Cells need oxygen for the more efficient of their energy metabolism processes (aerobic metabolism) - you don't need to try it to see how long we would last if we didn't get oxygen. Now, we already have a system of getting things to and from cells - the blood stream - and it is connected to the lungs in such a way that the oxygen from the air we breathe in comes into close contact with the blood stream. So no problem, right? We'll just breathe in, and the oxygen will get into the blood stream and be carried away to our tissues.
Not so fast. Unlike, say, carbon dioxide, oxygen is rather poorly soluble in blood. In other words, you can pass a lot of oxygen over the blood stream (as our lungs do), but very little will dissolve into that blood.
Hmm, what to do? I suppose we could just keep increasing the amount of blood flow to our tissues until the amount of oxygen carried there, however inefficiently, was sufficient. Unfortunately, this isn't very practical. Only about 3% of the oxygen carried in our blood streams is dissolved. To get this figure up to 100% of the requirements, we would therefore have to increase our cardiac outputs by around 3300%, or 33 times. This would be disastrous. Try imagining a heart 33 times as large, or pumping 33 times as quickly (a heart rate of, for instance, 2000!!), or arteries having to be 33 times as large... You get the picture. Furthermore, this strategy is somewhat self-destructive. All this increased energy expenditure would require... even more oxygen.
A more ingenious solution is to provide oxygen transport proteins - proteins that specialise in taking the oxygen from the air and holding it until it gets to the tissues. In this way, much more oxygen can be carried in the blood than if it all had to be dissolved in the blood.
So, is the problem solved? No, not by a long shot. Haemoglobin, the transport protein used by vertebrates, is composed of two components - a haem group (or heme group, to Americans), and a globin protein. The haem's job is to hold the oxygen molecule. It is made up of a protein ring (called a porphorin) with an iron atom at its centre.
Under the right conditions, when oxygen is near, the haem group grabs it and sandwiches it between the iron and a nitrogen atom from the surrounding prophorin ring.
So why isn't our job done yet? The problem is that oxygen is actually a very reactive thing. If it reacts with the iron, it will oxidise it (technically, it would convert it from the ferrous to the ferric state). Think about how iron rusts - that's fundamentally the same thing. If this happened to the iron of the haem group, the molecular oxygen (O2) would be lost to the body, as the process is essentially irreversible.
Enter the other component of the haemoglobin molecule - the globin part. It's primary function is to protect the oxygen binding iron atom from irreversible oxidisation. It does this by coiling around the haem group, forming a hydrophobic pocket that surrounds it, and thus stopping the oxidisation of the iron from occurring.
Whew - another close call. So, at last we've accomplished the creation of a molecule that can bind molecular oxygen, and keep it in that state for later use by the tissues. We can now watch as it whooshes by us, riding along in the blood stream... right past the tissues. Barely any oxygen is released. And so another snag becomes apparent. The oxygen transport protein must be very keen to grab hold of oxygen at the lungs, but it must equally be very keen to let go of this same oxygen when it gets to the tissues.
This does really appear to be a snag, doesn't it? Somehow, we've got to construct a molecule that automatically changes its oxygen binding affinity depending on whether it is at the lungs or at the tissues. But how...?
That'll be the topic of the next post.