Which one of the following organelles can be seen in a prokaryotic cell?

Correct — C, Ribosomes. The line that divides prokaryotes from eukaryotes is the membrane: a prokaryotic cell has no membrane-bound compartments, so it has no true nucleus, no mitochondria, no Golgi complex, no endoplasmic reticulum and no lysosomes. Its genetic material sits naked in the cytoplasm in a region called the nucleoid, and the jobs that membrane-bound organelles do in a eukaryote are handled at the cell membrane itself, often at an infolding of it called a mesosome. The ribosome is the one structure that survives this test, because a ribosome is not made of membrane at all — it is a particle of RNA and protein, and every cell that makes protein must have them. Prokaryotic ribosomes are of the 70S type, built from 50S and 30S subunits, against the 80S ribosomes of the eukaryotic cytoplasm; that difference is what several antibiotics exploit, attacking the bacterial ribosome while leaving human ribosomes alone. Note too that the 70S type reappears inside eukaryotic mitochondria and chloroplasts, which is a pillar of the endosymbiotic theory.

• (a)Mitochondria — A double-membrane organelle, and therefore impossible in a cell defined by having no membrane-bound compartments. Aerobic bacteria do respire, but they do it at the cell membrane and its mesosome infoldings.
• (b)Definitive nucleus — This is the very thing the word 'prokaryote' denies — 'before the nucleus'. The DNA lies free in the cytoplasm in an undefined region called the nucleoid, with no nuclear envelope and no nucleolus.
• (d)Golgi bodies — A stack of membranous sacs that packages and dispatches material in eukaryotic cells. Like the endoplasmic reticulum and lysosomes, it belongs entirely to the eukaryotic side of the line.

Prokaryotic cells are the bacteria, blue-green algae, mycoplasma and PPLO. They are generally smaller than eukaryotic cells and multiply faster; the four basic bacterial shapes are bacillus, coccus, vibrio and spirillum. All of them have a cell envelope, a cell membrane and cytoplasm containing ribosomes and inclusion bodies, and none of them has a membrane-bound organelle. Features unique to them include the mesosome, an infolding of the cell membrane, and inclusions that store reserve material.

The safe way through any question of this shape is to ask one thing about each option: is it made of membrane? Three of the four here are, and are therefore ruled out at a stroke without recalling anything specific about bacteria. That single test also answers the negative versions of the question, which appear at least as often — 'which structure is NOT present in a prokaryotic cell' has been asked in officially keyed NDA papers with the nucleus as the answer in one year and mitochondria in another. One wrinkle in the wording is worth noticing: some textbooks reserve the word organelle for membrane-bound structures and would then say a prokaryote has none at all. NCERT does not, calling ribosomes non-membrane bound organelles found in all cells, and the question is framed on that reading.

• Prokaryotes are represented by bacteria, blue-green algae, mycoplasma and PPLO; mycoplasmas are the smallest cells known.
• Prokaryotic cells have no membrane-bound organelles at all — no true nucleus, mitochondria, Golgi complex, endoplasmic reticulum or lysosomes.
• The genetic material lies in the cytoplasm in an undefined region called the nucleoid, and many bacteria also carry small extra DNA circles called plasmids.
• Ribosomes are non-membrane bound and are found in every cell; the prokaryotic ribosome is 70S, made of 50S and 30S subunits, against the 80S ribosome of the eukaryotic cytoplasm.
• The mesosome, a specialised infolding of the cell membrane, is characteristic of prokaryotes and takes on work that membranes of organelles do in eukaryotes.

• Assuming a bacterium must have mitochondria because it respires; it respires at the cell membrane instead.
• Reading 'organelle' as necessarily membrane-bound and concluding the question has no answer — NCERT counts the ribosome as a non-membrane bound organelle.
• Confusing the nucleoid with a nucleolus; the nucleoid is the DNA region of a prokaryote, the nucleolus a structure inside a eukaryotic nucleus.

Most often negatively — which structure is not present in a prokaryotic cell — and sometimes by naming the undefined nuclear region, for which the answer is the nucleoid.

• practice — not a real PYQ

  The ribosomes found in a bacterial cell are of which type?

  • (a)80S, with 60S and 40S subunits
  • (b)70S, with 50S and 30S subunits
  • (c)70S, with 60S and 10S subunits
  • (d)80S, with 50S and 30S subunits

  Answer(b) 70S, with 50S and 30S subunits — the eukaryotic cytoplasmic ribosome is 80S with 60S and 40S subunits, and the difference is what several antibiotics act on.

• practice — not a real PYQ

  A specialised differentiated form of the cell membrane, formed by its infolding and characteristic of prokaryotes, is called

  • (a)Mesosome
  • (b)Lysosome
  • (c)Peroxisome
  • (d)Centrosome

  Answer(a) Mesosome — it is an infolding of the bacterial cell membrane, and it takes on work that membrane-bound organelles do in a eukaryotic cell.