Carbon Monoxide Binding to Hemoglobin in Smoking

Steps:

• Cigarette smoke contains carbon monoxide (CO), which enters the bloodstream via the lungs.
• CO binds to hemoglobin in red blood cells with 200–250 times greater affinity than oxygen (O2).
• This forms carboxyhemoglobin, occupying binding sites on hemoglobin.
• Fewer sites remain available for O2, reducing oxyhemoglobin formation.

Why C is correct:

• CO's high affinity for hemoglobin's iron (Fe2+) blocks O2 attachment, directly limiting oxyhemoglobin per the binding equilibrium: Hb + O2 ⇌ HbO2.

Why the others are wrong:

• A: CO binds to the heme group's iron, not the globin protein chain.
• B: Carboxyhemoglobin forms from CO and hemoglobin; CO2 binds differently as bicarbonate, not replaced here.
• D: No "haemoglobin acid" exists; smoking doesn't produce acidic hemoglobin forms.

Final answer: C