Cholesterol helps maintain the rigidness of the membrane because it's an amphipathic molecule.

Why a bilayer?

What's on the inside of the cell and on the outside? Mostly water. As you can see here, the water-based interior of the cell has lots of components. These need to be kept inside the cell. And it is the nature of the phospholipid bilayer to keep the inside of the cell separate from the outside.

Explanation:

is in the form atp or adenosine triphosphate