Editing Lipid (section)

==Metabolism==
The major dietary lipids for humans and other animals are animal and plant triglycerides, sterols, and membrane phospholipids. The process of lipid metabolism synthesizes and degrades the lipid stores and produces the structural and functional lipids characteristic of individual tissues.

===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"/>

===Degradation===
[[Beta oxidation]] is the metabolic process by which fatty acids are broken down in the [[mitochondria]] or in [[peroxisomes]] to generate [[acetyl-CoA]]. For the most part, fatty acids are oxidized by a mechanism that is similar to, but not identical with, a reversal of the process of fatty acid synthesis. That is, two-carbon fragments are removed sequentially from the carboxyl end of the acid after steps of [[dehydrogenation]], [[hydration reaction|hydration]], and [[oxidation]] to form a [[keto acid|beta-keto acid]], which is split by [[thiolysis]]. The acetyl-CoA is then ultimately converted into [[adenosine triphosphate]] (ATP), CO<sub>2</sub>, and H<sub>2</sub>O using the [[citric acid cycle]] and the [[electron transport chain]]. Hence the citric acid cycle can start at acetyl-CoA when fat is being broken down for energy if there is little or no glucose available. The energy yield of the complete oxidation of the fatty acid palmitate is 106 ATP.<ref name = "Stryer_2007" />{{rp|625–6}} Unsaturated and odd-chain fatty acids require additional enzymatic steps for degradation.