Regarding assertion and reason, choose the correct option.
Assertion [A]: Partial pressure of oxygen is an important factor affecting binding.
Reason [R]: The binding of oxygen with haemoglobin is primarily related to partial pressure.
Assertion [A] is True and Reason [R] is True and is a correct explanation to [A].
Hemoglobin molecules have four subunits that can hold four oxygen molecules. When the first subunit binds an oxygen molecule, its configuration changes, and increases the affinity of the second subunit to oxygen. This increase in affinityincreases with each subunit that binds to oxygen. The fourth subunit will then have the highest affinity for oxygen. The affinity of hemoglobin for oxygen is dependent on the partial pressure of oxygen it is exposed to. Oxygen that has a partial pressure of about 50 mmHg saturate 50% of haemoglobin. Thus, an increasing p50 would suggest that the affinity of hemoglobin for oxygen is decreasing - it takes more oxygen to achieve 50% saturation.