The nucleus is separated from the cytoplasm by ........... which is absent in .......... and present only in ......... The words filling the blanks are respectively. 

The nucleus is separated from the cytoplasm by nuclear membrane which is an essential structure in eukaryotic cells. The nuclear membrane, also known as the nuclear envelope, is a double-layered membrane that surrounds the nucleus. Its primary function is to separate the chromosomes and genetic material within the nucleus from the rest of the cell's cytoplasm. This separation ensures proper regulation of genetic activities and allows selective exchange of materials through nuclear pores.

In contrast, prokaryotic cells lack this nuclear membrane. The nucleus is separated from the cytoplasm by nuclear membrane which is completely absent in prokaryotes. Instead, their DNA is concentrated in a specific area called the nucleoid region. The absence of a nuclear membrane in prokaryotes results in the genetic material floating freely within the cytoplasm, leading to a less compartmentalized cell structure.

On the other hand, eukaryotic cells have a well-defined nucleus where the nucleus is separated from the cytoplasm by nuclear membrane which is a defining feature. The nuclear membrane in eukaryotic cells contains small pores that facilitate the controlled exchange of molecules like nucleic acids and proteins between the nucleus and the cytoplasm. This specialized structure plays a vital role in maintaining cellular organization and efficient functioning.

To summarize, the main differences between prokaryotic and eukaryotic cells are as follows:

• Prokaryotes: Lack a nuclear membrane. The nucleus is not separated from the cytoplasm, and DNA resides in the nucleoid region.
• Eukaryotes: Possess a nuclear membrane. The nucleus is separated from the cytoplasm by nuclear membrane which is a double-layered structure allowing selective material exchange.

Therefore, the correct answer is: Nuclear membrane, prokaryotes, and eukaryotes.

Note: The key difference between prokaryotic and eukaryotic cells is the presence of membrane-bound organelles, particularly the nuclear membrane. In eukaryotes, the nucleus is separated from the cytoplasm by nuclear membrane which is absent in prokaryotes, providing a clear distinction between these two cell types.