What are the main characteristics of the phospholipid bilayer?

The phospholipid bilayer is the main component of the plasma membrane. Phospolipids consist of two parts with different properties. One part is often referred to as 'head' or 'water-loving', which is hydrophilic and turns towards water, interacts with it. The other part is the 'tail', which is hydrophobic and hides from the water. In the plasma membrane, there are two layers of these lipid molecules, one of them facing the extracellular environment, and the other facing the cytosol and the contents of the cell. Since both areas contain water, the heads stay outside, they interact with the water and the tails stay inside, hiding. This way, the plasma membrane maintains a certain shape. The bilayer also serves as a filter: very small polar molecules, water and certain types of fats can get through it easily, but bigger molecules require proteins to pass through.