Question

Every answer I can find about why phospholipids form a bilayer says the same thing: hydrophilic heads associate with water, hydrophobic tails do not, so they arrange themselves with heads facing outward. Makes sense.

What I have not been able to find an explanation for is why the two layers of a bilayer sheet stick together in a cell. Why aren't the phospholipids of the upper layer pulled away by passing water molecules, like how salt ions are pulled away from a salt crystal? Why doesn't the same thing happen in the bottom layer? And what keeps the two layers bonded when the fatty acid tails have (in my understanding) little to no attraction to each other?

Answer 1

The two layers of a phospholipid bilayer stick together due to hydrophobic interactions between the fatty acid tails and other forces such as Van der Waals interactions and electrostatic attractions between the phospholipids.

Step-by-step explanation:

The two layers of a phospholipid bilayer stick together due to the hydrophobic interactions between the fatty acid tails in the middle of the bilayer. While the hydrophilic heads of the phospholipids are attracted to water, the hydrophobic tails repel water and cluster together in the interior of the bilayer. This clustering of the hydrophobic tails creates a stable structure and prevents the layers from being pulled apart by passing water molecules. Additionally, there are other forces such as Van der Waals interactions and electrostatic attractions between the phospholipids that contribute to the stability of the bilayer.