The lipid not seen in plant cell membranes but present in animal cell membranes is cholesterol.

Here's why:

* Cholesterol is a sterol lipid that provides structural support and fluidity to animal cell membranes. It helps maintain the integrity of the membrane and regulate its permeability.

* Plant cells have a different type of sterol called stigmasterol which serves a similar function to cholesterol.

While both animal and plant cells have phospholipids, the presence of cholesterol is a defining characteristic of animal cell membranes.