Biology Respiration / Gaseous Exchange MDCAT 2016
PMDC Verified Question 33 of 34
Carboxyhemoglobin (\( 10 \text{-} 20\% \)) is formed when \( \text{CO}_2 \) combines with:
A
Haem portion of haemoglobin
B
Amino group of haemoglobin
C
Iron part of haemoglobin
D
Plasma proteins
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: Amino group of haemoglobin
Concept: Hemoglobin acts as a dual-transport molecule. While oxygen binds to the heme iron, carbon dioxide binds to a completely different site to prevent competition.

Solution:
  • About \( 20\text{-}23\% \) of carbon dioxide is transported in the blood bound directly to hemoglobin.
  • It binds to the terminal amino groups of the globin polypeptide chains, forming carbaminohemoglobin.
  • Note: Some older regional curricula erroneously refer to this as "carboxyhemoglobin", though modern medicine strictly reserves that term for Carbon Monoxide (\( \text{CO} \)) binding. Regardless of terminology, \( \text{CO}_2 \) targets the amino group.


Why other options are incorrect:
  • Haem / Iron part: These are the specific binding sites for Oxygen (\( \text{O}_2 \)) and Carbon Monoxide (\( \text{CO} \)), not Carbon Dioxide.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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