Editing Endomembrane system (section)

== History of the concept ==
Most lipids are synthesized in yeast either in the endoplasmic reticulum, lipid particles, or the mitochondrion, with little or no lipid synthesis occurring in the plasma membrane or nuclear membrane.<ref>{{cite journal | vauthors = Zinser E, Sperka-Gottlieb CD, Fasch EV, Kohlwein SD, Paltauf F, Daum G | title = Phospholipid synthesis and lipid composition of subcellular membranes in the unicellular eukaryote Saccharomyces cerevisiae | journal = Journal of Bacteriology | volume = 173 | issue = 6 | pages = 2026–34 | date = March 1991 | pmid = 2002005 | pmc = 207737 | doi = 10.1128/jb.173.6.2026-2034.1991 }}</ref><ref>{{cite journal | vauthors = Czabany T, Athenstaedt K, Daum G | title = Synthesis, storage and degradation of neutral lipids in yeast | journal = Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids | volume = 1771 | issue = 3 | pages = 299–309 | date = March 2007 | pmid = 16916618 | doi = 10.1016/j.bbalip.2006.07.001 }}</ref> [[Sphingolipid]] biosynthesis begins in the endoplasmic reticulum, but is completed in the Golgi apparatus.<ref>{{cite journal | vauthors = Futerman AH | title = Intracellular trafficking of sphingolipids: relationship to biosynthesis | journal = Biochimica et Biophysica Acta (BBA) - Biomembranes | volume = 1758 | issue = 12 | pages = 1885–92 | date = December 2006 | pmid = 16996025 | doi = 10.1016/j.bbamem.2006.08.004 | doi-access = free }}</ref> The situation is similar in mammals, with the exception of the first few steps in [[ether lipid]] biosynthesis, which occur in peroxisomes.<ref>{{cite journal | vauthors = Wanders RJ, Waterham HR | title = Biochemistry of mammalian peroxisomes revisited | journal = Annual Review of Biochemistry | volume = 75 | pages = 295–332 | year = 2006 | pmid = 16756494 | doi = 10.1146/annurev.biochem.74.082803.133329 }}</ref> The various membranes that enclose the other subcellular organelles must therefore be constructed by transfer of lipids from these sites of synthesis.<ref name=Voelker>{{cite journal | vauthors = Voelker DR | title = Organelle biogenesis and intracellular lipid transport in eukaryotes | journal = Microbiological Reviews | volume = 55 | issue = 4 | pages = 543–60 | date = December 1991 | doi = 10.1128/MMBR.55.4.543-560.1991 | pmid = 1779926 | pmc = 372837 }}</ref> However, although it is clear that lipid transport is a central process in organelle biogenesis, the mechanisms by which lipids are transported through cells remain poorly understood.<ref>{{cite journal | vauthors = Voelker DR | title = Bridging gaps in phospholipid transport | journal = Trends in Biochemical Sciences | volume = 30 | issue = 7 | pages = 396–404 | date = July 2005 | pmid = 15951180 | doi = 10.1016/j.tibs.2005.05.008 }}</ref>

The first proposal that the membranes within cells form a single system that exchanges material between its components was by Morré and Mollenhauer in 1974.<ref>{{cite book | vauthors = Morré DJ, Mollenhauer HH | date = 1974 | chapter = The endomembrane concept: a functional integration of endoplasmic reticulum and Golgi apparatus. | title = Dynamic Aspects of Plant infrastructure | veditors = Robards AW | pages = 84–137 | location = London, New York | publisher = McGraw-Hill}}</ref> This proposal was made as a way of explaining how the various lipid membranes are assembled in the cell, with these membranes being assembled through ''lipid flow'' from the sites of lipid synthesis.<ref name=Morre1975>{{Cite journal | vauthors = Morre DJ | year = 1975 | title = Membrane Biogenesis | journal = Annual Review of Plant Physiology | volume = 26 | issue = 1 | pages = 441–481 | doi = 10.1146/annurev.pp.26.060175.002301}}</ref> The idea of lipid flow through a continuous system of membranes and vesicles was an alternative to the various membranes being independent entities that are formed from transport of free lipid components, such as [[fatty acid]]s and [[sterol]]s, through the cytosol. Importantly, the transport of lipids through the cytosol and lipid flow through a continuous endomembrane system are not mutually exclusive processes and both may occur in cells.<ref name=Voelker/>