10.
Which one of the following cannot form a bilayer by itself?
a. Glycerophospholipids
b. Cholesterol
c. Phosphatidic acid
d. Glycerolipids
Cholesterol cannot form a bilayer by itself. This makes it the correct answer (option b) in the CSIR NET Life Sciences question on lipid membrane structures.
Option Analysis
Glycerophospholipids, like phosphatidylcholine, feature hydrophilic phosphate heads and hydrophobic fatty acid tails, enabling spontaneous bilayer assembly in aqueous environments through amphipathic interactions. Phosphatidic acid, the simplest glycerophospholipid with a negatively charged phosphate head, also forms stable bilayers despite its conical shape, as confirmed in membrane studies. Glycerolipids, often referring to phospholipids in this context, possess the required amphiphilic properties for bilayer formation similar to glycerophospholipids.
Cholesterol lacks this capability due to its rigid steroid ring structure with a single small hydroxyl group, preventing self-assembly into bilayers; it integrates into existing phospholipid bilayers to modulate fluidity.
Glycerophospholipids and related lipids drive cell membrane formation, but one key player stands out as unable to form a bilayer by itself. This concept tests amphipathic properties critical for CSIR NET Life Sciences exams.
Lipid Structures
Glycerophospholipids contain glycerol, two fatty acids, and a phosphate group with polar heads like choline, creating ideal cylindrical shapes for bilayers. Phosphatidic acid shares this backbone minus extra headgroup modifications, supporting bilayer stability. Glycerolipids, including phospholipids, exhibit amphiphilicity for membrane assembly.
Why Cholesterol Fails
Cholesterol’s flat steroid rings and single hydroxyl cannot generate the dual-layer organization; sterols require phospholipids for insertion and do not form bilayer phases alone. This reinforces cholesterol’s role in fluidity modulation, not primary structure.
Exam Relevance
In model membranes, only amphipathic lipids self-assemble bilayers, distinguishing cholesterol in questions like “which cannot form a bilayer by itself.”
