Phospholipids – Definition, Structure, Classification, Functions of Phospholipids

Overview

Phospholipids are derivatives of phosphatidic acid. They are frequently associated with biological membranes. A phospholipid molecule is similar to a fat molecule. It contains glycerol and fatty acid chains.

Phospholipids have two fatty acid chains. The nitrogen-containing groups replace the third chain. Phospholipids consist of two parts; head and tail. The polar phosphate and nitrogen groups are soluble in water (hydrophilic). They form the head.

The insoluble non-polar hydrophobic fatty acid part forms the tail. Phospholipids have double tendency. They are soluble at one end and insoluble at the other. So they are major structural components of cell membranes.

Definition of Phospholipids 

These are made up of fatty acid, glycerol or other alcohol, phosphoric acid, and a nitrogenous base. Phospholipids are the significant lipid constituents of cell membranes. Like fats, phospholipids are amphipathic in nature, i.e., each has a hydrophilic or polar head (phosphate group) and a long hydrophobic tail (containing two fatty acid chains).

Structure of Phospholipids

A phospholipid is made up of 2 fatty acid tails and a phosphate group head. Fats are long chains that are mostly made up of hydrogen and carbon, while phosphate groups include a phosphorus molecule with 4 oxygen particles connected. These two parts of the phospholipid are connected by means of a third molecule, glycerol.

Phospholipids have the ability to form cell membranes due to the fact that the phosphate group head is hydrophilic (water-loving) while the fatty acid tails are hydrophobic (water-hating). They automatically arrange themselves in a certain pattern in water because of these properties, and form cell membranes.

To form membranes, phospholipids line up beside each other with their heads on beyond the cell and their tails on the inside. The 2nd layer of phospholipids also forms with heads facing the within the cell and tails dealing with away. In this way, a double layer is formed with phosphate group heads on the outdoors, and fat tails on the inside. This double layer, called a lipid bilayer, forms the main part of the cell membrane.

The nuclear envelope, a membrane surrounding a cell’s nucleus, is likewise comprised of phospholipids arranged in a lipid bilayer, as is the membrane of mitochondria, the part of the cell that produces energy.

Classification of Phospholipids

There are 2 classes of phospholipids:

1. Glycerophospholipids or phosphoglycerides, which contain glycerol as alcohol.
2. Sphingophospholipids contain sphingosine as alcohol.

Glycerophospholipids or Phosphoglycerides

Phospholipids derived from glycerol are called phosphoglycerides or glycerophospholipids. In glycerophospholipid, the hydroxyl groups at C1 and C2 of glycerol are esterified with two fats. The C3 hydroxyl group of the glycerol is esterified to phosphoric acid and a resulting compound called phosphatidic acid.

Phosphatidic acid is a key intermediate in the biosynthesis of other glycerophospholipids. In glycerophospholipid, the phosphate group of phosphatidic acid ends up being esterified with the hydroxyl group of among the several nitrogen base or other groups.

Various types of glycerophospholipids are discussed listed below.

Phosphatidylcholine (lecithin)

These are glycerophospholipids including choline. These are the most abundant phospholipids of the cell membrane having both structural and metabolic functions. Dipalmitoyl lecithin is an important phosphatidylcholine discovered in the lungs, produced by lung type II epithelial cells. It acts as a lung surfactant and is essential for normal lung function. It decreases surface area tension in the alveoli, therefore prevents alveolar collapse (adherence of the inner surface areas of the lungs).

Phosphatidylethanolamine (Cephalin)

They vary from lecithin in having nitrogenous base ethanolamine in place of choline. Thromboplastin (coagulation factor III), which is needed to start the clotting procedure, is made up mainly of cephalins.

Phosphatidylserine

It includes the amino acid serine rather than ethanolamine and is found in most tissues.

Phosphatidylinositol

In phosphatidylinositol, inositol is present as the stereoisomer myoinositol. Phosphatidylinositol is a second messenger for the action of hormonal agents like oxytocin and vasopressin.

Plasmalogens

Plasmalogens are typically similar to other phospholipids but the fat at C1 of glycerol is linked through an ether, instead of an ester bond. There are 3 major classes of plasmalogens:

1. Phosphatidylcholines.
2. Phosphatidylethanolamines.
3. Phosphatidylserines

These are found in myelin and in cardiac muscle. Plasmalogen is a platelet-activating factor (PAF) and involved in platelet aggregation and degranulation.

Lysophospholipids

Lysophospholipids are produced when one of the two fatty acids is eliminated from glycerophospholipid. The most common of these are lysophosphatidylcholine (lysolecithin) and lysophosphatidylethanolamine.

Cardiolipin (Diphosphatidylglycerol)

Cardiolipin is made up of two molecules of phosphatidic acid connected by a particle of glycerol. Two molecules of phosphatidic acid esterified through their phosphate groups with a particle of glycerol are called cardiolipin. Cardiolipin is a significant lipid of the mitochondrial membrane and is essential for the maximum function of the electron transport process. This is just a human glycerophospholipid that has antigenic properties.

Sphingophospholipids

Phospholipids originated from alcohol sphingosine instead of glycerol are called sphingophospholipids, e.g., sphingomyelin.

Sphingomyelin

Sphingomyelin is the only phospholipid in membranes that are not derived from glycerol. Rather, the alcohol in sphingomyelin is sphingosine, amino alcohol. In sphingomyelin, the amino group of the sphingosine is connected to fat to yield ceramide (sphingosine fatty acid complex). In addition, the primary hydroxy group of sphingosine is esterified with phosphorylcholine. Sphingomyelin is among the principal structural lipids of membranes of nerve tissue.

Functions of Phospholipids

• Phospholipids are the major lipid constituents of cell membranes.
• They control the permeability of membranes in addition to the activation of some membrane-bound enzymes.
• Phospholipids are of value in insulating the nerve impulse (like the plastic or rubber covering around an electric wire) from the surrounding structures, e.g., sphingomyelins function as electrical insulators in the myelin sheath around nerve fibers.
• Phospholipids are important constituents of lipoproteins.
• Phospholipids serve as a lipotropic element. The lipotropic factor is the part that prevents fatty liver, i.e., the build-up of fat in the liver.
• These are good emulsifying agents that assist in the digestive absorption of lipids.
• Thromboplastin (coagulation factor III), which is needed to start the clotting process, is made up generally of cephalins.
• Phospholipid (lecithin) acts as lung surfactant, which avoids alveolar collapse.
• Lecithin represents a storage type of lipotropic element choline.
• Phosphatidylinositol acts as a second messenger for the activity of specific hormones.
• In mitochondria, cardiolipin is needed for maximum functions of the electron transportation procedure.
• Plasmalogens (platelet-activating element) associated with platelet aggregation and degranulation.