Question

Why are plasma membranes arranged as a bilayer rather than a monolayer?

Answer 1

Plasma membrane are composed of phospholipid, mainly. Phospholipid contain fatty acids and alcohol. Each molecule of phospholipid is consist of polar head and hydrophobic tail. Polar heads are hydrophilic and exposed to the aquous side and tails of phospholipid are hydrophobic which remain away from aquous side. Plasma membrane acts s a semipermeable barrier between exterior side and cytoplasm of the cell. Both outside and inside of the cell is consist of polar medium. So for a monolayer , it is not possible to be arranged in such a way where both the sides can be faced by hydrophilic molecules. But, if it is a bilayer, then on the both sides, the polar heads are oriented to the aquous medium and the hydrophobic tails remain embedded , far away from aquous medium. This is called lipid bilayer arrangement which is most stable conformation of phospholipid molecules. Within the lipid bilayer, the protein molecules remain embedded which acts as transporter molecules.