Which type of organism is characterized by having a simple cell structure without a membrane-bound nucleus?

Prokaryotes are defined by their simple cell structure, which lacks a membrane-bound nucleus. Instead of having a nucleus, the genetic material in prokaryotic cells is located in a nucleoid region, where it exists as a single circular chromosome. This structural characteristic allows prokaryotes to be more efficient in their cellular processes and reproduction.

Additionally, prokaryotes include organisms such as bacteria and archaea, which are some of the most ancient forms of life on Earth. Their simplicity allows for rapid reproduction and adaptability to various environments. This ability to thrive in diverse habitats contributes to their ecological success.

In contrast, eukaryotes, which feature complex cells with a membrane-bound nucleus and organelles, include animals, plants, fungi, and protists. The multicellular category refers to organisms composed of multiple cells, which can include eukaryotes but not prokaryotes. Autotrophs are organisms that produce their own food, typically through photosynthesis or chemosynthesis, and this classification can apply to both prokaryotes and eukaryotes. Thus, prokaryotes are uniquely identified by their simple cell structure lacking a nucleus.