Lipid

The lipids constitute the fat contents of the living beings. They are small molecules Hydrophobe S or amphipathic mainly made up of Carbone, Hydrogène and Oxygène and having a Densité lower than that of water. The lipids can be in a solid state, as in waxes, or liquid, as in the Huiles.

The Biochimie could supplement this definition by adding that the lipids can come entirely or partly of condensations of Thioester S based on Carbanion S and/or condensations of units Isoprène based on Carbocation S. This refers to the way in which the being alive synthesize the lipids. They exist several ways of synthesis which can gather in two categories according to “building block” used: a thioester or an isoprene. However, there does not exist yet of single definition of a lipid recognized by the whole of the scientific community. This is due probably to the fact that lipids form a whole of molecules to the structures and the functions extremely varied in the world of the alive one.

Classification

See also: Classification of the lipids

The lipids can be classified according to the structure of their carbonaceous Squelette (carbon atoms chained, cyclic, presence of non-saturations, etc). However, because of their diversity and of the difficulty of adopting a universal definition, there does not exist single classification of the lipids. According to IUPAC, for example, are included in the category “lipids” the fatty-acids and are derived, like their respective Ester S:

The category of the lipids defined by the IUPAC does not include the Cholestérol, classified in the terpènoïdes, that is to say:

However, the compounds such as cholesterol are actually regarded as lipids for a long time, including in the definition made by the IUPAC. Various studies were thus made to integrate the lipids of the fatty-acid type and cholesterol type in the same system of classification. The last in date defines 8 categories. In the majority of the cases, the phosphate is also related to a hydroxylated polar compound (e.g. the Choline, the Sérine, the éthanolamine). A well-known example of phosphoacylglycérol is the Lécithine, often used like Food additive. The developed general formula semi phosphoglycérides is:

\begin{array}{rl} & \ mathrm \ \ & {\ color {OliveGreen}|} \ \ \ mathrm \! \! \! \! & \ mathrm \ \ & {\ color {OliveGreen}|} \ \ \ mathrm \! \! \! \! & \ mathrm \ \ \end{array}

avec \ mathrm and \ mathrm of the chains acyles and X a hydroxylated compound.

It should be noted that when - X is a hydrogen H atom, the compound is called Phosphatidic acid.

Classify Sphingolipide S

The Sphingolipide S are derived from the sphinganines or from the sphing-4-ènines, more known under the name of Sphingosine S. These the last 2 compounds derive themselves from condensation from a Fatty-acid and Sérine. The semi-developed general formula of the sphinganines and the sphingosines is: OH | R-CH-CH-CH 2-OH | NH2 with R, a chain acyl, saturated or not.

The sphinglolipides can be divided into 9 subclasses:

  • sphing-4-ènines or sphingosines
  • the céramides
  • the phosphosphingolipides
  • the phosphonosphingolipides
  • neutral glycosphingolipides
  • acid glycosphingolipides
  • basic glycosphingolipides
  • amphoteric glycosphingolipides
  • the arsénosphingolipides

The most important subclass is undoubtedly that of the céramides. It is of sphinganines or sphingosines related to an fatty-acid by a bond amide. A well-known example of céramide is the Sphingomyéline, implied in the nervous transmission in the mammals. The semi-developed general formula of the céramides is: OH | R1-CH-CH-CH2-OH | R2-CO-NH with R1 and R2 two chains acyl, saturated or not.

Classify Stérol S

The sterols are lipids deriving from the core cyclopentanophenanthrenic . The various types of sterols are distinguished according to the number and the position of non-saturation and/or side chains. Sterols are divided into 6 subclasses:

Two well-known examples of sterols are the Cholestérol and the hormones stéroïdiennes.

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