Phospholipids: Definition, Structure, Functions, Types, Clinical Uses and Food Industry

Biological membranes have certain common structures such as the phospholipid double layer that acts by preventing the entry of polar ions and molecules.

Phospholipids are main components of cell membranes and therefore their antipathies and the characteristics of their fatty acids (carbon number, the presence of double bonds) confer many of their properties.

There are several lipids that can be part of this membrane structure. In us humans, we have the glycerophospholipids, galactolipids, sulpholipids, and sphingolipids .

Structure

A phospholipid is made up of a glycerol attached to two fatty acids and a phosphate group. The fatty acids attached to glycerin can vary, but generally one of them is saturated. Phosphate can bind to different chemical groups.

Phospholipids are amphiphilic lipids, which means that one side is hydrophilic (the glycerol phosphate portion) while the other is hydrophobic (fatty acids). Therefore, if they are placed in water, the phospholipids will organize in such a way that only the hydrophilic zone will be in contact with the water.

If they are forced to mix with this liquid, they will form a lipid bilayer in which the hydrophobic fatty acids will face each other. The plasma membranes of cells and organelles are furthermore constituted in this way.

Mixed with water, phospholipids can form tiny microscopic spheres called liposomes. Each liposome is a small hollow sphere delimited by a double layer of phospholipids.

Liposomes can be artificially synthesized. We started using them in medicine to transport certain drugs into cells. Liposomes can fuse with cell membranes and release the drug they contain into the cell.

They are also used to make synthetic vaccines. The cosmetic industry also makes significant use of it (skin creams).

Phospholipids can also form a single layer that covers tiny lipid droplets. These structures, called micelles, allow lipids to remain suspended in the water. The hydrophobic portion of the phospholipid is bound to fat, while the hydrophilic portion, directed outward, is bound to water.

Phospholipids are not true “fats” because they have one of the fatty acids replaced by a phosphate group. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; An exception to this rule is sphingomyelin, which is derived from sphingosine rather than glycerol.

The first phospholipid identified as such in biological tissues was lecithin or phosphatidylcholine, in egg yolk, by Theodore Nicolas Gobley, a French chemist and pharmacist, in 1847.

Functions of phospholipids

As components of the membrane, phospholipids are selectively permeable (also called semipermeable), which means that only certain molecules can pass through them to enter or exit the cell.

Molecules that dissolve in fat can pass easily, while molecules that dissolve in water cannot. Oxygen, carbon dioxide, and urea are some molecules that can easily cross the cell membrane.

Large molecules like glucose or ions like sodium and potassium can’t get through easily. This helps keep the cell contents working properly and separates the cell interior from the surrounding environment.

Phospholipids can be broken down in the cell and used for energy. They can also be divided into smaller molecules called chemokines, which regulate a variety of activities in the cell, such as the production of certain proteins and the migration of cells to different areas of the body.

In addition, they are found in areas such as the lung and in the joints, where they help lubricate cells.

Clinical uses

In pharmaceuticals, phospholipids are used as part of drug delivery systems, which are systems that help transport a drug throughout the body to the area it is intended to affect. They have high bioavailability, which means that they are easy for the body to absorb.

Valium is an example of a drug that uses a phospholipid-based drug delivery system.

In the food industry, phospholipids can act as emulsifiers, which are substances that disperse oil droplets in water so that oil and water do not form separate layers. For example, egg yolks contain phospholipids and are used in mayonnaise to keep it from separating.

Phospholipids are found in high concentrations in many other animal and plant sources, including soybeans, sunflowers, cottonseed, corn, and even cow brains.

Types

Glycerophospholipids

Glycerophospholipids, also called phosphoglycerides, are phospholipids derived from the parent compound phosphatidic acid. Membrane lipids are made up of two fatty acids attached to the first and second glycerol carbon by an ester bond and the polar group and is linked by a phosphodiester bond at the third carbon.

Sphingolipids

Sphingolipids are a class of membrane lipids made up of an amino alcohol molecule, sphingosine (long chain) or one of its derivatives, a long chain fatty acid and the polar head joined by a glycosidic linkage or phosphodiester linkage.

Among the sphingolipids there is one that is classified as a phospholipid, they are sphingomyelins. The polar group of this phospholipid contains phosphocholine or phosphoethanolamine, closely resembling phosphatidylcholines in three-dimensional structure, without having liquid charges in their head group.

In addition to being present in the plasma membranes of animals, sphingomyelins are elevated in myelins, hence their name.

Cardiolipin

Cardiolipin is a diphosphatidylglycerol, a glycerol molecule that binds to two phosphatidic acid molecules. It is a very important phospholipid that is present in the inner membrane of the mitochondria of plants and animals.

Cell membranes normally have their lipids replaced. The degradation of phospholipids occurs in lysosomes where there are specific enzymes for the hydrolysis of each one.

One or two fatty acids are removed by type A phospholipases, as a product we have a lysophospholipid. Lipophospholipates enter the scene to remove the remaining fatty acids.