Final answer:
The structure that supports and protects the plasma membrane in nearly all prokaryotes is the cell wall, which provides structural support, maintains shape, and protection from the environment.
Step-by-step explanation:
The structure present in nearly all prokaryotes that supports and protects the plasma membrane is the cell wall. The cell wall provides structural support, maintains cell shape, and offers protection against environmental changes.
While the makeup of the cell wall can vary between archaea and bacteria, its primary function across prokaryotes is to offer a sturdy barrier against the outside environment.
Additionally, the plasma membrane itself is a selectively permeable barrier controlling the entry and exit of substances, composed of a phospholipid bilayer with embedded proteins, unique in its composition between archaea and bacteria.