Q.34 The fluidity of a phospholipid membrane increases when the fatty acid
(A) chain length increases and degree of unsaturation decreases
(B) chain length decreases and degree of unsaturation increases
(C) chain length decreases and degree of unsaturation decreases
(D) chain length increases and degree of unsaturation increases

The fluidity of a phospholipid membrane increases when the fatty acid chain length decreases and degree of unsaturation increases, so the correct answer is (B).​

Correct answer: Option (B)

The fluidity of a phospholipid bilayer increases when fatty acid chain length decreases and degree of unsaturation increases. Shorter chains reduce van der Waals interactions, and more double bonds (unsaturation) introduce kinks that prevent tight packing, both making the membrane more fluid.​

So, the correct option is:

• (B) chain length decreases and degree of unsaturation increases

Concept: What increases membrane fluidity?

Membrane phospholipids have hydrophobic fatty acid tails whose properties strongly control fluidity. Two key factors are:​

• Chain length: Shorter hydrocarbon chains reduce the surface area for intermolecular interactions, lowering the melting temperature and increasing fluidity.​

• Degree of unsaturation: More double bonds (especially cis) create kinks in the tails, preventing tight packing, lowering melting temperature, and increasing fluidity.​

Thus, a membrane with many short, highly unsaturated fatty acids is more fluid at a given temperature than one with long, saturated chains.​

Explanation of each option

Option (A)

(A) chain length increases and degree of unsaturation decreases

• Longer fatty acid chains strengthen hydrophobic interactions and increase van der Waals forces, which makes the membrane more ordered and less fluid.​

• Decreasing unsaturation (fewer double bonds, more saturated tails) allows closer packing of phospholipids, increasing rigidity and the gel-like character of the bilayer.​

This combination decreases membrane fluidity, so option (A) is incorrect.​

Option (B)

(B) chain length decreases and degree of unsaturation increases

• Shorter chains interact less strongly, lowering the melting temperature and increasing fluidity.​

• Higher unsaturation introduces kinks in the tails, disrupting tight packing and further lowering the melting temperature, which increases fluidity.​

This is the only option that clearly increases membrane fluidity, so option (B) is correct.​

Option (C)

(C) chain length decreases and degree of unsaturation decreases

• Decreasing chain length tends to increase fluidity by weakening hydrophobic interactions.​

• However, decreasing unsaturation (i.e., making the fatty acids more saturated) promotes tighter packing and reduces fluidity.​

These effects oppose each other, and increased saturation usually makes the membrane more ordered, so this option does not represent a clear, strong increase in fluidity and is therefore incorrect.​

Option (D)

(D) chain length increases and degree of unsaturation increases

• Increasing chain length tends to decrease fluidity by strengthening van der Waals interactions and raising the melting temperature.​

• Increasing unsaturation tends to increase fluidity by introducing kinks and disrupting packing.​

Again, the effects are opposing, and longer chain length usually makes membranes more rigid, so this combination is not the best description of a net increase in fluidity; option (D) is incorrect.​

SEO summary for students

For exam preparation, remember this key rule: membrane fluidity increases with shorter fatty acid chains and more unsaturation (more double bonds). When answering MCQs, look for the option that combines decreased chain length with increased degree of unsaturation, which in this question is option (B).​