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Characteristics of phospholipids
Q.1- Which one of the following characteristics of phospholipids is correct?
A. They always contain choline and glycerol
B. They are an important source of energy during fasting
C. They are major components of membranes
D. They are uncharged at physiological pH
E. They are water-insoluble.
Details
The correct answer is -C.
The phospholipids are compound lipids. They contain in addition to fatty acids and glycerol/or other alcohol, a phosphoric acid residue, a nitrogen-containing base and other substituents.
Phospholipids may be regarded as derivatives of phosphatidic acid, in which the phosphate is esterified with the —OH of suitable alcohol.
They are amphipathic molecules containing a polar head and a hydrophobic portion (Figure).
Figure- Phospholipids are amphipathic molecules, the polar head is constituted by alcohol, phosphoric acid, and the nitrogenous base, the hydrophobic part is contributed by fatty acids.
Based on the nature of alcohol the phospholipids can be-
1)Glycerophospholipids- Glycerol is the alcohol group.
Examples-
- Phosphatidylcholine
- Phosphatidylethanolamine
- Phosphatidylserine
- Phosphatidyl Inositol
- Phosphatidic acid
- Cardiolipin
- Plasmalogen
- Platelet-activating factor
- Phosphatidyl Glycerol
2) Sphingophospholipids- Sphingol is the alcohol group
Example- Sphingomyelin
Functions of Phospholipids
- Components of cell membrane, mitochondrial membrane, and lipoproteins
- Participate in lipid absorption and transportation from the intestine
- Play an important role in blood coagulation
- Required for enzyme action- especially in the mitochondrial electron transport chain
- Choline acts as a lipotropic agent
- Membrane phospholipids act as a source of Arachidonic acid
- Act as a reservoir of the second messenger- Phosphatidyl Inositol
- Act as a cofactor for the activity of Lipoprotein lipase
- Phospholipids of myelin sheath provide insulation around the nerve fibers
- Dipalmitoyl lecithin acts as a surfactant
As regards other options
- They don’t always contain Choline and glycerol. As discussed above, in place of choline, ethanolamine, Inositol, serine, etc., maybe there and the alcohol is not glycerol always, it can be sphingosine also.
- They are not the source of energy (the functions have been described above).
They are major components of membranes since they are amphipathic and it is the correct answer.
D. They are uncharged at physiological pH. It is not true; the phospholipids are charged molecules at physiological pH.
E. They are water-insoluble. This is also incorrect; in fact, they are water-soluble due to the presence of polar head.