NDA (I) 2018 General Ability Test · Question 70 of 150

Correct — B, Mitochondria. The line that separates a prokaryotic cell from a eukaryotic one is the membrane. Prokaryotes have no membrane-bound compartments at all — no true nucleus, and no mitochondria, plastids, endoplasmic reticulum, Golgi apparatus or lysosomes. A bacterium still respires aerobically, but it does so using enzymes built into its own plasma membrane and infoldings of it, not inside a separate organelle. Of the four choices, mitochondria are the only membrane-bound organelle, and so the only one absent from prokaryotes.

• (a)Cell wall — Bacteria do have a cell wall, made of peptidoglycan rather than the cellulose of plants. It is the target of penicillin and the basis of the Gram stain, so it is one of the most studied features of the prokaryotic cell.
• (c)Plasma membrane — Every living cell has a plasma membrane — it is what defines the boundary of a cell. In a bacterium it does extra duty, carrying the respiratory enzymes that a eukaryote keeps inside its mitochondria.
• (d)Ribosome — Ribosomes are present in prokaryotes and are essential for making proteins. They are smaller than the eukaryotic kind, of the 70S type rather than 80S, and that difference is exactly what several antibiotics exploit — they block bacterial ribosomes while leaving human ones alone. Ribosomes are also not membrane-bound, which is why prokaryotes can keep them.

A prokaryotic cell has its genetic material lying free in the cytoplasm in a region called the nucleoid, with no nuclear membrane around it, and it has no membrane-bound organelles. A eukaryotic cell has a true nucleus enclosed by a nuclear envelope and a set of membrane-bound organelles that divide the cell's work between separate compartments. Bacteria and archaea are prokaryotes; protists, fungi, plants and animals are eukaryotes. Mitochondria are the sites of aerobic respiration, carry their own circular DNA and their own ribosomes, and are thought to descend from free-living bacteria taken up by an early eukaryotic cell.

There is a small wrinkle in the wording worth noticing rather than worrying about. The stem calls all four choices organelles, whereas strictly the cell wall and the plasma membrane are cell structures rather than organelles. It makes no difference to the answer, because the item is really asking which structure is missing from a bacterium, and the intended test is the membrane-bound rule. Keep that rule in the form 'no nucleus, no mitochondria, no chloroplasts, no endoplasmic reticulum, no Golgi, no lysosomes' and note the two things prokaryotes do keep — a cell wall and ribosomes. The mitochondrion's own DNA and its own 70S ribosomes are the strongest evidence for the endosymbiotic theory, and they make a neat bridge between this item and the questions on cell organelles that carry genetic material.

• Prokaryotic cells lack a nuclear membrane and all membrane-bound organelles.
• Bacteria do possess a cell wall, made of peptidoglycan, and ribosomes of the 70S type.
• In bacteria, aerobic respiration is carried out by enzymes on the plasma membrane and its infoldings.
• Mitochondria contain their own circular DNA and their own ribosomes, evidence for the endosymbiotic theory.
• The undefined nuclear region of a prokaryote is called the nucleoid.

• Assuming bacteria have no cell wall because they are not plants.
• Thinking prokaryotes cannot respire aerobically because they have no mitochondria.
• Overlooking that ribosomes are present in prokaryotes because they are not enclosed by a membrane.

NDA asks which structure is absent from a prokaryote or which is exclusive to a eukaryote, and has set both forms in recent papers.

• practice — not a real PYQ

  The genetic material of a bacterium lies in an undefined region of the cytoplasm called the

  • (a)nucleolus
  • (b)nucleoid
  • (c)nuclear envelope
  • (d)chromatin network

  Answer(b) nucleoid — the region where prokaryotic DNA sits without any membrane around it.

• practice — not a real PYQ

  Which of the following organelles other than the nucleus contains DNA?

  • (a)Golgi apparatus
  • (b)Lysosome
  • (c)Mitochondrion
  • (d)Centriole

  Answer(c) Mitochondrion — it carries its own circular DNA, a relic of its bacterial ancestry.