Final answer:
Unicellular organisms without membrane-bound organelles are mainly found in bacteria, classified as prokaryotes, which include both Bacteria and Archaea domains. Prokaryotes lack a nucleus and other organelles, distinguishing them from eukaryotes.
Step-by-step explanation:
Unicellular organisms that do not have membrane-bound organelles are primarily found in bacteria. These organisms are classified as prokaryotes, which are single-celled organisms of the domains Bacteria and Archaea.
Prokaryotic cells lack a nucleus and other membrane-bound organelles, which differentiates them from eukaryotic cells found in plants, animals, fungi, and protists. Bacteria are incredibly diverse and can inhabit a wide range of environments, including extreme conditions such as hot springs and frozen terrain.