A Levels Biology (9700)•9700/13/M/J/18

Explanation
E-cigarette use avoids CO, lowering carboxyhaemoglobin levels
Steps:
- Cigarette smoke contains carbon monoxide (CO), which binds to haemoglobin forming carboxyhaemoglobin.
- E-cigarette vapour lacks CO, so no such binding occurs.
- After a month of e-cigarette use, CO exposure is eliminated.
- Carboxyhaemoglobin concentration decreases due to absence of CO.
Why C is correct:
- Carboxyhaemoglobin is the complex of haemoglobin and CO (HbCO), directly formed by CO inhalation, absent in e-cigarettes.
Why the others are wrong:
- A. Carbaminohaemoglobin forms from CO2 binding to haemoglobin, unaffected by CO absence.
- B. Carbonic anhydrase is an enzyme catalyzing CO2 hydration, not impacted by CO levels.
- D. Hydrogencarbonate ions result from CO2 transport in blood, unrelated to CO exposure.
Final answer: C
Topic: Transport of oxygen and carbon dioxide
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