Q.41 Four statements about lipids are given below as options. Choose the statement(s)
which is/are CORRECT.
(A) Cholesterol is amphipathic
(B) Self–assembly of phospholipids in water is due to hydrophobic effect
(C) The temperature at which the gel phase changes to liquid crystalline phase increases
with an increase in the degree of unsaturation of fatty acyl tails
(D) The choline head group of lipids is positively charged
Correct statements: (A), (B), and (D).
Cholesterol exhibits amphipathic properties due to its hydrophilic hydroxyl group and hydrophobic steroid ring and tail. Phospholipids self-assemble in water primarily through the hydrophobic effect, where nonpolar tails avoid water. The choline head group in lipids like phosphatidylcholine carries a positive charge from its quaternary ammonium structure.
Option Analysis
(A) Cholesterol is amphipathic
Cholesterol contains a polar hydroxyl (-OH) group that interacts with water and a nonpolar steroid ring plus hydrocarbon tail that repels water, confirming its amphipathic nature.
(B) Self-assembly of phospholipids in water is due to hydrophobic effect
Hydrophobic interactions drive phospholipid tails to cluster away from water, forming bilayers or micelles spontaneously, as entropy increases when water molecules are released from ordered hydration shells around tails.
(C) Gel to liquid crystalline phase temperature increases with unsaturation
Unsaturation introduces kinks in fatty acyl tails, disrupting packing and lowering the gel-to-liquid crystalline transition temperature (Tm); for example, DSPC (saturated) has Tm of 55°C, while DOPC (diunsaturated) drops to -20°C.
(D) Choline head group of lipids is positively charged
In phosphatidylcholine, choline forms a quaternary ammonium ion (N(CH3)3+), providing a permanent positive charge despite the overall zwitterionic nature of the phospholipid.
CSIR NET aspirants often encounter lipids questions testing core concepts like cholesterol’s amphipathic nature and phospholipid behavior in membranes. This guide breaks down a key multiple-choice question on lipids properties, focusing on the CSIR NET lipids cholesterol amphipathic phospholipid self-assembly unsaturation transition temperature choline charge aspects for exam success.
Cholesterol: Amphipathic Molecule in Membranes
Cholesterol integrates into bilayers with its hydrophilic -OH group facing polar heads and hydrophobic rings embedding among tails, modulating fluidity. This makes option (A) correct, vital for CSIR NET life sciences prep.
Phospholipid Self-Assembly via Hydrophobic Effect
Phospholipids form bilayers as hydrophobic tails minimize water contact, driven by entropy gain—the hallmark hydrophobic effect. Option (B) holds true, explaining membrane formation.
Impact of Fatty Acyl Unsaturation on Transition Temperature
Increased unsaturation lowers the gel (ordered) to liquid-crystalline (fluid) phase transition temperature by introducing chain kinks that hinder packing. Thus, (C) is incorrect as Tm decreases, not increases.
Choline Head Group: Positively Charged Quaternary Amine
Phosphatidylcholine’s choline (N+(CH3)3) bears a positive charge, balanced by phosphate’s negative charge, forming a zwitterion. Option (D) is correct, key for lipid charge interactions.
Correct choices (A), (B), and (D) align with standard biochemistry for CSIR NET lipids cholesterol amphipathic phospholipid self-assembly unsaturation transition temperature choline charge topics. Practice similar MCQs to ace Unit 1 Biomolecules.
