Question

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

Short answer

The plasma membrane consists of phospholipids whose hydrophilic heads interact with water which is present both inside and outside the cell whereas the hydrophobic tails do not prefer the aqueous medium. This causes the spontaneous arrangement of the phospholipids into a bilayer.

Step-by-step solution

Introduction

The plasma membrane is a selectively permeable membrane that regulates the entry and exit of molecules into or out of the cell. It protects the cells from harmful pathogens and facilitates intercellular communication. The main components of plasma membranes are lipids (phospholipids and cholesterol), membrane proteins, and carbohydrates attached to some lipids and proteins.

Explanation

Cells have an aqueous medium both inside and outside the cell. The phospholipids that constitute the plasma membrane are amphipathic molecules with a hydrophilic head and a hydrophobic tail. The hydrophilic head consists of a negatively charged polar phosphate group that forms hydrogen bonds with water. The hydrophobic tails are uncharged and non-polar and therefore repel water. This leads to a spontaneous rearrangement of the phospholipids wherein the hydrophobic tails interact with each other and the hydrophilic heads face the aqueous medium. This is the most energetically favorable conformation.

A monolayer will place the hydrophobic tails in the cytoplasm. As the cytoplasm is made up of water, the hydrophobic tails will repel water. Thus, the interaction will be unstable and disturb the structure of the plasma membrane.