Why don't phospholipid bilayers dissolve?

Question

I just started learning about the structure and composition of cell's membrane and there is something that I fail to understand. The membrane is composed of a phospholipid bilayer. The phospholipid has a polar, hydrophilic head and a nonpolar, hydrophobic tail. My question: **Why doesn't the hydrophilic head dissolve in the extracellular fluid outside the cell?** I suppose that the extracellular fluid is watery, so how come that the water-loving head doesn't interact with the water from the extracellular fluid?

Answer 1

Extracellular fluid is water, so is cytosol inside the cell. The hydrophobic tails of the phospholipids are much longer than hydrophillic phosphate heads. So tails "want" to be as far away from water as possible and "heads" want to interact with water as much as possible. The double-layer of the membrane optimizes this arrangement and makes it more thermodynamically stable than any other combination. Think of it as a magnet that wants to align its +/- poles with the direction of the magnetic field. There are also electrostatic forces between the tails.

Also, things move very fast on cellular level, so individual phospholipids sometimes flip in opposite direction, but return to their low energy configuration.