Here's why:

* Lack of a Nucleus: Prokaryotes lack a membrane-bound nucleus, while eukaryotes have a distinct nucleus that houses their genetic material (DNA).

* Simple Cell Structure: Prokaryotes have a simpler cell structure compared to eukaryotes. They lack complex organelles like mitochondria and chloroplasts.

* Size: Prokaryotes are generally much smaller than eukaryotes.

* Genetic Material: Prokaryotic DNA is circular and located in a region called the nucleoid, while eukaryotic DNA is linear and found within the nucleus.

*Rhodospirillum rubrum* is a type of bacteria, and bacteria are all prokaryotes.