Q.23 At 50% saturation, each Hb molecule has ______ bound.

1. one O₂ molecule
2. two O₂ molecules
3. three O₂ molecules
4. four O₂ molecules

   Hemoglobin (Hb) binds oxygen cooperatively across its four heme sites, with saturation levels reflecting partial pressure of oxygen (PO2). At 50% saturation, known as the P50 point on the oxygen-hemoglobin dissociation curve, each Hb molecule statistically averages two O2 molecules bound due to symmetric cooperative binding.

   Correct Answer

   two O2 molecules

   Hemoglobin is a tetramer with four binding sites, and its sigmoidal dissociation curve shows 50% saturation (Y=0.5) at P50 (around 26-27 mmHg PO2), where exactly two sites are occupied on average across the population of Hb molecules.

   Option Analysis

   One O2 Molecule

   This would represent about 25% saturation (one of four sites filled), occurring at lower PO2 before significant cooperativity ramps up binding affinity for subsequent O2 molecules.

   Two O2 Molecules

   Correct for 50% saturation; after the first two O2 bind (moderate affinity), cooperativity boosts affinity for the next two, balancing the curve at the inflection point (P50).

   Three O2 Molecules

   This implies ~75% saturation, seen at higher PO2 (above 30-35 mmHg) where the curve steepens toward full loading in lungs.

   Four O2 Molecules

   Full saturation (100%) occurs at high arterial PO2 (~100 mmHg), fully occupying all sites in oxyhemoglobin.

   Option	Saturation Level	Typical PO2 Range (mmHg)	Binding Stage
   One O2	~25%	<20	Initial low affinity ​
   Two O2	50% (P50)	26-27	Cooperative midpoint ​
   Three O2	~75%	30-40	High affinity phase ​
   Four O2	100%	>80	Full saturation ​

   Clinical Relevance

   Understanding 50% saturation at two O2 molecules is key for interpreting arterial blood gases and anemia effects in exams like NEET-PG or USMLE, as it highlights hemoglobin’s efficiency in oxygen delivery.​

   ---