two fatty acids and phosphoric acid

Phosphoric acid

Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...

. Whereas the phosphate group is combined with choline

Choline

Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....

 in Lecithin, it is combined with the ethanolamine

Ethanolamine

Ethanolamine, also called 2-aminoethanol or monoethanolamine , is an organic chemical compound that is both a primary amine and a primary alcohol . Like other amines, monoethanolamine acts as a weak base...

in Cephalin.

The two fatty acids may be the same, or different, and are usually in the 1,2 positions (though can be in the 1,3 positions).