The most outermost structure in a plant cell is the cell wall.

Here's why:

* Cell Wall: This rigid structure surrounds the cell membrane and provides support and protection for the plant cell. It's primarily composed of cellulose, a strong and fibrous material.

* Cell Membrane: This thin, flexible structure lies just inside the cell wall and controls what enters and exits the cell.

So, while the cell membrane is the outermost layer in terms of direct contact with the cell's environment, the cell wall sits outside of it, making it the most outermost structure.