Phospholipids - Types & Functions

Definition:

Compound lipids that contain fatty acid, glycerol, or any other alcohol, phosphoric acid residue, and the nitrogenous base is called Phospholipids.
Phospholipids are amphipathic, i.e., they have both hydrophobic and hydrophilic portions in their molecule.
Glycerol, Phosphoric Acid, And Choline form the hydrophilic polar head.
Hydrocarbon Chains Of Fatty Acids form the hydrophobic non-polar tail.
Enzymes hydrolyzing phospholipids are called phospholipases.

Classification: on the type of alcohol present, Phospholipids can be classified into two groups

Glycerol is the alcohol
Made of glycerol + 2 fatty acids + phosphoric acid + nitrogenous base
An ester linkage links the two fatty acids to glycerol– C1 carbon contains saturated fatty acid, and C2 carbon contains unsaturated fatty acid.

NITROGEN CONTAINING PHOSPHOLIPIDS

a) 1.Phosphatidyl Choline (Lecithin):

Made of glycerol+ fatty acids + phosphoric acid + choline(nitrogenous base)
There are two types of lecithin molecules depending on the placement of the phosphoric acid choline complex on the α or β carbons,
If phosphoric acid & choline complex is present on α carbon, then it is called α lecithin.
If phosphoric acid & Choline's complex is present on β carbon, then it is called β lecithin
Lecithin exists as zwitterion at Pi – 6.7.

Functions: Lecithin is present prominently in cell membranes

It acts as a storage reserve of Choline.
It takes part in transmethylation reactions, nerve impulse transmissions(as acetylcholine), and acts as a lipotropic factor.
When the two fatty acids present in lecithin are palmitic acid, it is called dipalmitoyl lecithin.
Dipalmitoyl lecithin is present in lung alveoli and acts as a surfactant reducing the surface tension and enabling good gas exchange in the lungs.

a) 2.Phosphatidyl Ethanolamine(Cephalin) :

b) 3.Phosphatidylserine:

4.Plasmalogens:

An ether linkage links the fatty acids to glycerol molecule – C1 carbon of glycerol contains unsaturated fatty acid,
These phospholipids have long chain alcohol groups attached to glycerol.
Phosphoric acid is attached to Choline or ethanolamine.
Found in biomembranes of brain and muscle.

NON-NITROGEN CONTAINING PHOSPHOLIPIDS

c) 1.Phosphatidylinositol (lipositol)

Made of glycerol+ fatty acids+ phosphoric acid + myoinositol.
It is an essential constituent of the cell membrane
The enzyme phospholipase-C cleaves the phosphatidylinositol into diacylglycerol and inositol triphosphate.
Both diacylglycerol and inositol triphosphate act as second messengers in hormone action.

2.Phosphatidylglycerol

3.Di-Phosphatidylglycerol (cardiolipin) :

Two molecules of phosphatidic acid are held to one glycerol molecule
The name cardiolipin was derived as it was first isolated from the heart muscle(myocardium).
It is important for mitochondrial function and is an essential component of the inner mitochondrial membrane.
Plays a vital role in the electron transport chain
It is the only phospholipid with antigenic properties.
Antibodies can develop against this phospholipids in some people with autoimmune conditions, leading to antiphospholipid antibody syndrome.
Decreased levels of cardiolipin lead to mitochondrial dysfunction, aging, hypothyroidism,cardioskeletal myopathy.

II.Sphingophospholipids:

Sphingomyelin :

Sphingosine + fatty acid (attached by an amide linkage) form ceramide.
Ceramide is further attached to phosphoric acid and a nitrogenous base(Choline) to form sphingomyelin.
Sphingomyelin is a major constituent of the myelin sheath that is present in nervous tissue.
The relevant proportion of lecithin and sphingomyelin (L/S RATIO) is significant in biological fluids like amniotic fluid.
L/S RATIO is used to evaluate fetal lung maturity.
Before 34 weeks of gestation, the L/S RATIO is equal.
L/S RATIO gradually increases after 34 weeks and reaches >2 at term, indicating adequate fetal lung maturity.
In premature infants, the L/S ratio <1 leads to respiratory distress syndrome / hyaline membrane disease.

Functions Of Phospholipids :

They are an integral component of cell membranes.
They are components of the myelin sheath(sphingomyelin).
Because of amphipathic nature, they actively take part in the formation of lipoproteins.
They are essential components of electron transport chain(cardiolipin)
They act as second messengers mediating hormone action(phosphatidylinositol)
They play a role in blood coagulation(cephalin)
They act as surfactants in the lungs(lecithin)
They act as neurotransmitters(Acetylcholine)
They act as lipotropic factors.
They help in reverse cholesterol transport.
Arachidonic acid derived from the phospholipids acts as a precursor in the synthesis of eicosanoids.

Easy Biochemistry Notes