What molecule is found in the membrane of animal cells but not plant cells?

Cholesterol is a structural component of the cell membranes found in the animal cells but not plant cells. 

Cholesterol has a rigid ring structure, which is responsible for providing cell membranes in animals with critical structural integrity and flexibility. The flexibility enables animal cells to alter their shape. 

While plant cells lack cholesterol, they contain sterols, a related type of compound that serves a similar function as cholesterol. However, the cell wall is more rigid, resulting in plant cells being less flexible.