Lipids: What Are They and What Are They For?

Lipids are molecules with multiple forms and properties. From cell membranes to sex hormones, these compounds leave no physiological ground uncovered.

Reviewed and approved by el biólogo Samuel Antonio Sánchez Amador.

Written by Samuel Antonio Sánchez Amador

Last update: 26 May, 2023

Lipids are a group of organic molecules, for the most part, composed of carbon and hydrogen —in addition to oxygen, nitrogen, sulfur, and phosphorus, among others. It’s important to note that, together with proteins and carbohydrates, lipids are considered one of the main macronutrients in the diet.

According to dietary associations, these nutrients are the ones that act as the most efficient energy reservoir in the body. Each gram of fat provides twice the calories of glycogen—a carbohydrate—and takes up less space. So it isn’t surprising to learn that 99% of a fat tissue cell is made up of lipids.

We’re facing a group of organic molecules that contain an exciting complexity, since from their chemical structure to various dietary concepts, lipids have many essential functions for the survival of organisms. In this opportunity, we’ll tell you everything about them.

Lipids: chemical structure

A mnemonic rule that is usually taught in schools to remember the chemical structure of lipids is the following: CHONSP. This strange word refers to the following terms:

• Carbon
• Hydrogen
• Oxygen
• Nitrogen
• Sulfur
• Phosphorus

The first three (CHO) are essential in the lipid “skeleton”, while N, S and P are present in some cases and not in others. To understand the chemical composition of lipids, it’s necessary to go to their essential functional unit: the fatty acid.

Fatty acids

A fatty acid is defined as a biomolecule formed by a linear hydrocarbon chain. This means that, in effect, its skeleton is made up of carbon and hydrogen. Although they’re found free in small amounts in nature, in general these molecules are obtained by hydrolysis of more complex lipids.

At one end of this chain we find a CH3 group, which corresponds to three free bonds occupied by hydrogen atoms. At the other extreme is a COOH carboxyl group, which will act as a “glue” for the union of the fatty acid with glycerin or propanetriol, giving rise to the most famous type of lipid, which we will see in later lines.

Although it may be difficult for us to imagine this biomolecule, informative portals show us its three-dimensional form in an interactive way. Be that as it may, a general formula for the fatty acid would look like this:

CH ₃ –(CH ₂ ) _n –COOH

The ” n ” outside the parentheses in the formula may have caught your eye, right? This refers to the number of repetitions of the basic unit of the hydrocarbon chain. In general, fatty acids have between 12 and 24 carbons, with 16 and 18 being especially typical.

Types of fatty acids and properties

We can’t continue discovering more about the world of lipids without first describing the types of fatty acids and their properties. According to educational portals, there are two main groups. We can summarize them briefly:

• Saturated: All the bonds that join the atoms in the chain are simple.
• Unsaturated: These have a double or triple bond in their structure. In this group, the most common are monounsaturated fatty acids, with a single double bond between carbons 9 and 10.

Now we’re going to point out some of the physicochemical properties of fatty acids:

1. They’re amphipathic: The hydrocarbon chain is hydrophobic and the carboxyl group (COOH) is hydrophilic. This gives fatty acids an insoluble character to water and, in addition, allows the formation of various cellular structures in living beings —such as the lipid bilayer.
2. Melting point: This is defined as the amount of energy needed for the material to transform into a liquid state. This value varies greatly between saturated and unsaturated fatty acids. For example, stearic acid has a melting point of 71 degrees and oleic acid has a melting point of 16.3 degrees.
3. Esterification: If a fatty acid is combined with an alcohol, an ester is obtained and a water molecule is released. This process is essential for the formation of certain types of lipids.
4. Saponification: The union of a fatty acid with a strong base —such as NaOH— gives rise to soap and a water molecule.

Lipid types

We’ve told you in previous lines that the basic functional unit of the lipid is the fatty acid, but the truth is that not all have it. According to this criterion —that is, if they’re saponifiable or not—we can distinguish two different types of lipids. We’ll tell you about them in the following lines.

1. Saponifiable lipids

These are lipids that, through a process of hydrolysis (the rupture of the molecule in the presence of water) fatty acids are produced. Therefore, in the presence of strong bases, they can give rise to the soaps described above. In this group we can find a molecular variety of vertigo, so we’ll try to summarize the conglomerate in a few terms.

In the first place we have simple lipids, that is, esters of fatty acids and alcohol. These are divided, in turn, into two groups.

• Acylglycerides: Based on the union of a glycerin molecule with one (monoglyceride), two (diglyceride) or three (triglyceride) fatty acids. Here we have fats, sebum and oils, that is, the energy reserves in living beings.
• Cerides: These are esters of a fatty acid with a long-chain linear monohydric alcohol. A very clear example is the wax produced by bees.

In the second instance, we find complex lipids, that is, those that contain other elements beyond carbon, hydrogen, and oxygen. This is where the NSP part of the above-described mnemonic (CHONSP) comes in. According to the elements that they present in their chemical structure, these lipids are divided into the following categories:

• Phospholipids: 1 glycerin molecule + 2 fatty acids + 1 phosphoric acid. They’re the essential units of the structure of the cell membrane of living beings.
• Phosphoglycerides: These are esters that contain phosphoric acid instead of a fatty acid.
• Sphingolipids: 1 alcohol (sphingosine) + 1 fatty acid + 1 phosphoric acid + 1 amino alcohol. Some of them have essential roles in cell signaling.
• Glycolipids: 1 ceramide + 1 short-chain carbohydrate. Here we find gangliosides and cerebrosides.

If we want to get something across with this information, it’s the incredible variety of lipids present in nature. Even so, there’s still one more category to describe: non-saponifiable lipids.

2. Non-saponifiable lipids

These are the ones that aren’t composed of fatty acids, which means they can’t produce soaps through the saponification reaction. We’ll quickly mention the different types within this category:

• Terpenes: Lipids derived from isoprene hydrocarbon. Depending on the number of isoprene molecules they contain, they’ll receive different names —from monoterpenes to polyterpenes. Vegetable essential oils, carotenoids, and rubber are examples of terpenes.
• Sterols: Lipids derived from cyclopentanoperhydrophenanthrene. The name alone makes us dizzy, so we will limit ourselves to saying that sex hormones and cholesterol are compounds that fall into this group.
• Prostaglandins: Lipids derived from omega-3 and omega-6 20-carbon essential fatty acids. They’re mediators in inflammation processes and certain interactions of the immune system.

Lipid functions

Now that we’ve explained the classification of lipids, we’re left to explore what their functions are. As we’ve already said, they’re a perfect energy reserve, since fats store more than twice as many calories as other organic compounds.

Despite what’s commonly believed, fats in themselves aren’t bad. The World Health Organization (WHO) recommends a fat intake that accounts for between 20-30% of the total caloric energy in the diet. Even so, there are certain observations to take into accountconcerningo this statement.

Medical portals such as the US National Library of Medicine emphasize that saturated fatty acids raise the levels of “bad cholesterol”. For this reason, these compounds shouldn’t account for more than 6% of the daily caloric intake. The products with the most beneficial lipids for the body are those with monounsaturated fatty acids, such as olive oil.

Beyond nutrition

Despite the nutritional importance of lipids, these organic molecules have much more diverse uses. To close this article, we’ll show you a list with some of them:

• Thermal insulators: Fats that protect living beings from inclement temperatures outside and allow them to balance their own.
• Mechanical protection: Fatty tissues protect bones and organs from blows and external forces.
• Lipids constitute between 50-60% of brain mass.
• They’re essential for tissue growth and regeneration.
• They’re part of the membranes of living beings at a cellular level.
• Biocatalytic function: Sex hormones, lipid vitamins, and bile acids are formed from lipids.

Molecules essential for life

From an evolutionary point of view, life as we know it today wouldn’t be possible without lipids. The cell is the functional unit of living beings and, without phospholipids, it couldn’t produce a cell membrane that differentiates it from the environment.

From a nutritional point of view, lipids form the group of macronutrients, along with proteins and carbohydrates. Finally, they’re also biocatalyst molecules of essential compounds for the human being. From sex hormones to the activation of the immune system, lipids form an essential part of our body.