B2.3 Cell specialization

The question highlights three features: biconcave shape, no nucleus, and high deformability. We must explain why these are advantageous to red blood cell function.

Red blood cells lack a nucleus at maturity. This maximises the internal volume available for haemoglobin, the oxygen-carrying protein, allowing more $O_2$ to be transported per cell.

Capillaries can be as narrow as 5–8 µm in diameter, while red blood cells are approximately 7–8 µm. Their flexible membrane allows them to squeeze through these narrow vessels without rupturing.

The option stating that the absence of a nucleus increases space for haemoglobin and allows passage through narrow capillaries correctly identifies both the structural and functional advantages.

We need to identify which option correctly explains the functional advantage of the red blood cell's anucleate, biconcave, and deformable structure.

Engulfing bacteria (phagocytosis) is a function of white blood cells (neutrophils/macrophages), not red blood cells. This option is biologically incorrect for red blood cells.

Mature red blood cells cannot divide by mitosis at all — they lack a nucleus and therefore have no DNA to replicate. This option is factually wrong.

Red blood cells actually have no mitochondria, so they cannot produce ATP via aerobic respiration. The membrane's deformability is unrelated to ATP production.

By elimination, 'The absence of a nucleus increases space for haemoglobin and allows passage through narrow capillaries' is the only biologically accurate explanation.

All somatic cells in an individual are derived from the same zygote through mitosis. Mitosis produces genetically identical daughter cells, so every somatic cell carries the same DNA sequence (genome).

Although neurons and pancreatic cells have identical DNA, they are structurally and functionally very different. This is because different subsets of genes are transcribed and translated in each cell type — a process called differential gene expression.

A neuron expresses genes for neurotransmitter synthesis and ion channels, while a pancreatic acinar cell expresses genes for digestive enzymes. The different proteins produced give each cell its unique function.

Therefore: identical DNA sequences (same genome from mitosis) but different proteins (due to differential gene expression). The correct option is the first one.

The question tests whether students understand that DNA is constant across cells in one individual, while protein expression varies between specialised cell types.

Somatic cells arising from mitosis do not have different DNA sequences under normal circumstances. Mutations could cause minor differences, but the core genome is identical. This option is incorrect.

If both cells produced identical proteins, they would be functionally identical. Clearly a neuron and a pancreatic cell are not — they have entirely different structures and functions. This option is incorrect.

This option contradicts both principles — it incorrectly states DNA differs, and it incorrectly implies proteins are the same. This is biologically impossible in normal development.

'Both cells have identical DNA sequences but produce different sets of proteins' is the only option consistent with the principle of differential gene expression.