NDA (II) 2023 General Ability Test · Question 67 of 146

Correct — D, Mitochondria. The dividing line between the two cell types is the membrane-bound organelle, and the mitochondrion is one. Prokaryotes have none — a bacterium runs its respiration on enzymes sitting in the plasma membrane and its infoldings, because it has no mitochondrion to house them. The other three structures are shared. Every cell alive has a plasma membrane and nucleic acids, and a cell wall is found in bacteria as readily as in plants, so none of those can separate a eukaryote from a prokaryote.

• (a)Cell wall — A cell wall is common to both groups. Bacteria have walls of peptidoglycan, fungi of chitin and plants of cellulose, while animal cells have none at all — so the wall neither marks eukaryotes nor excludes prokaryotes.
• (b)Plasma membrane — The plasma membrane is the one structure every living cell must have. It is the boundary that defines a cell, present in bacteria and in human cells alike, so it cannot be exclusive to either.
• (c)Nucleic acid — DNA and RNA are universal to cellular life. Prokaryotes simply keep their DNA loose in a region called the nucleoid instead of enclosing it in a nuclear membrane — the molecule is the same, only its packaging differs.

Prokaryotic cells lack a nuclear membrane and lack membrane-bound organelles altogether, so no mitochondria, plastids, endoplasmic reticulum, Golgi apparatus or lysosomes. Eukaryotic cells have a true nucleus and a full set of such organelles, which lets them keep different chemical jobs in separate compartments. Both share a plasma membrane, cytoplasm, ribosomes and nucleic acids.

Test each option by asking whether a bacterium has it. Bacterium with a plasma membrane, yes. With DNA, yes. With a cell wall, yes. With a mitochondrion, no — and that single 'no' is the answer. Note also what is not on the list. The nucleus would have been an equally correct answer, and NDA has in fact used it as the key in an earlier session, so do not read the absence of 'nucleus' here as a reason to doubt mitochondria.

• Prokaryotes have no membrane-bound organelles — no nucleus, no mitochondria, no plastids, no endoplasmic reticulum.
• Bacterial DNA lies in an unbounded region of the cytoplasm called the nucleoid.
• Lacking mitochondria, bacteria carry out respiration using enzymes on the plasma membrane and its infoldings.
• Ribosomes occur in both groups, but the prokaryotic ribosome is 70S against the 80S ribosome in the eukaryotic cytoplasm.
• Cell walls occur in bacteria, fungi and plants, so a wall cannot be used to tell the two cell types apart.

• Picking the cell wall because plants have one, forgetting that bacteria have one too.
• Picking nucleic acid because prokaryotes lack a nucleus — the nucleus and nucleic acid are different things.
• Assuming a structure absent from animal cells must therefore be exclusive to eukaryotes.

Asked either way round — which structure is exclusive to eukaryotes, or which structure is absent from prokaryotes — and sometimes as a straight comparison table item.

• practice — not a real PYQ

  The region of a bacterial cell in which the DNA lies without a surrounding membrane is called the

  • (a)nucleolus
  • (b)nucleoid
  • (c)nucleosome
  • (d)nuclear pore

  Answer(b) nucleoid — the undefined nuclear region of a prokaryotic cell.

• practice — not a real PYQ

  Which one of the following is present in both prokaryotic and eukaryotic cells?

  • (a)Golgi apparatus
  • (b)Ribosome
  • (c)Lysosome
  • (d)Nuclear membrane

  Answer(b) Ribosome — both cell types have ribosomes, though of different size (70S and 80S).