<P> In prokaryotes endomembranes are rare, although in many photosynthetic bacteria the plasma membrane is highly folded and most of the cell cytoplasm is filled with layers of light - gathering membrane . These light - gathering membranes may even form enclosed structures called chlorosomes in green sulfur bacteria . </P> <P> The organelles of the endomembrane system are related through direct contact or by the transfer of membrane segments as vesicles . Despite these relationships, the various membranes are not identical in structure and function . The thickness, molecular composition, and metabolic behavior of a membrane are not fixed, they may be modified several times during the membrane's life . One unifying characteristic the membranes share is a lipid bilayer, with proteins attached to either side or traversing them . </P> <P> 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 . Sphingolipid biosynthesis begins in the endoplasmic reticulum, but is completed in the Golgi apparatus . The situation is similar in mammals, with the exception of the first few steps in ether lipid biosynthesis, which occur in peroxisomes . The various membranes that enclose the other subcellular organelles must therefore be constructed by transfer of lipids from these sites of synthesis . 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 . </P> <P> 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 . 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 . 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 acids and sterols, 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 . </P>

Lipid synthesis is a primary function of which organelle
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