Lipid

In biology, a **lipid** is a substance o biological oreegin that is soluble in nonpolar solvents. It comprises a group o naiturally occurrin molecules that include fats, wauxes, sterols, fat-soluble vitamins (sic as vitamins A, D, E, an K), monoglycerides, diglycerides, triglycerides, an phospholipids. The main biological functions o lipids include storin energy, seegnalin, an actin as structural components o cell membranes. Lipids hae applications in the cosmetic an fuid industries as weel as in nanotechnology.

Scientists whiles broadly define lipids as hydrophobic or amphiphilic smaw molecules; the amphiphilic naitur o some lipids allous them tae form structurs sic as vesicles, multilamellar/unilamellar liposomes, or membranes in an aqueous environment. Biological lipids originate entirely or in pairt frae twa distinct teeps o biochemical subunits or "biggin-blocks": ketoacyl an isoprene groups. Uisin this approach, lipids mey be dividit intae aicht categories: fatty acids, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, an polyketides (derived frae condensation o ketoacyl subunits); an sterol lipids an prenol lipids (derived frae condensation o isoprene subunits).

Awtho the term "lipid" is whiles uised as a synonym for fats, fats are a subgroup o lipids cried triglycerides. Lipids an aa encompass molecules sic as fatty acids an thair derivatives (includin tri-, di-, monoglycerides, an phospholipids), as weel as ither sterol-conteenin metabolites sic as cholesterol. Awtho humans an ither mammals uise various biosynthetic pathweys baith tae brak doun an tae synthesize lipids, some essential lipids canna be made this wey an maun be obtained frae the diet.

## History
In 1815, Henry Braconnot clessifee'd lipids (*graisses*) in twa categories, *suifs* (solit creashes or tauch) and *huiles* (fluid iles). In 1823, Michel Eugène Chevreul developit a mair detailed clessification, includin iles, creashes, tauch, wauxes, rosets, balsams an volatile iles (or essential iles).

The wird "lipid", that stems etymologically frae the Greek *lipos* (fat), wis introduced in 1923 bi Gabriel Bertrand. Bertrands includit in the concept nae anerly the tradeetional fats (glycerides), but an aw the "lipoids", wi a complex constitution.

## Categories o lipids

### Fatty acids

Fatty acids, or fatty acid residues whan thay are pairt o a lipid, are a diverse group o molecules synthesised bi cheen-elangation o an acetyl-CoA primer wi malonyl-CoA or methylmalonyl-CoA groups in a process cried fatty acid synthesis. Thay are made o a hydrocarbon chain that terminates wi a carboxylic acid group; this arrangement confers the molecule wi a polar, hydrophilic end, an a nonpolar, hydrophobic end that is insoluble in watter. The fatty acid structur is ane o the maist fundamental categories o biological lipids, an is commonly uised as a biggin-block o mair structurally complex lipids. The caurbon cheen, teepically atween fower an 24 caurbons lang, mey be saturatit or unsaturatit, an mey be attached tae functional groups conteenin oxygen, halogens, nitrogen, an sulfur.

### Glycerolipids
Glycerolipids are componed o mono-, di-, an tri-substitutit glycerols, the best-kent bein the fatty acid triesters o glycerol, cried triglycerides. The wird "triacylglycerol" is whiles uised synonymously wi "triglyceride". In thir compoonds, the three hydroxyl groups o glycerol are ilk esterifee'd, teepically bi different fatty acids. Acause thay function as an energy store, thir lipids comprise the bouk o storage fat in ainimal tishies. The hydrolysis o the ester bonds o triglycerides an the release o glycerol an fatty acids frae adipose tishie are the ineetial steps in metabolisin fat.

### Glycerophospholipids

Glycerophospholipids, uisually referred tae as phospholipids (tho sphingomyelins are an aw classified as phospholipids), are ubiquitous in naitur an are key components o the lipid bilayer o cells, as weel as bein involved in metabolism an cell seegnalin. Neural tishie (includin the harn) conteens relatively heich amounts o glycerophospholipids, an alterations in thair composeetion haes been implicatit in various neurological disorders. Glycerophospholipids mey be subdividit intae distinct classes, based on the naiture f the polar headgroup at the *sn*-3 poseetion o the glycerol backbone in eukaryotes an eubacteria, or the *sn*-1 poseetion in the case o archaebacteria.

### Sterol lipids

Sterol lipids, sic as cholesterol an its derivatives, are an important component o membrane lipids, alang wi the glycerophospholipids an sphingomyelins. The steroids, aw derived frae the same fused fower-raing core structur, hae different biological roles as hormones an seegnalin molecules. The aichteen-caurbon (C18) steroids include the estrogen faimily whauras the C19 steroids comprise the androgens sic as testosterone an androsterone. The C21 subcless includes the progestogens as well as the glucocorticoids and mineralocorticoids. The secosteroids, comprisin various forms o vitamin D, are chairacterised bi cleavage o the B raing o the core structur. Ither ensaumples o sterols are the bile acids an thair conjugates, that in mammals are oxidised derivatives o cholesterol an are synthesised in the liver. The plant equivalents are the phytosterols, sic as β-sitosterol, stigmasterol, an brassicasterol; the latter compoond is an aw uised as a biomerker for algal growthe. The predominant sterol in fungal cell membranes is ergosterol.

### Prenol lipids

Prenol lipids are synthesised frae the five-caurbon-unit precursors isopentenyl diphosphate an dimethylallyl diphosphate that are produced mainly via the mevalonic acid (MVA) pathwey. The simple isoprenoids (linear alcohols, diphosphates, etc.) are formed bi the successive addeetion o C5 units, an are clessifee'd accordin tae nummer o thir terpene units. Structurs conteenin greater nor 40 caurbons are kent as polyterpenes. Carotenoids are important semple isoprenoids that function as antioxidants an as precursors o vitamin A. Anither biologically important cless o molecules is ensaumplifee'd bi the quinones and hydroquinones, that conteen an isoprenoid tail attached tae a quinonoid core o nan-isoprenoid oreegin. Vitamin E and vitamin K, as well as the ubiquinones, are ensaumples o this class. Prokaryotes synthesize polyprenols (cried bactoprenols) in that the terminal isoprenoid unit attached tae oxygen remains unsaturatit, whauras in ainimal polyprenols (dolichols) the terminal isoprenoid is reduced.

### Saccharolipids

Saccharolipids descrive compounds in that fatty acids are airtit directly tae a succar backbane, formin structurs that are compatible wi membrane bilayers. In the saccharolipids, a monosaccharide substitutes for the glycerol backbane present in glycerolipids and glycerophospholipids. The maist familiar saccharolipids are the acylated glucosamine precursors o the Lipid&nbsp;A component o the lipopolysaccharides in Gram-negative bacteria. Teepical lipid&nbsp;A molecules are disaccharides o glucosamine, that are derivatised wi as many as seiven fatty-acyl chains. The meenimal lipopolysaccharide required for growthe in ''E. coli'' is Kdo2-Lipid A, a hexa-acylatit disaccharide o glucosamine that is glycosylated wi two 3-deoxy-D-manno-octulosonic acid (Kdo) residues.

### Polyketides
Polyketides are synthesised bi polymerisation o acetyl an propionyl subunits bi clessic enzymes as weel as iterative an multimodular enzymes that share mechanistic featurs wi the fatty acid synthases. Thay comprise a lairge nummer o seicontar metabolites an naitural products frae ainimal, plant, bacterial, fungal an marine soorces, an hae great structural diversity. Mony polyketides are cyclic molecules that's backbanes are eften faur modifee'd bi glycosylation, methylation, hydroxylation, oxidation, or ither processes. Mony commonly uised anti-microbial, anti-parasitic, an anti-cancer agents are polyketides or polyketide derivatives, sic as erythromycins, tetracyclines, avermectins, an antitumor epothilones.

## Metabolism
The major dietar lipids for humans an ither ainimals are ainimal an plant triglycerides, sterols, an membrane phospholipids. The process o lipid metabolism synthesizes an degrades the lipid stores an produces the structural an functional lipids chairactereestic o individual tishies.

### Biosynthesis
In ainimals, whan thare is an owersupply o dietar carbohydrate, the excess carbohydrate is convertit tae triglycerides. This involves the synthesis o fatty acids frae acetyl-CoA an the esterification o fatty acids in the production o triglycerides, a process cried lipogenesis. Fatty acids are made bi fatty acid synthases that polymerise an then reduce acetyl-CoA units. The acyl cheens in the fatty acids are extendit bi a cycle o reactions that add the acetyl group, reduce it tae an alcohol, dehydrate it tae an alkene group an then reduce it again tae an alkane group. The enzymes o fatty acid biosynthesis are dividit intae twa groups, in ainimals an fungi aw thir fatty acid synthase reactions are cairied oot bi a single multifunctional protein, while in plant plastids an bacteria separate enzymes perform ilk step in the pathwey. The fatty acids mey be subsequently convertit tae triglycerides that are packaged in lipoproteins an secretit frae the liver.

The synthesis o unsaturatit fatty acids involves a desaturation reaction, whaurbi a dooble bond is introduced intae the fatty acyl cheen. For example, in humans, the desaturation o stearic acid bi stearoyl-CoA desaturase-1 produces oleic acid. The doobly unsaturatit fatty acid linoleic acid as weel as the treeply unsaturatit α-linolenic acid canna be synthesised in mammalian tishies, an are tharefore essential fatty acids an maun be obteened frae the diet.

Triglyceride synthesis taks place in the endoplasmic reticulum bi metabolic pathweys in that acyl groups in fatty acyl-CoAs are transferred tae the hydroxyl groups o glycerol-3-phosphate an diacylglycerol.

Terpenes an isoprenoids, includin the carotenoids, are made bi the assemmly an modification o isoprene units donatit frae the reactive precursors isopentenyl pyrophosphate an dimethylallyl pyrophosphate. Thir precursors can be made in different weys. In ainimals an archaea, the mevalonate pathwey produces thir compoonds frae acetyl-CoA, while in plants an bacteria the non-mevalonate pathwey uises pyruvate an glyceraldehyde 3-phosphate as substrates. Ane important reaction that uises these activatit isoprene donors is steroid biosynthesis. Here, the isoprene units are jynt thegither tae mak squalene an then fauldit up an formed intae a set o raings tae mak lanosterol. Lanosterol can then be convertit intae ither steroids sic as cholesterol an ergosterol.

### Degradation
Beta oxidation is the metabolic process bi that fatty acids are braken doun in the mitochondria or in peroxisomes tae generate acetyl-CoA. For the maist pairt, fatty acids are oxidised bi a mechanism that is seemilar tae, but nae identical wi, a reversal o the process o fatty acid synthesis. That is, twa-caurbon fragments are remuived sequentially frae the carboxyl end o the acid after steps o dehydrogenation, hydration, an oxidation tae form a beta-keto acid, that is split bi thiolysis. The acetyl-CoA is then ultimately convertit intae ATP, CO2, an H2O uising the citric acid cycle an the electron transport chain. Hence the citric acid cycle can start at acetyl-CoA whan fat is bein braken doun for energy if thare is little or na glucose available. The energy yield o the complete oxidation o the fatty acid palmitate is 106 ATP. Unsaturatit an odd-cheen fatty acids require addeetional enzymatic steps for degradation.

## References

### Bibliografie

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