[AhmedEzatAlzawalaty]

i know the function of heme in haemoglobin but what is the function of globulin polypeptide.

[Yggdrasil]

1) it surrounds the fairly nonpolar heme group in a hydrophilic protein so that it is soluble and able to exist in a physiological environment

2) it fine tunes the geometry of binding to reduce the affinity of the heme for carbon monoxide.  Carbon monoxide binding to heme is still fairly strong, but it would be even stronger without certain amino acid side chains that sterically hinder carbon monoxide binding but not oxygen binding

3)  it confers cooperativity/ultrasensitivity to oxygen binding.  Instead of the typical hyperbolic binding curve expected from a bimolecular association, hemoglobin shows a sigmoidal binding curve.  This sigmoidal shape of the binding curve is important because it makes hemoglobin more sensitive to oxygen concentrations.  This is important because the partial pressure of oxygen in the lungs is not that different from the partial pressure of oxygen in the body.  Hemoglobin must be sensitive enough to sense the difference in oxygen concentration so that it will bind oxygen in the lungs and release it in the body.

[AhmedEzatAlzawalaty]

i wanna know how haemoglobin binds to the first oxygen molecule difficult but then due to allosteric effectors the next binding of oxygen molecules will be easier why next binding is easier than first?

[Yggdrasil]

It's fairly hard to explain in through a medium like this.  For a good explanation, you should look it up in a biochemistry textbook such as Biochemistry by Voet and Voet.  Here is a short summary:

In deoxyhemoglobin, the iron is coordinated to the four nitrogens of the heme group and the nitrogen of a histidine residue that lies below the heme.  In this state, the iron lies slightly out of the plane of the heme group.  However, upon binding to oxygen, the iron now is coordinated by six ligands and the geometry of ligand coordination changes to bring iron into the plane of the heme ring.  This exerts a pulling force on the histidine residue which causes a large, lever-like movement of a helix within hemoglobin, chainging the overall shape of hemoglobin.  These shape changes are communicated to the other hemoglobin subunits through the binding interfaces, causing the other subunits to adopt the higher affinity state where the iron lies within the plane of the heme group.

For visuals, please see movies here: http://en.wikipedia.org/wiki/Hemoglobin#Binding_of_ligands