Here's why:

* Cell wall: This rigid structure provides support and protection to the prokaryotic cell. It's composed of peptidoglycan, a unique polymer found only in bacteria.

* Plasma membrane: This inner membrane is responsible for controlling the passage of molecules into and out of the cell. While essential, it's not the outermost boundary.

* Capsule: Some prokaryotes have a capsule, a sticky layer outside the cell wall that helps them adhere to surfaces and evade the host's immune system. However, it's not present in all prokaryotes and is not considered the outermost boundary.

Let me know if you have any more questions!