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 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, they are major components of membranes.

The phospholipids are compound lipids. They contain in addition to fatty acids and glycerol/or other alcohol, a phosphoric acid residue, 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 a suitable alcohol.

They are amphipathic molecules containing a polar head and a hydrophobic portion (Figure-1) .

Phospholipids

Figure-1- Phospholipids are amphipathic molecules, the polar head is constituted by alcohol, phosphoric acid and the nitrogenous base, the hydrophobic part is constituted by fatty acids.

Based on nature of alcohol the phospholipids can be-

1) Glycerophospholipids- Glycerol is the alcohol group.

Examples-

  • Phosphatidyl choline
  • Phosphatidyl ethanolamine
  • Phosphatidyl  serine
  • Phosphatidyl inositol
  • Phosphatidic acid
  • Cardiolipin
  • Plasmalogen
  • Platelet activating factor
  • Phosphatidyl Glycerol

2) Sphingophospholipids- Sphingosine is the alcohol group

Example- Sphingomyelin (Figure-2)

Sphingomyelin

Figure-2- Sphingomyelin contains, sphingosine as the alcohol,phosphoric acid, a nitrogenous base and a long chain fatty acid.

Functions of Phospholipids

  • Components of cell membrane, mitochondrial membrane and lipoproteins
  • Participate in lipid absorption and transportation from intestine
  • Play important role in blood coagulation
  • Required for enzyme action- especially in mitochondrial electron transport chain
  • Choline acts as a lipotropic agent
  • Membrane phospholipids acts as source of Arachidonic acid
  • Act as reservoir of second messenger- Phosphatidylinositol
  • Act as 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

A. They don’t always contain Choline and glycerol. As discussed above, in place of choline, ethanolamine, Inositol, serine etc., may be there and the alcohol is not a glycerol always, it can be sphingosine also.

B. They are not source of energy (the functions have been described above).

C. 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.

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