Q.24 Which one of the following will NOT form lipid bilayer?

(A) Cholesterol

(B) Phosphatidyl ethanolamine

(C) Triacylglycerol

(D) Phosphatidyl serine

Triacylglycerol will NOT form a lipid bilayer. This is because it lacks the amphipathic structure required for self-assembly into bilayers, unlike the other options which possess polar heads and hydrophobic tails.​

Option Analysis

Lipid bilayers form through hydrophobic interactions where amphipathic molecules arrange polar heads toward water and tails inward.​

• (A) Cholesterol: This sterol integrates into phospholipid bilayers, modulating fluidity and thickness by aligning its hydroxyl group with heads and rings/tail with chains, but does not form stable bilayers alone.​

• (B) Phosphatidyl ethanolamine: A phospholipid with a polar ethanolamine head and two hydrophobic tails that readily forms bilayers, though it favors hexagonal phases under certain conditions.​

• (C) Triacylglycerol: Neutral lipid with three fatty acid chains esterified to glycerol, lacking a distinct polar head, so it forms droplets instead of bilayers.​

• (D) Phosphatidyl serine: Phospholipid with a polar serine head and two tails that assembles into stable bilayers, contributing to membrane asymmetry.​

Lipid bilayers are fundamental to cell membranes, formed by amphipathic lipids with hydrophilic heads and hydrophobic tails. In competitive exams like CSIR NET Life Sciences, questions like “Which one of the following will NOT form lipid bilayer?” test understanding of lipid structures and self-assembly. This article breaks down Cholesterol, Phosphatidyl ethanolamine, Triacylglycerol, and Phosphatidyl serine to reveal why triacylglycerol cannot form a lipid bilayer.​

Lipid Bilayer Basics

Cell membranes rely on lipid bilayers for selective permeability. Phospholipids spontaneously form bilayers in aqueous environments due to their cylindrical shape, positioning heads outward and tails inward via hydrophobic effect. Non-bilayer-prone lipids like those with conical shapes disrupt this.​

Why Triacylglycerol Fails to Form Lipid Bilayer

Triacylglycerol (triglyceride) has glycerol esterified with three fatty acids, creating a bulky, hydrophobic molecule without a polar head. It aggregates into energy-storing droplets, not bilayers, as it cannot satisfy amphipathicity. Limited solubility (around 3%) in bilayers leads to blisters or phase separation, not stable structures.​

Phosphatidyl Ethanolamine and Phosphatidyl Serine: Bilayer Formers

Both are glycerophospholipids. Phosphatidyl ethanolamine (PE) features a small ethanolamine head promoting tight packing and bilayer formation, though it induces curvature. Phosphatidyl serine (PS) with its serine head forms fluid bilayers vital for signaling and apoptosis.​

Cholesterol’s Role Without Independent Bilayer Formation

Cholesterol embeds in bilayers, rigidifying fluid phases and preventing gel phases, but pure cholesterol forms crystals or domains, not bilayers alone.​

Master this for CSIR NET: Triacylglycerol is the answer due to absent polar head.​