Which type of organisms lack a nucleus and membrane-bound organelles?

Prokaryotes are defined as organisms that lack a nucleus and membrane-bound organelles. This means their genetic material is not enclosed within a membrane but instead is found in a region called the nucleoid. Prokaryotic cells are generally simpler in structure compared to eukaryotic cells, which do have a defined nucleus and various organelles such as mitochondria and endoplasmic reticulum.

Additionally, prokaryotes are typically unicellular organisms, which further differentiates them from multicellular organisms that consist of multiple eukaryotic cells. Viruses, while they may lack a cellular structure altogether, are not classified as living organisms and do not fit into this category of cellular organization. Thus, identifying prokaryotes as the organisms that lack a nucleus and membrane-bound organelles is accurate and reflects their fundamental biological characteristics.