Prokaryotes and Eukaryotes
By now, you know that cell theory holds good for all living forms. However, if you examine under the microscope, the cells of a bacterium, a blue green alga or a mycoplasma on the one hand and an amoeba, or a Chlarnydomonas or an individual cell of a multicellular organism, whether plant or animal. on the other, you will note a distinct difference between the two groups. You will not see a well formed nucleous in the first group of organism as they do not have a discrete nucleous and their nuclear material is not surrounded by a nuclear membrane.
You will however notice that members of the other group consisting of amoeba or Chlamydomonas or an individual cell of a multicellular organism whether plant or animal are characterised by a distinct well formed membrane bound nucleus, called karyon (also written as caryon). From an evolutionary point of view, absence of the membrane-bound nucleus is considered primitive while its presence is deemed advanced. Therefore, biologists have coined two separate terms to denote the two categories of organisms i.e., the prokaryotes or procaryotes whose cells lack a membrane-bound nucleus and eukaryotes or eucaryotes whose cells have discrete nuclei.