The sigmoid nature of Hb-O2 dissociation curve is because of:

Ans. is 'a' i.e. Binding of one O2 molecule increases the affinity for next Hb also .Lehninger writes : "Hemoglobin must bind oxygen efficiently in lungs and release oxygen in the tissues, myoglobin, or any protein that binds oxygen with a hyperbolic binding curve, would be ill-suited to this function. A protein that binds O2 with high affinity would bind it efficiency in the lungs but would not release much of it in the tissues. If the protein oxygen with a sufficiently low affinity to release it easily in the tissues, it would not pick up much of oxygen in lungs.Haemoglobin solves this problem by undergoing a transition from a low affinity state (the *T state) to a high affinity state (the (R state) as more oxygen molecules are bound. As a result, the Haemoglobin hos a hybrids-shaped, or sigmoid*, binding curve for oxygen*. "