Final answer:
The gradual replacement of shorter-chain fatty acids by longer-chain fatty acids in membrane phospholipids is likely due to the effect on membrane fluidity, with unsaturation playing a crucial role in maintaining fluidity despite longer chain lengths.
Step-by-step explanation:
The likely reason for the gradual replacement of shorter-chain fatty acids by longer-chain fatty acids in membrane phospholipids has to do with maintaining proper membrane fluidity. Shorter-chain fatty acids increase membrane fluidity due to their decreased van der Waals forces and increased space between the fatty acid tails. Conversely, longer-chain fatty acids can decrease membrane fluidity as they have more interactions between their tails, making the membrane denser and more rigid. However, when referring to unsaturated fatty acids, the presence of double bonds introduces kinks that increase the fluidity of the membrane by maintaining space between the phospholipid molecules, even when they contain longer chains. Therefore, the presence of double bonds and the level of saturation are also important factors to consider when evaluating the effect of fatty acid chain length on membrane fluidity.