Here's why:

* All living cells need a cell membrane. The cell membrane is essential for regulating what enters and exits the cell. It acts as a barrier, but also has specialized channels and pumps to allow for the transport of nutrients, waste products, and other molecules.

* Plants also have a cell wall. While the cell membrane is present in plant cells, they also have a rigid cell wall made of cellulose that provides structural support. This wall is located outside the cell membrane.

So, while plants have an additional layer (the cell wall), the cell membrane is a fundamental structure that is present in both animal and plant cells.