The Lipid Compositions of Plants and Microorganisms
Comprehensive discussion of the enormous literature on compositions of animal, plant and microbial tissues would be a daunting task, and it is only possible here to summarize some of the more significant features of lipid composition, with data from some highly selective (and simplified) analyses. Further data are available for specific lipids in many of the web pages in this part of the website. There are short cuts to finding structural, compositional and biochemical information on individual lipid classes in the LipidWeb here and at the end of this web page.
One problem in comparing data from different sources is the method of presentation, which is often dependent on the nature of the analytical methods used. For example, it is easier technically to analyse the phospholipids and glyco(sphingo)lipids following isolation as distinct groups separately from the simple lipids, and often there is no attempt subsequently to integrate the data. To add to the confusion, results of analyses of simple lipids are often reported in terms of weight percent of each lipid class, since the data are acquired in this form, while the results for phospholipids are most frequently recorded as molar percent, especially when phosphorus analysis is used as the means of quantification. Many of the data listed below are from publications that may appear relatively old but are nonetheless reliable. The results from modern lipidomic studies, which are usually based entirely on analyses of molecular species by mass spectrometry, are often too complex to summarize in a simple tabular form although they are often much more comprehensive. This is discussed further in our web page dealing with compositions of animal lipids. For this reason, I have chosen to list fatty acid positional distributions in glycerolipids as structural determinants rather than molecular species data.
1. Lipid Class Compositions of Plant Tissues
The compositions of the lipids of plant tissues have been reviewed , and some results are listed in Table 1. Those plant tissues that serve as major food materials have received most study obviously. Triacylglycerols tend to be the most abundant class of storage lipid in tissues that are rich in lipids, such as the commercially important oil seeds. One rare exception is jojoba oil, which consists mainly of wax esters. Many storage tissues in plants have starch as the main constituent rather than lipid, and in potato tubers and apples, the complex glycolipids and phospholipids are the only lipids present and at low levels. As well as those lipids listed in the table, sterols, sterol esters, acylated sterol glycosides, phytoglycolipids (complex sphingolipids), ceramide, glucosylceramide, phosphatidic acid, N-acylphosphatidylethanolamine and phosphatidylserine, among others, may be present. It should be recognized that seeds and tubers do not have a homogeneous lipid distribution, the endosperm, germ, bran and other organelles each having a distinctive composition.
Table 1. The lipid class compositions (weight % of the total lipids) of various plant tissues.
|Lipid class||Potato tuber||Apple fruit||Soybean seed||Clover leaves||Rye grass||Spinach chloroplasts|
The glycosyldiacylglycerols, i.e., mono- and digalactosyldiacylglycerols and sulfoquinovosyldiacylglycerol, are the most abundant lipid classes in leaf (photosynthetic) tissues. Glycerophospholipids, such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylglycerol, are present, and other complex lipids are occasionally reported. Phosphatidylglycerol appears to be especially characteristic of photosynthetic tissue, and it can be the main phospholipid in certain green algae, but triacylglycerols are virtually absent from leaves.
As in animal tissues, each of the membranes or organelles in the leaf has a characteristic lipid composition. Spinach chloroplasts have received a great deal of attention, because they can be prepared relatively easily for biochemical experiments, and like the intact leaf they contain appreciable amounts of the glycolipids and a smaller proportion of glycerophospholipids. In contrast, as in animal tissues, the plasma membrane has a high content of phosphatidylcholine, while the mitochondria contain cardiolipin. When phosphorus is limiting as a nutrient, the proportion of glycosyldiacylglycerols relative to phospholipids tends to increase.
Sphingolipids: It is increasingly being recognized that plant membranes contain much higher proportions of complex sphingolipids than had been realized hitherto, but this is not yet reflected in published total analyses to my knowledge. For example, the plasma membrane in leaf tissue was once considered to contain roughly 10% glucosylceramides, with much of the remainder being the complex glycerophospholipids. Now it is recognized that approximately 40% of this membrane consists of highly complex sphingolipids in addition to the glucosylceramides, mainly glycosylinositol phosphoceramides, which were technically difficult to analyse prior to the development of modern mass spectrometry methods. Details of these are available on the appropriate web pages and are not duplicated here.
Surface lipids: A further region of plants with a distinctive composition is the epidermis or cuticle where the lipids are rich in waxes, while the cutin and suberin layers contain complex polyesters of hydroxy and dicarboxylic fatty acids. Leaf surface lipids are largely non-polar waxes, and it has been argued that these may be among the most abundant lipids on Earth, considering that leaves cover much of the land surface.
2. Fatty Acid Compositions of Plant Tissues
The fatty acid compositions of the seed oils of importance to commerce have been reviewed , and data for a few typical analyses are listed in Table 2. Maize (corn), sunflower and safflower oils are of nutritional value since they contain appreciable amounts of the essential fatty acid, linoleic acid, but excessive amounts of linolenic acid, as in soybean oil, can lower the commercial value of an oil because it is then more susceptible to rancidity problems caused by autoxidation. It is therefore a common industrial practice to subject the oil to hydrogenation, although this causes new problems because it can produce trans fatty acids, which are potentially harmful to consumers. On the other hand, the high content of linolenic acid in linseed oil increases its value for many industrial applications. There are no such problems with olive oil, an important lipid constituent of the ‘Mediterranean’ diet, with its high content of oleic acid.
Table 2. The fatty acid compositions (weight % of the total) of some seed oils .
|Fatty acid||Seed oil|
|a newer cultivars can contain much less erucic acid|
Palm oil contains a higher proportion of saturated fatty acids than most seed oils, while cocoa butter consists largely of molecular species with saturated fatty acids in positions sn-1 and 3 and oleic in position sn-2, an important factor for its use as an ingredient of chocolate. Palm kernel and coconut oils are noteworthy for a high content of saturated fatty acids of short to medium chain-length. Rapeseed is one of the few oil crops capable of being grown in northern climates. In its native form, it tends to have a high content of erucic acid (22:1(n-9)), which may have some properties that may be harmful to the consumer, although this is still a matter for controversy; new cultivars with negligible levels of this component (‘Canola’) are now widely grown. Cottonseed oil resembles maize oil in its composition, but it also contains small amounts of the cyclopropene fatty acid, ‘sterculic’ or 9,10‑methyleneoctadecenoic acid, which has well-established toxicity properties and must be removed during refining.
There are many seed oils which may have limited or negligible commercial value at present but contain fatty acids with unusual substituent groups, which are of great interest to biochemists or have industrial applications, including ricinoleic acid (D‑(-)12‑hydroxy-octadec-cis-9-enoic acid) from castor oil (90%) of Ricinus communis. The picture will become more complex as new genetically modified seed oils are introduced.
Each of the glycerolipid classes in a plant tissue can have a characteristic fatty acid composition, and for illustrative purposes, some results for spinach leaf lipids are listed in Table 3.
Table 3. The fatty acid compositions (weight % of the total) of the individual glycerolipids of spinach leaves .
|Fatty acid||Lipid class|
|a MGDG, monogalactosyldiacylglycerol; DGDG, digalactosyldiacylglycerol; SQDG, sulfoquinovosyldiacylglycerol; PG, phosphatidylglycerol; PC, phosphatidylcholine; PI, phosphatidylinositol; PE, phosphatidylethanolamine.|
Glycosyldiacylglycerols tend to consist mainly of the unsaturated fatty acids, linoleic acid and especially α-linolenic acid (18:3(n‑3)); a hexadecatrienoic acid (16:3(n‑3)) may be present also with certain species. On the other hand, the glycerophospholipids contain higher proportions of saturated fatty acids, generally palmitic acid, with the unsaturated components. Phosphatidylglycerol is unique in that it contains a substantial amount of an unusual fatty acid, i.e., trans-3-hexadecenoic acid. The fatty acid compositions of plant tissues can vary with climatic and other cultivation conditions and with the stage of development of the tissue, and major differences among species occur, although the results listed in Table 3 are reasonably typical in general. There are no definitive reports of ether lipids in plants.
3. Fatty Acid Positional Distributions within Glycerolipids in Plant Tissues
In addition to each lipid class in a tissue having a distinctive fatty acid composition, each position of the glycerol moiety tends to have a unique fatty acid composition that is determined during the biosynthesis of a lipid mainly by the specificities of various acyltransferases. Again, the positional distributions of fatty acids in particular lipid classes can vary markedly between tissues and species and can have some metabolic importance. It should be noted that modern mass spectrometric methods cannot distinguish between positions sn-1 and sn-3 of a triacylglycerol, i.e., determine the stereospecific distributions, and rather, they can only determine the composition of position sn‑2 and the average of those of positions sn-1 and sn-3, which should be termed regiospecific distributions. Stereospecific analysis of triacyl-sn-glycerols requires a combination of chromatographic and enzymatic methodology and is becoming something of a lost art. Mass spectrometry can distinguish between the primary (sn-1) and secondary (sn-2) positions of glycerophospholipids.
Once more, the triacylglycerols of the commercial seed oils are those to have been subjected most frequently to detailed structural analyses. In general, there tends to be little difference between the compositions of positions sn-1 and sn-3 of the glycerol moiety, but the saturated fatty acids are concentrated in the primary positions and the unsaturated are in greatest abundance in position sn-2. In some instances, there appears to be a higher proportion of longer-chain fatty acids (C20 to C22) in position sn-3 than in position sn-1, and sometimes the more unusual fatty acids are concentrated in position sn-3. Some comparative data and a more comprehensive discussion are available in our web page dealing with the compositions of triacylglycerols per se.
The positional distributions of fatty acids in many of the glycerophospholipids of plants seem to resemble those of animal tissues in that the saturated fatty acids are concentrated in position sn-1 and the unsaturated in position sn-2, but phosphatidylglycerol from spinach leaves and the ‘model’ plant Arabidopsis thaliana is unusual in that the major molecular species contains linolenic acid in position sn-1 and trans-3-hexadecenoic acid in position sn-2 [9,10]. Data for both glycoglycerolipids and glycerophospholipids are listed in Table 4.
Table 4. Composition (mol %) of fatty acids in positions sn-1 and sn-2 of mono- and digalactosyldiacylglycerols, phosphatidylcholine, phosphatidylethanolamine and phosphatidylglycerol from leaves of Arabidopsis thaliana .
|Lipid class||Position||Fatty acids|
In those plants containing 16:3(n-3), the monogalactosyldiacylglycerols consist mainly of the 16:3-18:3 combination (with all the 16:3 in position sn‑2), while the digalactosyldiacylglycerols have the more common 18:3-18:3 species. The distinctive compositions of the phosphatidylglycerol and monogalactosyldiacylglycerol was once believed to result from the existence of a primitive prokaryotic biosynthetic pathway alongside the more usual eukaryotic pathway in the chloroplasts , but this has been challenged. This is discussed further in our web page on glycosyldiacylglycerols.
4. Lipids of Bacteria
Lipid Class Compositions
As each family tends to have a distinct and characteristic lipid composition, it is not easy to generalize about the lipids of bacteria and there are many lipid classes that are unique to particular genera. The fatty acid components are often very different from those of animal or plant tissues, and there are some that appear to be restricted to certain species of bacteria only. For this reason, I have not attempted to tabulate data here, and much more information on microbial lipids is available elsewhere [12,13]. The nature and composition of microbial lipids have proved to be of great taxonomic value, and their study has assisted towards an understanding of the molecular basis of evolution.
Simple glycerolipids such as triacylglycerols are often the most abundant lipids in algae and fungi, but bacteria do not normally accumulate storage lipids of this type, although they are known to be present in a few species; diacylglycerols may be formed transiently as intermediates in the biosynthesis of glycerophospholipids. Sterols are found in yeasts, fungi and algae, but in essence are absent from bacterial membranes, although hopanoids take their place in some organisms.
Most of the common glycerophospholipids of plants and animals that are described above can be present in some but not all microorganisms, and phosphatidylethanolamine is often the most abundant lipid class in many bacterial species (including both Gram-negative and Gram-positive bacteria) and may be accompanied by phosphatidylserine and phosphatidylglycerol. The 'model' bacterium Escherichia coli does not contain phosphatidylcholine, sphingomyelin or cholesterol, and the cell wall instead contains high concentrations of lipopolysaccharides such as lipid A. Indeed, phosphatidylcholine is rarely being found in bacteria, although it is often the main complex lipid in eukaryotes. The N‑mono- and N,N‑dimethyl derivatives of phosphatidylethanolamine are found in some bacterial genera. In bacteria, phosphatidylglycerol especially is widely distributed and it is found in most genera; uniquely it is the only glycerophospholipid in the membranes of certain cyanobacteria. Phosphatidylglycerol is usually accompanied by cardiolipin, which is often a major lipid component of bacteria, although in essence it is only found as a constituent of mitochondrial membranes in eukaryotes, and derivatives of phosphatidylglycerol in which the glycerol moiety is esterified to an amino acid (lipoamino acids), such as lysine, ornithine or alanine, are common in Gram-positive bacteria.
The plant glycosyldiacylglycerols are present as major constituents of the membranes of algae and cyanobacteria, although glucose may replace a galactose unit in some species. Diglycosyldiacylglycerols, in which the carbohydrate moiety may be two glucose, two mannose or less often two galactose units, are found as minor components of the membranes of Gram-positive and occasionally of Gram-negative bacteria, while the plant sulfolipid, sulfoquinovosyldiacylglycerol, is present in small amounts in certain photosynthetic bacteria.
Complex phosphorylceramides containing inositol are major components of yeast lipids, but sphingolipids are rarely found in bacteria. On the other hand, there always appear to be exceptions to any rule of this kind, and ceramide phosphorylethanolamine and ceramide phosphorylglycerol have been detected in some anaerobic bacteria, while free ceramides have been found in some species of Bacteriodes. Aminolipids containing alkylamines but lacking the hydroxyl groups of the sphingoid bases occur in some bacteria, while others contain sulfonolipids (capnoids), which appear to be related structurally to the long-chain bases; capnine is 2‑amino-3-hydroxy-15-methylhexadecane-1-sulfonic acid and is found in the free form or as the N-acyl derivative in some species of gliding bacteria.
Many bacterial species contain certain glycerophospholipids and glycolipids (including glycophospholipids), which are apparently found nowhere else in nature, and there are short cuts to web pages dealing with these here... Glycoglycerophospholipids are common constituents of bacterial membranes, and although phosphatidylinositol itself is rarely found, various mannoside derivatives do occur in some species. Glycosylated forms of phosphatidylglycerol are perhaps the most common of all, and glucosaminylphosphatidylglycerol has been found in both Gram-negative and Gram-positive bacteria. Certain rumen bacteria contain lipids with complex glycerophosphoryl groups and glycosylglycerol groups, linked by a C32 dicarboxylic acid (‘diabolic acid’) that may span the membrane bilayer. The lipoteichoic acids of the cell walls of Gram-positive bacteria are polymers of glycerol-1-phosphate and other complex organic groups linked to a diglycosyldiacylglycerol.
Relatively simple glycolipids, such as di- and triacylglucosides, and an analogous rhamnolipid are present in some bacteria. Much more complex lipid polysaccharides, such as Lipid A, occur in the cell envelope of certain microbial species, with phosphatidylinositol as the anchor moiety for others in the membrane. Similarly, the cell walls of the Mycobacteria, such as M. tuberculosis, contain a wide range of distinctive lipid classes, including trehalose derivatives, phosphate esters, peptidolipids and phenolic lipids, and these may be esterified to a bewildering array of unusual and often unique fatty acids (see below).
A further group of distinctive bacterial lipids are the acylornithines (lipoamino acids). In these, the amine group of ornithine is attached via an amide bond to a fatty acid with a 2- or 3-hydroxyl group, which can in turn be esterified to a further fatty acid; the carboxyl group of the ornithine can be linked to an aliphatic alcohol in some instances. The precise nature of the aliphatic groups and of the linkages can be characteristic of particular species.
The glycerophospholipids of bacteria exist not only in the diacyl forms but also as ether lipids, most often the plasmalogen forms, and many different complex and unique di- and tetra-ether phospholipids and glycolipids, in which the aliphatic moieties are isoprenoid structures, are present in archaebacterial extremophiles .
Fatty Acid Compositions
Polyunsaturated fatty acids of the kind found in plant lipids occur in algae (green and brown), fungi and cyanobacteria, but are not often present in bacteria (some marine species are exceptions where biosynthesis is via a polyketide mechanism). In general, bacterial lipids tend to contain appreciable amounts of C14 to C18 straight-chain saturated and monoenoic fatty acids, but the common C18 monoenoic acid is not oleic acid, but cis-vaccenic acid (18:1(n-7)). Thes can occur with odd-chain, branched-chain (mainly iso- and anteiso-methyl, but 10-methyloctadecanoic or ‘tuberculostearic’ acid is characteristic of some species), cyclopropane and 3-hydroxy fatty acids, which are only rarely synthesised by eukaryotes. The presence of a methyl branch or of a cyclopropane ring in the fatty acids in a membrane increases its fluidity in an analogous manner to that of double bonds in polyunsaturated fatty acids in the membranes of higher organisms. In comparing the detailed fatty acid compositions of bacteria, it is important to recognize that they can vary greatly with culture conditions and with stage of growth.
The Mycobacteria and certain related species contain a highly distinctive range of very-long-chain α-branched β-hydroxy fatty acids, the mycolic acids, which occur in the bacterial cell walls in the free form, as wax esters and as components of other complex lipids that are often unique.
5. Lipidomic Analyses
As cautioned earlier and in our web page dealing with compositions of animal lipids, it is not easy to present the results of modern lipidomics analyses in a concise tabular fashion, but the newer mass spectrometric methods are making an immense contribution to our understanding of lipid composition and function. The book  is an essential guide to analytical methodology.
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- Allen, C.F., Good, P., Davis, H.F. and Fowler, S.D. Plant and chloroplast lipids I. Separation and composition of major spinach lipids. Biochem. Biophys. Res. Commun., 15, 424-430 (1964); DOI.
- Browse, J., Warwick, N., Somerville, C.R. and Slack, C.R. Fluxes through the prokaryotic and eukaryotic pathways of lipid synthesis in the '16:3' plant Arabidopsis thaliana. Biochem. J., 235, 25-31 (1986); DOI.
- Haverkate, F. and van Deenen, L.L.M. Isolation and chemical characterization of phosphatidyl glycerol from spinach leaves. Biochim. Biophys. Acta, 106, 78-92 (1965); DOI.
- Moreau, P., Bessoule, J.J., Mongrand, S., Testet, E., Vincent, P. and Cassagne, C. Lipid trafficking in plant cells. Prog. Lipid Res., 37, 371-391 (1998); DOI.
- Ratledge, C. and Wilkinson, S.G. (editors), Microbial Lipids. Volumes 1 and 2 (1988 and 1989, respectively) (Academic Press, London).
- Sohlenkamp, C. and Geiger, O. Bacterial membrane lipids: diversity in structures and pathways. FEMS Microbiol Rev., 40, 133-159 (2016); DOI.
- Caforio, A. and Driessen, A.J.M. Archaeal phospholipids: Structural properties and biosynthesis. Biochim. Biophys. Acta, 1862, 1325-1339 (2017); DOI.
- Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis (4th edition), 446 pages (Oily Press, Woodhead Publishing and now Elsevier) (2010) - see Science Direct.
- Ohlrogge, J., Thrower, N., Mhaske, V., Stymne, S., Baxter, M., Yang, W., Liu, J., Shaw, K., Shorrosh, B., Zhang, M. and Wilkerson, C. PlantFAdb: a resource for exploring hundreds of plant fatty acid structures synthesized by thousands of plants and their phylogenetic relationships. Plant J., 96, 1299-1308 (2018); DOI.
Recommended Web Sites
In addition to the LIPID MAPS® Lipidomics Gateway - PlantFAdb: Phylogenetic relationships between hundreds of fatty acids synthesised by thousands of plants (see ref  above).
|© Author: William W. Christie|
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