What are the two layers of the plasma membrane?

The plasma membrane consists of two layers of phospholipids.

The phospholipids that make up the membrane are composed of two parts – a head and two tails. The head forms the hydrophilic or water-loving portion of the phospholipid. It may be charged or polar. The long, fatty acid tails form the hydrophobic or water-fearing part of the phospholipid. These are nonpolar.

The polar hydrophilic layer faces outwards and is in contact with the aqueous fluid both inside and outside the cell. As water is a polar molecule, it readily forms electrostatic interactions with the polar phospholipid heads. The nonpolar hydrophobic layer is on the interior of the membrane facing inwards. It interacts readily with other nonpolar molecules but poorly with water. Staying in the interior of the membrane shields this layer from the water surrounding the outside of the cell.

The phospholipid bilayer formed by these hydrophilic and hydrophobic interactions creates an efficient barrier between the cell’s interior and exterior regions, preventing water and other polar substances from easily crossing into the hydrophobic core of the membrane.