Which organelle is typically absent in prokaryotic cells but abundant in eukaryotes?

• A. Ribosome
• B. Nucleus
• C. Cell membrane
• D. Cytoplasm

Answer: (B)

Understanding the difference in cellular organization between prokaryotes and eukaryotes is what this question examines. Prokaryotic cells lack a membrane-bound nucleus; their DNA sits in a region called the nucleoid without a surrounding envelope, so there isn’t a distinct nucleus separating genetic material from the rest of the cell. In contrast, eukaryotic cells have a defined nucleus enclosed by a nuclear membrane that houses their chromosomes and regulates access to the genome. This membrane-bound compartment is what makes the nucleus present in eukaryotes and absent in prokaryotes, and it reflects the more complex level of internal organization in eukaryotic cells. The other structures listed are found in both cell types: ribosomes are present in prokaryotes and eukaryotes (though different sizes), the cell membrane encloses the cell in both, and the cytoplasm fills the interior of both.