Oxygen shows cooperative binding to hemoglobin. cooperative binding has the following effects on the binding and release of oxygen:
oxygen binding to hemoglobin: when one molecule of oxygen binds to one of hemoglobin's four subunits, the other subunits change shape slightly, increasing their affinity for oxygen.
oxygen release from hemoglobin: when four oxygen molecules are bound to hemoglobin's subunits and one subunit releases its oxygen, the other three subunits change shape again. this causes them to release their oxygen more readily.
these two graphs show how cooperative binding differs from a hypothetical situation where binding is not cooperative.
the x-axis shows the partial pressure of oxygen (po2). this is a measure of the amount of oxygen present in a tissue. the blue arrows on the x-axis show the partial pressure of oxygen in various tissues of the body.
the y-axis shows the oxygen saturation of hemoglobin (o2 saturation). this is the percentage of oxygen-binding sites on hemoglobin molecules that are actually bound to oxygen.