# Lipids

Lipids and lipoids (from the Greek λίπος lípos "fat", stress on the second syllable: Lip i de ) are collective names for wholly or at least largely water-insoluble ( hydrophobic ) natural substances , which, on the other hand, are very good at hydrophobic (or lipophilic ) due to their low polarity Dissolve solvents such as hexane . Their insolubility in water is mainly due to the long hydrocarbon residues that most lipids have. They are roughly divided into saponifiable and unsaponifiable lipids.

In living organisms, lipids are mainly used as structural components in cell membranes , as energy stores or as signal molecules . Most biological lipids are amphiphilic , i.e. have a lipophilic hydrocarbon residue and a polar hydrophilic head group, which is why they form micelles or membranes in polar solvents such as water . The term “fat” is often used as a synonym for lipids, but fats ( triglycerides ) are only a subgroup of lipids.

The lipids can be divided into seven substance classes: fatty acids , triglycerides ( fats and fatty oils), waxes , phospholipids , sphingolipids , lipopolysaccharides and isoprenoids (steroids, carotenoids etc.). Non-natural or synthetic substances in these classes are usually not referred to as lipids.

## Fatty acids, triacylglycerides (fats and fatty oils) and waxes

The triacylglycerols ( triglycerides ) make up the main part of dietary lipids with more than 90 percent. They are an important supplier of energy (1 g fat contains 38.9  kJ of energy, 1 g sugar only 17.2 kJ). In addition, triglycerides form the body's most important energy store (sugar, i.e. glucose, is also converted into fat for storage, and is only stored in comparatively small amounts as a short-term buffer as glycogen in the liver and muscles), they are good protection against the cold in the skin and also protect them from injuries. All important organs are protected by a layer of fat.

### Fatty acids

Both myristic acid (a saturated fatty acid ) and myristoleic acid (an unsaturated fatty acid ) have 14 carbon atoms. In contrast to myristic acid, myristoleic acid has a double bond.

Fatty acids are mostly unbranched monocarboxylic acids that consist of a hydrocarbon chain with a carboxy group at one end (see picture).

A distinction is made between saturated fatty acids, in which there are no double bonds , and unsaturated fatty acids, which have one or more double bonds (in nature usually in the cis position and not in conjugation with one another). The simplest saturated fatty acid is butyric acid with only four carbon atoms .

Important representatives of the unsaturated fatty acids are oleic acid (monounsaturated) and arachidonic acid (fourfold unsaturated). The animal organism can only synthesize unsaturated fatty acids with restrictions. Therefore, all those fatty acids that have to be consumed with food are called essential fatty acids (see below). The more double bonds a fatty acid contains, the lower its melting point.

### Triacylglycerine (fats and fatty oils)

General structure of triacylglycerols.
The side chains R 1 , R 2 and R 3 represent alkyl radicals of the fatty acids.

See the main articles fats and fatty oils, and triacylglycerols .

As mentioned above, triacylglycerols represent the largest group of dietary lipids. They consist of glycerol and three fatty acids esterified with glycerol . If they are liquid at room temperature (20 ° C) they are referred to as oils, if they are solid as fats. As already mentioned, these are important energy stores for animals and plants. If triacylglycerols are split through saponification , glycerol and the corresponding salts of fatty acids are formed.

### Waxes

#### Real waxes

Components of beeswax as a substitute for waxes

Waxes are single esters of fatty acids and as such differ from the triple esters of fats and oils. Both the acid and alcohol parts of waxes have long saturated alkyl groups. In contrast to triglycerides, waxes are less “oily” and also harder and more porous.

#### Waxes as a class of substances

Another definition ( German Society for Fat Science ) sees waxes as a class of substances that is defined exclusively by their mechanical-physical properties. According to this definition, waxes are kneadable at 20 ° C, solid to brittle and hard, they have a coarse to finely crystalline structure, in terms of color they are translucent to opaque (opaque), but not glass-like, above 40 ° C they melt without decomposition, a little above At their melting point they are slightly liquid (not very viscous ), have a strongly temperature-dependent consistency and solubility and can be polished under slight pressure.

## Membrane-forming lipids

Different structures that phospholipids can adopt in aqueous solutions. The circles are hydrophilic heads and the wavy lines are the hydrophobic fatty acid chains.

Membrane-forming lipids are those that have a hydrophilic and a hydrophobic part - i.e. are amphiphilic . This allows them, as comparatively polar lipids , to form either micelles (spherical aggregates of amphiphilic molecules that spontaneously assemble in a dispersion medium ) or double lipid layers in polar solvents such as water, depending on their properties - the hydrophilic part always interacting with the polar solvent . With the exception of the membranes of archaea , all biomembranes , which separate the content of a cell from the environment , are made up of these double lipid layers . Membrane-forming lipids are therefore one of the basic requirements for cell formation and thus for life.

### Phospholipids

General structure of the phosphoglycerides
The residues R 1 and R 2 determine the fatty acids, the residue X determines the class. If X = H, phosphatidic acid is present

Phospholipids form the main component of biomembranes. A distinction is made between phosphoglycerides and sphingomyelins . The structure of the phosphoglycerides is derived from the phosphatidic acid , which is similar to the triglycerides, with the difference that there is a phosphoryl group on the C3 hydroxyl group instead of the acyl residue. Like triacylglycerides, these belong to the glycerolipids . Sphingomyelins, on the other hand, differ from glycerolipids in their sphingosine backbone. The phosphoric diester group of all phospholipids is hydrophilic (that is, interacts with water) and is called the "head" . The acyl residues or the non-polar part of the sphingosine are called the “tail” and are hydrophobic . This opposing character leads to the formation of lipid bilayers , in which the hydrophobic part of the membrane lipids point inwards and the hydrophilic part outwards. The most important phospholipids involved in the construction of biomembranes are the phosphoglycerides phosphatidylcholine (also lecithin ), phosphatidylethanolamine , phosphatidylserine and sphingomyelins . The latter are both phospho- and sphingolipids. Phosphatidylethanolamine and phosphatidylserine are also known as cephalins . An important group of phosphoglycerides, especially in the intracellular transmission of extracellular signals ( signal transduction ), are the phosphatidylinositols , which occur in various stages of phosphorylation ; as a head group they have a phosphoinositol .

### Sphingolipids

General structure of the sphingolipids
Different residues (R) result in different subgroups.
Hydrogen - Ceramide
Phospho choline or phospho ethanolamine - sphingomyelins
saccharide - glycolipids

Sphingolipids are also part of cell membranes. Their basic structure consists of a fatty acid and sphingosine . They are divided into the groups of ceramides , sphingomyelins and glycolipids. Sphingolipids are found in nerve tissue , they play an important role in signal transmission and the interaction of individual cells .

### Glycolipids

Glycolipids are phosphate-free, sphingosine-containing lipids with a carbohydrate component glycosidically bound to the 1-hydroxy group of the sphingosine . They often form the outside of biological membranes, with their carbohydrate content being presented on the cell membrane . It is believed that these play a role in the communication and interaction between individual cells. Glycolipids are divided into cerebrosides , gangliosides and sulfatides .

### Ether lipids

Ether lipids are the main component of the cell membranes of archaea , but not in bacteria or eukaryotes . In archaea, the ether lipids are composed of glycerol with isoprenoids . Eukaryotes also produce a small amount of ether lipids.

## Isoprenoids

As isoprenoid compounds which on isoprene build. Compounds that count among the lipids are the steroids, terpenes and terpenoids as well as the carotenoids. Naturally occurring steroids belong to the triterpenoid derivatives (triterpenoid means that it consists of 30 carbon atoms), as they are all biosynthesized from squalene . Carotenoids are counted among the tetraterpenoid derivatives (40 carbon atoms), they are derived from lycopene .

### Steroids

Basic structure of all steroids, the Steran skeleton

All steroids have a basic structure of four, usually trans -connected carbon rings, three hexagonal and one pentagonal. The best known representative of the steroids is to sterols counting cholesterol . Among other things, it is also an essential component of all cell membranes with the exception of the inner membrane of the mitochondria and can therefore also be counted among the membrane lipids in a broader sense. It is usually in esterified form as cholesterol esters of fatty acids. The spectrum of the fatty acids of the cholesterol esters in a living being is strongly dependent on its diet.

Bile acids , which are involved in fat digestion , have a hydrophobic and a hydrophilic part, so they can coat fats and thus facilitate their absorption in the digestive tract .

The steroids also include the sex hormones produced in the ovaries and testes . They control reproduction and the development of the secondary sexual characteristics . The female sex hormones are progesterone and estrogen , the male androgens (e.g. testosterone and androsterone ).

Further examples are other zoo, myco and phytosterols and their esters such as z. B. ergosterol , vitamin D and cardiac glycosides (e.g. digitalis and strophantine ). Phytosterols such as B. β-sitosterol , stigmasterol and campesterol and their esters occur increasingly in human serum in a vegetarian diet .

### Carotenoids

β-carotene

Carotenoids are polymerization products of isoprene , which are produced exclusively in plants, bacteria and fungi and are responsible for the yellow to reddish color of plants (e.g. carrots and tomatoes ). Their physiological tasks are light absorption and protection against oxidative stress , since they can function as radical scavengers . Carotenoids can also be ingested by animals through food and are thus responsible for the color of egg yolk and butter , among other things . They mostly consist of unsaturated hydrocarbon chains and their oxidation products, and are made up of eight isoprene units. Thus, it is tetraterpenes with a skeleton size of 40 carbon atoms . They are divided into carotenes and xanthophylls , with xanthophylls , in contrast to carotenes, containing oxygen-containing groups. The best-known and most common carotenoid is β-carotene, also known as provitamin A. It is converted into retinal (a vitamin A) in the organism of humans and some animals , which is an important starting compound for rhodopsin (a visual pigment) is necessary for the visual process.

## Analytics

For the qualitative and quantitative analysis of the physico-chemically very different substance classes of lipids, chromatographic methods are preferably used. With the aid of thin layer chromatography and HPLC , all lipid classes can be separated from one another. However, the use of gas chromatography requires the separation of the phospholipids, since these cannot be evaporated without being decomposed. By coupling the chromatographic separation process with mass spectrometry , highly specific and highly sensitive qualitative and quantitative determinations of individual substances of the various lipid classes are possible.

Also, the solid phase extraction is used for the separation of lipid classes.

## Biological functions

The biological functions of lipids are just as diverse as their chemical structure. They serve as

• Fuel ( β-oxidation of fatty acids)
• Energy storage (triacylglycerols)
• Membrane building blocks (phospholipids)
• Signal molecules (diacylglycerol; IP 3 cascade)
• Hormones (eicosanoids; prostaglandins etc.)
• Fat-soluble vitamins (vitamins A, D, E, K)
• Cofactors (Dolichol)
• Pigments (carotenoids)

While some lipids can be formed by the human body in the fat metabolism itself , others have to be taken in with food. Hence these are called essential lipids .

### Essential fatty acids

So-called essential fatty acids are polyunsaturated and must be ingested with food, since in mammals and humans no double bonds can be introduced between their end and the ninth carbon atom during fatty acid synthesis. They include the omega-6 fatty acids and the omega-3 fatty acids. The representatives of the essential omega-3 fatty acids include linolenic acid , eicosapentaenoic acid and docosahexaenoic acid ; Corresponding omega-6 fatty acids include linoleic acid and arachidonic acid . Eicosanoids are synthesized from arachidonic acid ; these are important tissue hormones and mediators in the body. Omega-9 fatty acids are not essential as they can be synthesized from omega-3 and omega-6 fatty acids. Possible sources of omega-3 and omega-6 fatty acids in food are fish, flaxseed, soybean oil, hemp oil, pumpkin seeds or walnuts.

Essential fatty acids play an important role in many metabolic processes. There is evidence that deficiencies or imbalances in the intake of essential fatty acids are the cause of numerous diseases.

### Fat-soluble vitamins

The fat-soluble vitamins are:

## Lipidomics

The research of all lipids that occur in a cell or an organism is called lipidomics ( English lipidomics ). It is comparable to proteomics , which deals with the research of all proteins occurring in the organism and the cell. The aim of this subdisciplinary science is both the recording of all lipids and the determination of their functions and protein-lipid interactions in a biological, physiological or physical context. In lipidomics, techniques such as mass spectroscopy (MS), nuclear magnetic resonance spectroscopy (NMR) or fluorescence spectroscopy are used to characterize lipids . Mass spectroscopic methods appear particularly suitable which have a high sensitivity and in which the ionization of the molecules does not cause them to largely disintegrate. A suitable and gentle ionization method for this is nano- electrospray ionization mass spectrometry. Research in the field of lipidomics aims to determine the role of lipids in many metabolic diseases such as obesity, atherosclerosis, stroke, high blood pressure and diabetes. The rapidly growing area of ​​lipidomics complements the areas of genomics and proteomics and, together with them, defines systems biology .

## Packing parameters

Lipids can be characterized using the packaging parameters:

${\ displaystyle P_ {l} = {\ frac {V} {a_ {0} l_ {c}}}}$
 ${\ displaystyle V}$ Volume of the cylinder occupied by the fatty acid chains of the lipid tail ${\ displaystyle a_ {0}}$ largest cross-sectional area of ​​the hydrophilic lipid head group ${\ displaystyle l_ {c}}$ Length of the hydrophobic lipid tail

Lipids with only one fatty acid chain and a large head ( detergents ) have a packing parameter P l <⅓. With double-chain lipids such as phosphatidylethanolamines , the fatty acid chains take up a conical volume and P l = 1. Cardiolipins and cholesterol have a packing parameter> 1.

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Wikibooks: Organic Chemistry for Students / Fette  - Learning and Teaching Materials

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