phospholipid question?

Question

Can anyone tell me why sphingomyelin is one of the main phospholipid components of animal plasma membranes but is not found in the membranes of mitochondria or chloroplasts.

I understand that the lipid content of mitochondria and chloroplasts is much lower than the lipid content of the plasma membrane but I can't find a reason as to why the lipid that is present does not include sphingomyelin.

Is there a functional difference between sphingomyelin and other phospholipids (ie phosphoglycerides)??


Any help would be appreciated, thanks!

Answer 1

Sphingomyelin is involved in signal transduction and the "sphingomyelin cycle". Basically, sphingomyelin is a precursor (almost a reserve you can say) for ceramide, and ceramide is a key point in MANY pathways, the most important being type 1 apoptotic signal transduction.

Answer 2

Sphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons. Like all sphingolipids, it consists of sphingosine bonded to one fatty acid and one polar head group, which, in sphingomyelins, is either phosphocholine or phosphoethanolamine. The fatty acid is attached to the second carbon of sphingosine via an amide linkage. The function of sphingomyelin remained unclear until recently when it was found to have a function in signal transduction. Sphingomyelin is only found in the exoplasmic face of the cell membrane.

Answer 3

This is purely speculation.

Both chloroplasts and mitochondria have their own DNA, and synthesize their own proteins. Based on this, and the high similarity of their genome to bacterial genomes, the endosymbiont theory was formulated that basically states that a likely origin for both organelles, was a symbiotic relation in some sort of protoeukaryotic cell.

That said, I would check and see if any prokaryotes produce sphingomyelin or not. You could also search the mitochondrial and chloropast genomes to see if they are even capable of synthesizing sphingomyelin.