Hemoglobin-Oxygen dissociation curve is sigmoid shape due to -
Ans. is 4c' i.e., Co-operativity phenomenonMolecular basis of sigmoid shapeo Each molecule of hemoglobin can combine with upto four molecules of oxygen. Combination with the first molecule alters the conformation of the hemoglobin molecule in such a way as to facilitate combination with the next oxygen molecule. In light of this, if we look at the curve, as the PO2 starts rising from 0 mm Hg upwards, initially all hemoglobin molecules in blood starts combining with their first oxygen molecule. This is the most difficult molecule to combine with. Hence saturation rises only slowly with initial rise in PO2. As PO2 rises further, hemoglobin molecules combine with their second, third and fourth molecules, which are progressively easier to combine with. Hence saturation rises steeply between PO2 of 15 mm Hg and 40 mm Hg. When PO2 rises still further, oxygen finds most of the hemoglobin molecules carrying four molecules of oxygen each. Since no molecules of hemoglobin can carry more than four molecules of oxygen, there is not much scope for more O2 combining with hemoglobin. Hence the curve becomes almost flat again beyond the PO2 of 60 mm Hg.o Thus, the primary reason for the sigmoid shape of the oxygen-hemoglobin dissociation curve is that out of the four molecules of oxygen that can combine with a hemoglobin molecules, the first combines with the greatest difficulty and binding of an oxygen molecules increases affinity to next O2 molecule. This phenomenon is termed as cooperative binding or cooperativity, i.e., a molecule of O2 binds to a hemoglobin tetramer more readily if other O2 molecules are already bound.
Binding of 1st O2 molecule decreases the affinity to next molecule
Hb can bind to 2 molecules of O2
Co-operativity phenomenon
All of the above
Gas Transport