How many oxygen molecules can be held by one molecule of haemoglobin?

A molecule of Haemoglobin can hold four molecules of oxygen. I don't know whether this is exactly right, but it's the nearest i've gotten Clare!

http://wiki.answers.com/Q/How_many_oxygen_molecules_can_carried_by_one_haemoglobin

Each haemoglobin molecule can bind with 4 oxygen molecules.

https://en.wikipedia.org/wiki/Hemoglobin

250 million X 4 = < 1 billion

http://wiki.answers.com/Q/How_many_oxygen_molecules_can_carried_by_one_haemoglobin

I don't have a handy link, and you've got 4 answers already -- but I remember from my biology classes that it's 4 molecules.

Also, Runaway_drive -- AWESOME profile pic!

There are four haem or heme units in a molecule and each contains an iron ion at the centre. This s where the binding/unbinding takes place, but I can't find how many molecules can bind to each. https://en.wikipedia.org/wiki/Heme

As the link posted by Runaway_drive and Kedix says, each site bonds to 1 oxygen atom. (Careful reading shows that the figure quoted by Kedix is the number of oxygen atoms carried by a red blood cell, which has approximately 250 million haemoglobin molecules in it.

"Oxygen binds in an "end-on bent" geometry where one oxygen atom binds Fe and the other protrudes at an angle."

https://en.wikipedia.org/wiki/Hemoglobin