Lipids or Fats: Definition, Function, Types, Classification of Fatty Acids and Differences Between Healthy and Unhealthy Fats

They are one of the four introductory classes of molecules that the body needs to perform its vital functions.

The other classes are nucleic acids, proteins, and carbohydrates. Different types of lipids include waxes, phospholipids, and steroids.

Lipids are any of a diverse group of organic compounds that include fats, oils, hormones, and specific membrane components that clump together because they do not interact appreciably with water.

One type of lipid, triglycerides, is sequestered as fat in adipose cells, which serve as energy storage reservoirs for organisms and also provide thermal insulation.

Some lipids, such as steroid hormones, serve as chemical messengers between cells, tissues, and organs, and others communicate signals between biochemical systems within a single cell.

Cell membranes and organelles (structures within cells) are microscopically thin structures of two layers of phospholipid molecules.

Membranes work to separate individual cells from their environments and to classify the interior of the compartment into structures that carry out particular functions.


So important is this compartmentalization function that membranes, and the lipids that make them up, must have been essential to the origin of life itself.

Water is the biological medium, the substance that makes life possible. Almost all the molecular components of living cells, whether found in animals, plants, or microorganisms, are soluble in water.

Molecules such as proteins, nucleic acids, and carbohydrates have an affinity for water and are called hydrophilic (“water-loving”).

Lipids, however, are hydrophobic (“fearful of water”).

Some lipids are amphipathic: part of their structure is hydrophilic, and another part, usually a more extensive section, is hydrophobic.

Amphipathic lipids exhibit a unique behavior in water: they spontaneously form ordered molecular aggregates, with their hydrophilic ends on the outside, in contact with water, and their hydrophobic parts on the inside, protected from moisture.

This property is key to their role as the building blocks of cell membranes and organelles.

Although biological lipids are not large macromolecular polymers (e.g., proteins, nucleic acids, and polysaccharides), many are formed by the chemical bonding of several small constituent molecules.

Many of these molecular building blocks have a similar or homologous structure.

Homologies allow classifying lipids into some main groups: fatty acids, fatty acid derivatives, cholesterol, and their derivatives, and lipoproteins.

Function of lipids

Lipids are typically small, nonpolar, and generally insoluble in water.

Lipids have several different and essential functions in the body.

Phospholipids form membranes, which separate the internal contents of a cell from its environment. Due to the lipid content, these membranes are fluid and not too rigid.

Lipids also serve as a fuel source in the body; fat molecules are capable of storing even more energy than carbohydrates.

Fat also has a cushioning function for internal organs, protecting and isolating them.

One type of lipid, steroids, behave as communication substances in the body. For example, estrogen and testosterone are essential steroids that are hormones. These travel around the body and activate processes to develop and reproduce.

Lipids or fats

When organisms store glucose molecules for long periods, they generally store them as fats rather than carbohydrates.

Fats are large molecules made up of carbon, hydrogen, and oxygen, just like carbohydrates, but their hydrogen to oxygen ratio is greater than 2: 1.

For this reason, fats contain more carbon-hydrogen bonds that store more energy than carbohydrates.

Also, fats are nonpolar and insoluble molecules, working well as storage molecules.

Lipids include a wide variety of molecules, which are soluble in oil but insoluble in water.

This insolubility is because almost all bonds in lipids are nonpolar carbon-carbon or carbon-hydrogen bonds.

Three important categories of lipids are:

  • The oils, fats, and waxes.
  • Phospholipids.
  • Steroids.

Types of lipids

Lipids are compound molecules; they are made up of more than one component. Oils and fats are built from two different types of subunits:


Glycerol is a three-carbon molecule, and each carbon has a hydroxyl group (- OH). The three carbons form the backbone of the fat molecule.

Fatty acids

For a lipid, the monomer structure depends on the type of lipid. With fats, the monomer is something called a fatty acid.

Fatty acids have long hydrocarbon chains (consisting only of carbon and hydrogen atoms) that end in a carboxyl group (- COOH).

A fatty acid with only single bonds between its carbon atoms can contain more hydrogen atoms than a fatty acid with double bonds between its carbon atoms.

This is a straight chain of carbon and hydrogen (hydrocarbon) atoms with a carboxyl group attached to one end.

When three fatty acids combine and are attached to each glycerol backbone, the resulting fat molecule is called a triglyceride because there are three fatty acids.

The long chain of hydrocarbons makes the molecule predominantly hydrophobic, so it is not soluble in water.

Most of the fats in the diet are triglycerides, a rich type of lipid.

The difference between fats and oils is the number of double bonds in their fatty acids.

A fatty acid that carries as many hydrogen atoms as possible.

Classification of fatty acids

Fats made of fatty acids with double bonds are unsaturated because the double bonds replace some of the hydrogen atoms. If fat has more than one double bond, it is called polyunsaturated.

Saturated fatty acids

The simplest fatty acids are unbranched linear chains of CH 2 groups linked by carbon-carbon single bonds with a terminal carboxylic acid group.

The term saturated indicates that the maximum possible number of hydrogen atoms is attached to each molecule’s carbon.

Many saturated fatty acids have a standard or trivial name, as well as a chemically descriptive systematic name.

Systematic names are based on numbering the carbon atoms, starting with the acidic carbon.

Although the chains of saturated fatty acids are usually between 12 and 24 carbons long, several shorter chain fatty acids are biochemically essential.

For example, butyric acid (C 4) and caproic acid (C 6) are lipids found in milk.

Palm kernel oil, an essential dietary source of fat in some world regions, is rich in fatty acids containing 8 and 10 carbons (C 8 and C 10).

Unsaturated fatty acids

Unsaturated fatty acids possess one or more carbon-carbon double bonds.

The term unsaturated indicates that less than the maximum possible number of hydrogen atoms are attached to each carbon in the molecule.

The number of double bonds is indicated by the generic name: monounsaturated for molecules with one double bond or polyunsaturated for molecules with two or more double bonds.

Oleic acid is an example of a monounsaturated fatty acid.

The prefix cis-9 in the systematic name palmitoleic acid indicates that the position of the double bond is between carbons 9 and 10.

Two possible conformations, cis and trans, can be taken up by the two CH 2 groups immediately adjacent to the double-bonded carbons.

In the cis configuration, which occurs in all biological unsaturated fatty acids, the two adjacent carbons are on the same side of the double-bonded carbons.

In the trans configuration, the two adjacent carbons are on opposite sides of the double bond carbons.

Fatty acids that contain more than one carbon-carbon double bond (polyunsaturated fatty acids) are found in relatively minor amounts.

A CH 2 group almost always separates multiple double bonds (―CH 2 ―CH = CH – CH 2 ―CH = CH – CH 2 -), a regularly spaced motif that is the result of the biosynthetic mechanism. The bonds are introduced in the hydrocarbon chain.

The most common polyunsaturated fatty acids are linoleic and arachidonic. Arachidonic acid (C 20) is of particular interest as the precursor to a family of molecules known as eicosanoids (from the Greek Oikos, “twenty”), which includes prostaglandins, thromboxanes, and leukotrienes.

Under certain conditions, these compounds produced by cells have powerful physiological properties.

Animals cannot synthesize two crucial fatty acids, linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid), which are the precursors of eicosanoids precursors must obtain in the diet from plant sources.

For this reason, these precursors are called essential fatty acids.

Trans polyunsaturated fatty acids, although not biosynthetically produced by mammals, are produced by microorganisms in the intestines of ruminant animals such as cows and goats and are also produced synthetically by partial hydrogenation of fats and oils in the manufacture of margarine (the called trans fats).

There is evidence that ingesting trans fats can have detrimental metabolic effects.

Fats in food


Saturated fat has only one single bond in its carbon chains. This means that saturated fat has as many hydrogen atoms attached as possible. This type of grease is generally solid at room temperature because its chains pack well.

It can be found in foods of animal origin such as butter, red meat, cheese, sausages, milk, and its derivatives (yogurt, ice cream) made with whole milk dairy.


Unsaturated fat has at least one or more double bonds in its long carbon chains.

In general, these foods are liquid when they are at room temperature and are found in foods of plant origin such as vegetable oils: olive, sunflower, or corn oil, also in dried fruits such as walnuts, almonds among others, and seeds like sesame, sunflower, flax.


One type of unsaturated fat is monounsaturated fat. They only have one double bond in their long carbon chain.

They are present in seed oils such as sunflower oil and rapeseed oil. They are also found in walnuts, almonds, and avocados.


Polyunsaturated fat has at least one fatty acid chain with more than one double bond.

There are two families within these:

  • Omega 3s are found in soybean oil, rapeseed oil, dried fruits (walnuts), and fatty fish such as salmon, herring, tuna, mackerel, anchovy, and sardines.
  • Omega 6 is found in foods such as soybean oil, corn oil, sunflower oil, and dried fruits (walnuts, among others).

Trans fats are generally created in commercial food production.

The word “trans” refers to how the atoms are arranged around the double bond.

They look similar to saturated fats and can be packed together closely.

This type of fat is found in foods like fried foods, baked goods like biscuits, muffins, cookies, and other processed foods.

“Healthy” fat versus “bad” fat

We can consume many fats, such as the so-called good fats, which are even necessary for the body to function correctly.

When buying food at the grocery store, look for foods with unsaturated fats.

These fats are suitable for the body because they lower total cholesterol in the blood.

The fats that should be eaten in moderation are saturated and trans.

Trans fats are not broken down by our body’s enzymes, which is a big problem.

Saturated and trans fats can increase the risk of heart disease and raise total blood cholesterol levels.

The World Health Organization recommends a fat consumption with a percentage between 15 and 30% of the total daily caloric intake.

Special care must be taken in the quality and type of fat consumed to reduce the risks of cardiovascular diseases, the consumption of saturated fats (a percentage below 10%), and trans-fatty acids (for below 1%).

You should limit the intake of saturated fats, promote the consumption of unsaturated fats and reduce the consumption of trans fatty acids to the lowest possible percentages.

This is achieved by carefully selecting the type of fat that is consumed, buying the right foods, and regulating the size of the servings that are consumed.