Chemical structure and classification of fatty acids in plants

Fatty Acid Composition of Vegetable Oils and Its Characteristics

Natural fats are of plant or animal origin, however, their chemical composition differs fundamentally. While animal fats consist primarily of saturated and monounsaturated acids, vegetable oils are rich in unsaturated fatty acids. In practical agronomy, the composition of this fraction determines the quality of the resulting oil, and the level of free fatty acids serves as an indicator of harvest preservation.

• Unsaturated fatty acids in vegetable oils — 80–90%
• Saturated fatty acids in vegetable oils — 10–20%
• Saturated acids in animal fats — 40–60%
• Monounsaturated acids in animal fats — 30–50%
• Oleic acid in olive oil — 79%
• Linoleic acid in sunflower oil — 75%

The main part of fatty acids in a plant is bound within complex lipids — triglycerides and phospholipids. Free fatty acids account for only a small fraction of their total volume. Their excessive accumulation in plant tissues indicates destructive processes.

The length of the hydrocarbon chain of fatty acids in plants usually ranges from C14 to C22. In higher plants, the number of carbon atoms in saturated fatty acids is always even, whereas an odd number is found only in some marine algae. Most often, saturated acids C16 (palmitic) and C18 (stearic) accumulate in plants. Among monounsaturated acids, C16 (palmitoleic) and C18 (linolenic) are the most common.

Unsaturated fatty acids contain one (monoenoic) or several (polyenoic) double bonds. In a plant organism, they follow strict rules of spatial organization. When analyzing the structure of these acids, the following patterns are distinguished:

• double bonds are almost always in the cis-conformation;
• if there is only one bond, it is usually located between the 9th and 10th carbon atoms, where the terminal carbon atom of the carboxyl group is taken as position 1;
• additional bonds are located between the 10th carbon and the methyl end of the chain, and at least one –CH₂ group is necessarily located between each pair of double bonds.

Unbranched fatty acids with an odd number of carbon atoms are extremely rare in the plant world. Also, specific acids containing cyclic structures have been isolated from plants: sterculic (with a cyclopropene ring) and chaulmoogric (with a cyclopentene ring). For comparison, lactobacillic acid, which contains a cyclopropane ring, has been found in only three organisms.

Chemical Structure and Classification of Plant Lipids

By chemical structure, plant lipids are derivatives of monobasic higher carboxylic acids, alcohols, and aldehydes. They are united into a single structure by ether, ester, phosphoester, or glycosidic bonds. Molecules are formed as a result of the esterification of fatty acids with compounds containing hydroxyl groups. Usually, this base is the trihydric alcohol glycerol or the amino alcohol sphingosine.

The properties of the finished lipid directly depend on its chemical composition. Thanks to the combination of various elements, the cell is capable of synthesizing compounds for specific physiological tasks. In addition to fatty acids, the following can attach to the alcohol component:

• amino groups;
• residues of other acids;
• carbohydrates.

A characteristic feature of the lipid molecule is its two-part structure. It consists of a polar "head," which carries an electric charge and takes up no more than a quarter of the total length of the molecule, and long uncharged hydrocarbon "tails." A glycerol residue most often acts as the connecting link between them.

The polar heads of all natural lipids have an important feature: they are either negatively charged or neutral (due to the simultaneous presence of positive and negative charges). Exclusively positively charged heads are not found in the plant world. Due to the vast diversity of these compounds, a unified classification of lipids has not yet been established. In laboratory and field practice, they are divided into groups by evaluating the degree of hydrophobicity or solubility in various media.

According to the most common classification by R.P. Evstigneeva et al. (1976), all lipids are divided into three groups.

Neutral lipids. These include derivatives of higher fatty acids, alcohols, and aldehydes with the general formula

Such compounds have hydrophobic properties. These are triglycerides (R–acyl), neutral plasmalogens (R–OCH=CHR), alkyldiacylglycerides (R′–alkyl), and glycosyldiglycerides (R″– glycosyl residue). Neutral lipids also include diol lipids, simple and complex esters of cholesterol, and glycolipids of various natures.

Phospholipids (glycerophospholipids). The molecules of these compounds are derivatives of glycerol and contain hydrophobic residues R and R′, as well as a hydrophilic component X.

The hydrophilic component of phospholipids consists of phosphoric or phosphonic acids and associated alcohols (choline, ethanolamine), amino acids, and polyhydric alcohols.

Sphingolipids. They are distinguished by the presence in the molecule of an amino alcohol residue, which is called a sphingosine base. The general formula for sphingolipids in sphingolipid molecules as a substituent X can be:

• phosphorylethanolamine;
• a mono- or oligosaccharide residue.

According to the classification by I.V. Savitsky (1973), lipids are divided into four groups:

• Simple lipids. These are esters of fatty acids with various alcohols. These include fats – esters of fatty acids with glycerol (glycerides) and waxes – esters of fatty acids with monohydric aliphatic alcohols. Waxes include true waxes (esters of cetyl or another straight-chain higher alcohol and palmitic, stearic, oleic, or another higher fatty acid), as well as esters of cholesterol, vitamin A, vitamin D, and higher fatty acids.
• Complex lipids. These are esters of fatty acids with alcohols that contain other substituents on the hydroxyl group. These include:
  • a) phospholipids. Molecules of these compounds, as a rule, contain fatty acids, glycerol, phosphoric acid, and nitrogenous bases;
  • b) glycolipids (cerebrosides). Their molecules do not contain glycerol and phosphate, but they contain both fatty acids and a carbohydrate component;
  • c) sulfolipids. They are characterized by the presence of sulfur in the molecule;
  • d) lipoproteins – complex compounds of lipids with proteins;
  • e) lipopolysaccharides – complex complexes of lipids with polysaccharides.
  • a) fatty acids (saturated and unsaturated);
  • b) mono- and diglycerides;
  • c) glycerol, sterols, some steroids (vitamin D group), alcohols containing a β-ionone ring (vitamin A group);
  • d) fatty aldehydes;
  • e) the lipid part of lipoproteins.

  We present another classification of lipids, proposed by A. White, F. Handler, E. Smith, et al. (1981):

  • Glycerol-containing lipids. These are neutral fats (mono-, di-, and triacylglycerols, simple esters of cholesterol, glycosylglycerides) and phosphoglycerides (phosphatides, diphosphatidylglycerides, and phosphoinositides).
  • Lipids not containing glycerol: sphingolipids (ceramides, sphingomyelins, glycosphingolipids), aliphatic alcohols and waxes, terpenes, and steroids.
  • Lipids associated with substances of other classes: lipoproteins, proteolipids, phosphatidopeptides, lipoamino acids, and lipopolysaccharides.

  Lipids can be divided into two groups: 1) fats and 2) fat-like substances, so-called lipoids.

  Simple lipids include substances whose molecules consist of fatty acid (or aldehyde) residues and alcohols. These include fats (triglycerides and other neutral glycerides), waxes (esters of fatty acids and fatty alcohols), and diol lipids (esters of fatty acids and ethylene glycol or other dihydric alcohols).

  Complex lipids include derivatives of orthophosphoric acid (phospholipids) and lipids containing sugar residues (glycolipids). Lipids also include some substances that are not derivatives of fatty acids – sterols, ubiquinones, terpenes.

  Based on their structure and ability to undergo hydrolysis, lipids are divided into saponifiable and non-saponifiable. Saponifiable lipids form several structural components upon hydrolysis, and when reacting with alkalis, they form salts of fatty acids (soap).

  The group of non-saponifiable lipids includes steroids and terpenoids – the main component of essential oils obtained from plants:

  • lemon;
  • peppermint;
  • geranium;
  • turpentine, etc.

  The following classification is most acceptable for plant lipids: