Biology · Body Fluids and Circulation · NEET
Correct. In MOST mammals the mature RBC is enucleate (devoid of nucleus). NCERT says clearly: RBCs are devoid of nucleus in most of the mammals and are biconcave in shape. It has no nucleus, and also lacks mitochondria and other organelles at maturity. Note the word MOST, camel and llama RBCs are still enucleate but oval.
Two reasons NEET tests. (1) The space the nucleus would take is now free to be packed with more haemoglobin, so each cell carries more oxygen. (2) No mitochondria means the RBC does NOT use up the oxygen it carries (it makes energy by anaerobic glycolysis), so all oxygen is delivered to tissues. This is the idea behind the 2017 PYQ answer: all internal space is available for oxygen transport.
A biconcave disc (dented on both sides) has MORE surface area to volume than a sphere of the same size, so gases diffuse in and out faster. The thin, flexible disc can also fold and squeeze through capillaries narrower than itself. So the shape helps both gas exchange speed and movement.
Birds, reptiles, amphibians and fishes have nucleated RBCs. In the frog (NCERT animal), RBCs are nucleated and contain haemoglobin. Only mammals typically drop the nucleus. NEET loves this contrast, so remember: mammals = enucleate, other vertebrates = nucleated.
It cannot divide (no nucleus, no DNA) and cannot repair itself, which is why it lives only about 120 days. New RBCs are made fresh in the red bone marrow, not by RBCs splitting. The next concept, RBC formation, lifespan and erythropoietin, covers this.
Adult human RBCs are enucleate. Which of the following statement(s) is/are most appropriate explanation for this feature? (a) They do not need to reproduce (b) They are somatic cells (c) They do not metabolize (d) All their internal space is available for oxygen transport
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Enucleate. In most mammals the mature RBC loses its nucleus, so it is called enucleate. Other vertebrates like frogs and birds keep nucleated RBCs.
The biconcave disc shape increases surface area for faster gas exchange and makes the cell flexible enough to pass through narrow capillaries.
No. A mature mammalian RBC has no mitochondria, so it does not consume the oxygen it carries and makes energy by anaerobic glycolysis.
More internal space for haemoglobin, so more oxygen is carried. This exact point was the correct answer in NEET 2017.
No. Camel and llama RBCs are still enucleate, but they are oval rather than biconcave, an exception NEET may ask.
