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Acylglycerides (or fats)

The acylglycerides or fats or are saponifiable lipids formed by the esterification of one, two or three molecules of fatty acids with a glycerol molecule (propanetriol), releasing one, two or three water molecules. They are also called glycerides or glycerolipids or acylglycerols.

Three types are distinguished, according to the number of fatty acids that make up the acylglyceride molecule:

  • Monoacylglyceridescontain a fatty acid molecule.
  • Diacylglycerideswith two fatty acid molecules.
  • Triacylglycerideswith three fatty acid molecules.

Formación de triglicéridos

By Iacopo Leardini. Versión en español de Alejandro Porto. (File:Trigliceridi.gif) [CC0], via Wikimedia Commons

Fats, at room temperature, can be:

  • Oils: when the acylglyceride has at least one unsaturated fatty acid, its state is liquid at room temperatureFor example, olive oil is an ester of three oleic acids with a glycerin.
  • Tallowwhen all the fatty acids are saturated, the acylglyceride is solid at room temperature. For example, beef, horse or goat fat.
  • Butters: when the acylglyceride is semisolid (short chain fatty acids), such as pork fat.

In cold-blooded animals and vegetables there are oils, and in warm- blooded animals there are tallow or butters.

The acylglycerides are not soluble in water because the groups hydroxyl  (-OH) of glycerol, polar , are linked by an ester linkage to the groups carboxyl  (-COOH) of fatty acids. The triacylglycerols are considered neutral fats because they are nonpolar and insoluble in water. The monoacylglycerides and diacylglycerides have weak polarity by the radicals hydroxyl left free in the glycerin.

The triacylglycerols constitute a form of energy storage in animal cells and plant . They can be stored in large quantities in confined spaces without disturbing cellular homeostasis. That is why it is a more effective form of energy reserve than that of carbohydrates.

Acylglycerides when they react with bases, produce soap by saponification .

The saponification  is the formation of soap from saponifiable lipids by chemical reaction between a triacylglycerol and a base (NaOH or KOH). In the reaction, glycerin is released and three molecules of sodium or potassium salts (soaps) corresponding to fatty acids are formed.

Animated image: Formation of a triglyceride.

Video: What is fat ?. The type of fat you eat is more important in your diet than the amount.

Functions of acylglycerides

Acylglycerides are the main energy reserve in both animals and plants. In plants, they accumulate in vacuoles, and in animals, in adipocytes, specialized cells of adipose tissue.

Although starch and glycogen are the main sources of energy due to their rapid mobilization as they are soluble in water, fats provide much more energy (9 Kcal / g).

Acylglycerides also act as thermal insulators. For example, by forming a layer of fat under the skin of some homeothermic animals that live in cold climates to be able to live with low temperatures. Other hibernators accumulate fat under the skin and around organs to use as food during the hibernation period.

They are good mechanical shock absorbers. They absorb the energy of blows and, therefore, protect sensitive structures or structures that suffer continuous friction.

Fundamental ideas about fats or acylglycerides

The triglycerides, fats or triacylglycerols, are formed by the esterification of one molecule of glycerol with three molecules of fatty acids, which may be identical or different.

The triacylglyceride is formed by the union of the carboxyl groups (–COOH) of three fatty acids with the three hydroxyl groups (-OH) of a glycerol molecule, forming three ester bonds and the release of three water molecules

Depending on how these fatty acids are, triacylglycerides can be:

  • Saturated fats, when the fatty acids are saturated, without presenting double bonds between the carbons of the hydrocarbon chain.
  • Monounsaturated fats, when the fatty acids are monounsaturated, containing a double bond.
  • Polyunsaturated fats, when the fatty acids are polyunsaturated, containing several double bonds.
The main function of triacylglycerides is that of energy reserve, in animals and plants. They also act as thermal insulators, as a food store and as mechanical protection for certain organs, such as the heart and kidneys.
Triacylglyceride can be saponified when subjected to alkaline hydrolysis with  NaOH and KOH solutions, producing soap, a sodium or potassium salt (stearate), and glycerin, which is an alcohol.
Have you ever asked in college entrance exams for the difference in triglycerides and phosphoglycerides or phospholipids:
  • Triglycerides: formed by the union of glycerin and three fatty acids.
  • Phosphoglycerides or phospholipids: formed by the union of glycerin, two fatty acids, phosphoric acid and an amino alcohol (serine).