Structure of Lipids
Fig 4.2. Structure of phosphatidate
Phosphatidate is the parent compound for the formation of the different glycerophospholipids. To
the phosphate group different head alcohol may be attached. If choline is attached it is called
phosphatidyl choline (lecithin), if ethanolamine is attached it is called phosphatidyl ethanolamine.
The second largest membrane lipids are sphingolipids, which contain two non-polar and one
polar head groups. Their alcohol is the amino alcohol sphingosine.
Sphingolipids have subclasses viz., sphingomyelins, cerebrosides and gangliosides. Out of these
only sphingomyelin contains phosphorus.
Sphingomyelins contain as head group phosphocholine or hosphoethanolamine. Below is an
example of a sphingomyelin.
Gangliosides:
These are glycolipids most of which are complex containing oligomers of sugars
on head groups. One unit shall definitely be N-acetyl neuraminic acid (sialic acid) 6% of grey
brain matter is ganglioside.
Cerebrosides:- These are glycolipids which have no phosphate group but neutral head group and
contain one or two sugar groups usually glucose or Galactose Functions of phospholipids
- Phospholipids are components of membrane; impart fluidity and pliability to the membrane.
- Dipalmitoyl choline (lecithin) acts as surfactant and lowers the surface tension in alveoli of lungs. Lecithin along with sphingomyelin maintains the shape of alveoli and prevents their collapse due to high surface tension of the surrounding medium. Some premature infants can’t secrete lecithin; therefore suffer from respiratory distress syndrome.
- Intra cellular signals (for second messengers) like inositol triphosphate and diacylglycerol are generated from membrane PL, during the action of hormones.
- PL anchors certain proteins to cell membranes. PL being amphipathic can interact with nonpolar and polar substances. They link proteins to nonpolar membranes.
- Solubilization of cholesterol is done by amphipathic nature of PL.
- Lipids are transported as lipoproteins, which require PL