A mature mammalian erythrocyte lacks nucleus and mitochondria but is efficient in its functioning. Explain by giving suitable reasons.

1. The ABSENCE OF A NUCLEUS leaves more room for HAEMOGLOBIN, so each cell carries more oxygen, and it allows the cell to take up its BICONCAVE shape, which increases the surface area for diffusion and lets the cell bend through capillaries narrower than itself.
2. The ABSENCE OF MITOCHONDRIA means the cell does not respire aerobically, so it CONSUMES NONE OF THE OXYGEN it is carrying — the entire load is delivered to the tissues.

The question asks why the cell is efficient DESPITE these absences, so each absence must be shown to be an ADVANTAGE. That reframing is the whole point of the question.

The mitochondria argument is the more elegant and the one candidates miss: a cell whose only job is to TRANSPORT oxygen would be a poor carrier if it burned some of the cargo on the way.

THE COST of this specialisation: with no nucleus the cell cannot repair itself or divide, so it lives only about 120 days and is then destroyed in the SPLEEN — the "graveyard of RBCs".

Note the word MATURE in the question. Young mammalian red cells DO have a nucleus; it is lost as they mature, which is why the qualifier is there.