18. Of what are phospholipids important components?

A. the double bond in hydrocarbon chains

B. the plasma membrane of animal cells

C. the ring structure of steroids

D. the waxy covering on leaves

B. The plasma membrane of animal cells

Phospholipids make up the cell membrane of animal cells. It is a bilayer, consisting of a hydrophobic tail and hydrophilic head. Thus, forming a flexible cell membrane.