NDA (I) 2017 General Ability Test · Question 67 of 149

Correct — A, no nucleus, no mitochondria and no endoplasmic reticulum. A mammalian red blood cell throws out its nucleus in the last stage of maturation in the bone marrow and loses its mitochondria, endoplasmic reticulum and Golgi apparatus along with it. What is left is a flexible bag of haemoglobin — the protein fills about a third of the cell's volume — which is exactly what maximises oxygen-carrying capacity. Having no mitochondria also means the cell earns its energy by glycolysis and lactic-acid fermentation, so it consumes none of the oxygen it is ferrying, and having no nucleus means it cannot repair itself, which is why it survives only about 100 to 120 days.

• (b)nucleus, mitochondria and endoplasmic reticulum — This describes an ordinary body cell. A red cell precursor in the marrow does have all three, but the mature circulating cell has expelled the lot — that expulsion is the defining event of red-cell maturation in mammals.
• (c)nucleus, mitochondria but no endoplasmic reticulum — The nucleus is the first thing the cell loses, not the last thing it keeps. A nucleated red cell in a human blood film is abnormal and is itself a reportable finding.
• (d)no mitochondria but endoplasmic reticulum is present — Half right and therefore tempting — mitochondria are indeed absent, but so is the endoplasmic reticulum. Only traces of ribosomal material linger in the youngest circulating cells, the reticulocytes, and even those disappear within a day or two.

Red blood cells are specialised transport cells. In mammals they mature by discarding the nucleus and the membrane-bound organelles, ending up as biconcave discs whose shape gives a large surface area for gas exchange and enough flexibility to squeeze through capillaries narrower than the cell itself. Birds, reptiles, amphibians and fish keep nucleated red cells, so the statement in this question is a mammalian rule rather than a universal one.

The four options are the four possible combinations, so the item is really testing one idea — that the mature mammalian red cell is stripped bare. Reason it out from function rather than memory: a cell whose whole job is to carry oxygen has no use for organelles that would consume oxygen or take up space that haemoglobin could occupy.

• The nucleus is expelled during maturation, together with mitochondria, endoplasmic reticulum and Golgi apparatus.
• Without mitochondria the cell makes ATP by glycolysis and lactic-acid fermentation, using none of the oxygen it carries.
• A human red cell lives about 100 to 120 days and is then broken down, mainly by macrophages in the spleen.
• Haemoglobin accounts for roughly one-third of the cell's volume; red cells of birds, reptiles and fish do retain a nucleus.

• Applying the rule to all animals; only mammals have red cells without nuclei, and even in mammals the marrow precursors are nucleated.
• Remembering that mitochondria are absent but forgetting that the endoplasmic reticulum is gone too, which lands you on option (d).

NDA repeatedly asks which structure is present or absent in a named cell type, so keep a short list of the exceptions — red cells, sieve tube cells, prokaryotes and viruses.

• practice — not a real PYQ

  The absence of mitochondria in a mature mammalian red blood cell means that the cell obtains its energy chiefly by

  • (a)aerobic respiration
  • (b)glycolysis followed by lactic-acid fermentation
  • (c)photosynthesis
  • (d)beta-oxidation of fatty acids

  Answer(b) glycolysis followed by lactic-acid fermentation — so the cell spends none of the oxygen it is transporting.

• practice — not a real PYQ

  Which one of the following animals has nucleated red blood cells in its circulating blood?

  • (a)Human being
  • (b)Cow
  • (c)Pigeon
  • (d)Goat

  Answer(c) Pigeon — birds, reptiles, amphibians and fish keep the nucleus; mammals lose it.