what are lipids

The lipids are hydrophobic and water-insoluble moleculescomposed mainly of carbon, oxygen and hydrogen and generally attached to chains of carbohydrates called fatty acids.

In this way, most lipids at the biological level are classified as saponifiable lipids, that is, they are formed by fatty acids.

lipids are called simple lipids when only carbon, oxygen and hydrogen molecules are found in its composition, such as fats, oils and waxes.

On the other hand, they are called complex lipidsto those whose structure contains other elements in addition to those that make up simple lipids, such as, for example, the phospholipids of the plasmatic membrane, which also contain a modified phosphate group.

Lipid functions

The different types of lipids that exist in the body, in general, have the main function of storing energy. In this sense, each gram of lipid contains twice the energy of a carbohydrate, for example.

In the animal kingdom, lipids also have the function of providing thermal insulation and it is a fundamental unit for the formation of:

  • vitamins and their absorption, such as vitamins A, D, K and E,
  • hormones such as testosterone and estradiol,
  • bile acids that aid digestion,
  • plasma membranes, composed of specialized lipids called phospholipids.

In addition, in humans, some lipids, such as essential fatty acids, regulate inflammation and mood, reduce the risk of sudden death from heart attacks, lower blood triglycerides, lower blood pressure and It prevents formation of blood clots.

On the other hand, lipids in the form of wax help in the waterproofing function of leaves in plants and feathers in birds.

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Examples and types of lipids

At a biological level, the most important simple lipids are divided into fats, oils and waxes, and within the complex lipids we can find phospholipids and steroids.

The fats

Fats are one of the best known types of lipids. They are considered simple lipids because they are made up of carbon, oxygen and hydrogen and they are called saponifiable lipids because they are made up of fatty acids.

Fats are made up of a glycerol backbone and at least one fatty acid linked by an ester bond (C=O). Depending on the amount of fatty acid tails, they are classified as monoacylglycerides (1 fatty acid), diacylglycerides (2 fatty acids), or triacylglycerides (3 fatty acids).

Fats are characterized by their single-bonded saturated fatty acids that give them strength, such as the specialized fat cells called adipocytes that make up adipose tissue and butter.


Oils are simple, saponifiable lipids. They are characterized by being liquid due to their tails of unsaturated fatty acids with double bonds in the cis configuration. Examples of them we can find essential fatty acids, also known as omega fatty acids.


Waxes are simple and saponifiable lipids whose structure is generally made up of long chains of fatty acids linked to alcohols (glycerin) by means of ester bonds (C=O). Waxes can be found in plant leaves and bird feathers that give it hydrophobic properties.


Phospholipids are complex lipids, since, in addition to its glycerin skeleton and its 2 fatty acid tails, it has a modified phosphate group. Phospholipids are specialized lipids and are major components of the plasma or cell membrane.

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They make up the phospholipid bilayer of the cell membrane, where the fatty acid tails form the hydrophobic part of the layer between the hydrophilic heads of the phosphate groups.


Steroids are complex lipid molecules, as their structure is made up of 4 fused carbon rings. Steroids share the hydrophobic characteristics of lipids, such as their insolubility in water. Examples of steroids are cholesterol, synthesized mainly by the liver, and the raw materials of sex hormones such as testosterone.

Chemical structure of lipids

lipid structure

Most lipids, whether fats, oils, waxes, or phospholipids, are composed of a glycerol backbone (C3H8EITHER3) or also known as glycerin, an alcohol composed of 3 hydroxyl groups (OH).

In general, the hydroxyl groups of glycerol are attached to fatty acids by ester bonds (C=O) in a reaction called dehydration synthesis. Lipids made up of fatty acids are called saponifiable lipids.

Depending on the amount of fatty acids that bind to the glycerol molecule, the following types of lipids will be obtained:

  • Monoacylglycerides: 1 fatty acid tail attached to 1 glycerin molecule,
  • diacylglycerides: 2 fatty acid tails attached to 1 glycerin molecule,
  • Triacylglycerides: 3 fatty acid tails attached to 1 glycerin molecule

fatty acid structure

Fatty acids form the tails of saponifiable lipids, which make up the majority of lipids. Fatty acids are long carbohydrate chains (4 to 36 carbons) attached to a carboxyl group.

Fatty acids are classified into saturated and unsaturated:

saturated fatty acids

Saturated fatty acids are composed of single bonds between neighboring (C) carbons. It is called saturated because it is saturated with hydrogen molecules (H), that is, the carbons are attached to as many hydrogens as possible.

Single bonds generate straight, tight tails that are characteristic of solid fats with high melting points, such as butter.

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unsaturated fatty acids

The structure of unsaturated fatty acids is formed by double bonds which means that they have fewer hydrogens (H). Unsaturated fatty acids that contain 1 double bond are called monounsaturated and those that have multiple double bonds are called polyunsaturated.

Depending on the type of configuration of the double bonds of fatty acids, there are cis double bonds and trans double bonds.

The cis double bonds which are characterized by bonding to 2 hydrogens on the same side. These types of bonds are typical of liquid lipids or oils, as they have a low melting point, such as olive oil.

Another example are the essential fatty acids, so called because they are necessary in the diet of the human body, since it does not synthesize them naturally. Essential fatty acids are unsaturated and contain at least 2 cis bonds. Among them we can find those derived from alpha-linolenic acid (ALA), known as omega-3, and linoleic acid (LA), called omega-6.

The trans double bonds, on the other hand, are characterized by joining their 2 hydrogens but which are located on opposite sides. This type of fatty acids are obtained from industrial procedures called partial hydrogenation that converts double bonds into single bonds in order to give oils solid properties like those of saturated fats, such as vegetable shortenings.

See also Biomolecules and See also Food Pyramid.