What defines a prokaryote?

A prokaryote is defined as a single-celled organism that lacks both membrane-bound organelles and a true nucleus. This is a fundamental characteristic that distinguishes prokaryotic cells from eukaryotic cells, which do possess a well-defined nucleus and various membrane-bound organelles.

In a prokaryotic cell, the genetic material is not enclosed within a nuclear membrane; instead, it resides in a region of the cell called the nucleoid. Additionally, prokaryotes are typically simpler and smaller in structure compared to eukaryotes. The absence of membrane-bound organelles also means that their cellular processes are not compartmentalized like those in eukaryotic cells.

The other options suggest characteristics that do not align with the definition of prokaryotes. Specifically, the first option describes eukaryotic cells, which have a true nucleus. The second option again refers to eukaryotes, which contain membrane-bound organelles such as mitochondria and the endoplasmic reticulum. The last option describes a multicellular organism, which is not a characteristic of prokaryotes, as they are predominantly single-celled organisms.