Labeling CO2 transport in red blood cells

Steps:

• Recall CO2 enters RBCs and reacts with water via carbonic anhydrase to form carbonic acid.
• Carbonic acid dissociates into H+ and HCO3-, with H+ buffered by haemoglobin.
• Haemoglobin binds O2 as oxyhaemoglobin and CO2 as carbaminohaemoglobin.
• Match labels: enzyme at 3, O2-Hb at 9, CO2-Hb at 11.

Why C is correct:

• Carbonic anhydrase (label 3) catalyzes CO2 hydration per the reaction CO2 + H2O ⇌ H2CO3, essential for bicarbonate formation; oxyhaemoglobin (9) and carbaminohaemoglobin (11) are standard Hb complexes in O2/CO2 transport.

Why the others are wrong:

• A: Carbaminohaemoglobin does not involve ions; it's direct Hb-CO2 binding.
• B: Facilitated diffusion (1) applies to glucose, not CO2 entry; hydrogencarbonate ions (7) form inside RBCs, not labeled there.
• D: Carbonic anhydrase is in RBC cytoplasm (not 5); red blood cell (12) is the whole structure, not a specific component.

Final answer: C