Editing Lipid (section)

===Biosynthesis===
In animals, when there is an oversupply of dietary carbohydrate, the excess carbohydrate is converted to triglycerides. This involves the synthesis of fatty acids from [[acetyl-CoA]] and the [[esterification]] of fatty acids in the production of triglycerides, a process called [[lipogenesis]].<ref name = "Stryer_2007" />{{rp|634}} Fatty acids are made by [[fatty acid synthase]]s that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the acetyl group, reduce it to an alcohol, [[dehydration reaction|dehydrate]] it to an [[alkene]] group and then reduce it again to an [[alkane]] group. The enzymes of fatty acid biosynthesis are divided into two groups, in animals and fungi all these fatty acid synthase reactions are carried out by a single multifunctional protein,<ref name="Chirala_2004"/> while in plant [[plastid]]s and bacteria separate enzymes perform each step in the pathway.<ref name="White_2005"/><ref name="Ohlrogge_1997"/> The fatty acids may be subsequently converted to triglycerides that are packaged in [[lipoproteins]] and secreted from the liver.

The synthesis of [[unsaturated fatty acid]]s involves a [[desaturase|desaturation]] reaction, whereby a double bond is introduced into the fatty acyl chain. For example, in humans, the desaturation of [[stearic acid]] by [[stearoyl-CoA desaturase-1]] produces [[oleic acid]]. The doubly unsaturated fatty acid [[linoleic acid]] as well as the triply unsaturated [[alpha-Linolenic acid|α-linolenic acid]] cannot be synthesized in mammalian tissues, and are therefore [[essential fatty acid]]s and must be obtained from the diet.<ref name = "Stryer_2007" />{{rp|643}}

Triglyceride synthesis takes place in the [[endoplasmic reticulum]] by metabolic pathways in which acyl groups in fatty acyl-CoAs are transferred to the hydroxyl groups of glycerol-3-phosphate and diacylglycerol.<ref name = "Stryer_2007" />{{rp|733–9}}

[[Terpene]]s and [[terpenoid|isoprenoids]], including the [[carotenoid]]s, are made by the assembly and modification of isoprene units donated from the reactive precursors [[isopentenyl pyrophosphate]] and [[dimethylallyl pyrophosphate]].<ref name="Kuzuyama_2003"/> These precursors can be made in different ways. In animals and [[archaea]], the [[mevalonate pathway]] produces these compounds from acetyl-CoA,<ref name="Grochowski_2006"/> while in plants and bacteria the [[non-mevalonate pathway]] uses pyruvate and [[glyceraldehyde 3-phosphate]] as substrates.<ref name="Kuzuyama_2003"/><ref name="Lichtenthaler_1999"/> One important reaction that uses these activated isoprene donors is [[steroid biosynthesis]]. Here, the isoprene units are joined together to make [[squalene]] and then folded up and formed into a set of rings to make [[lanosterol]].<ref name="Schroepfer_1981"/> Lanosterol can then be converted into other steroids such as cholesterol and ergosterol.<ref name="Schroepfer_1981"/><ref name="Lees_1995"/>