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Carbon dioxide and oxygen are transported in the blood under different conditions. Carbon dioxide is more soluble than oxygen, so some of it is directly present in the blood plasma. It can also be present as bicarbonate ions which increase blood acidity and signal if there is too much CO2 in the blood and not enough O2.

Finally, both CO2 and O2 can bind to haemoglobin which is present in red blood cells. Its function is the oxygenation of tissues, as oxygen is central to aerobic respiration, the metabolic process of creating chemical energy for all cell functions. Its smaller cousin, myoglobin, stores rather than transports oxygen in the body, and instead of being found in the blood, resides in muscle tissue. It only has one haem group so can only bind one oxygen molecule, while haemoglobin binds up to 4.

Haemoglobin is a type of protein.  It is present in many varied organisms on our planet, and has a similar chemical structure in all of these organisms. In humans, haemoglobin is found in red blood cells. Haemoglobin's function is the transport of oxygen around the body. Oxygen must reach all parts of our bodies because it is required in the process of cellular respiration (to produce ATP - the main molecule involved in releasing energy for all uses). 

Haemoglobin is a big deal, so naturally, it has a quaternary structure (multiple protein chains linked together to form a greater functional unit which also includes inorganic molecules). Just stare in awe at this beauty:
Erythrocyte is a very easy to remember name for a red blood cell. As you can see, there are 4 protein chains: 2 alpha chains and 2 beta chains. At the centre of each chain there is a haem (or heme) group which is an inorganic group containing one iron ion. The million pound question, of course, is how many iron ions are there in one haemoglobin molecule? (No, you will not be lucky enough to get that question for 10 marks in any exam.)

While haemoglobin has 4 distinct chains, myoglobin is only made of one chain, hence can carry 1 oxygen molecule rather than 4.

If you think this is all nice and easy, read on to be utterly disappointed. 

Despite the fact that haemoglobins are similar in structure and function across a variety of organisms, they are adapted to different needs, as organisms span the large breadth of the biosphere. Some organisms need to be able to make use of very little oxygen available in their environment.

Since there are 4 iron ions in each haemoglobin molecule, and iron ions are directly responsible for binding the oxygen molecules (hence a lack of iron may cause anaemia), a haemoglobin molecule which has 4 oxygens bound to its haem groups is called saturated. The % saturation of haemoglobin overall in an organism is used to determine haemoglogin's ability to bind oxygen. This ability is affected by the partial pressure of oxygen (partial because it is only 21% of air).

This is a graph showing the oxygen dissociation curve:

As you can see, the higher the partial pressure of oxygen (which reflects the amount of oxygen), the higher the % saturation of haemoglobin. Haemoglobin binds oxygen in the lungs where the partial pressure of oxygen is high. Of course, more oxygen than could be taken up by haemoglobin is irrelevant because the limiting factor becomes the number of haemoglobin molecules (hence the curve plateaus at about 8 KPa).

A very important point to take from the graph is that at a low partial pressure of oxygen (for example in respiring tissues), the % saturation of haemoglobin is decreasing. What this means is that haemoglobin has the unique ability to release oxygen where it is needed most. Abundance of oxygen in the lungs? Haemoglobin FETCHES! Tissues depleted of oxygen? Haemoglobin SPITS! Of course you will use terms such as oxygen binding and releasing, and percentage saturation of haemoglobin in your exams, won't you.

Let's go back to one of the first points on this topic. I said that haemoglobin action varies slightly between different organisms, depending on their individual needs and environment. This is a crucial point because those evil examiners will put a scary looking double, or even triple curve graph in front of you and ask you to explain what is going on. Looking at the above graph, you can see that at 6 KPa the % saturation is 80. This may suit an organism fine, but others may well have shifted curves, where the points at which haemoglobin binds and releases oxygen are different. These curves may well be shifted to either the left or the right. This is what the graph might look like:

If we take, say, 80% saturation, the corresponding partial pressure of oxygen is about 35 mm Hg for high altitude deer mice, and about 50 mm Hg for low altitude deer mice. What does this mean? It means that high altitude deer mice's haemoglobin binds oxygen at a lower partial pressure than that of low altitude deer mice. Essentially, their haemoglobin is able to make better use of less oxygen. Why?

Well, let's think, what do we know of these mice? Some live at high altitudes, and others live at low altitudes. What do we know about altitudes that is relevant? We know that there is less oxygen at higher altitudes in the atmosphere. Hence the haemoglobin of high altitude deer mice has evolved to bind oxygen at lower partial pressures - where there is less of it available. This confers a clear advantage in terms of survival.

Myoglobin has an even higher affinity for oxygen, as its role is storing it in muscle. 

Right, onto the last bit now (phew). You will be expected to know the effect of carbon dioxide (CO2) on the oxygen dissociation curve. CO2 being acidic decreases blood pH if in increasing quantity, and it results in carbamino compounds being produced. These compounds bind to haemoglobin and shift the curve to the right. Bicarbonate ions also contribute to a shift to the right, as they release protons into the blood plasma. This is known as the Bohr effect.

The Bohr effect is advantageous to tissues because it enables a better uptake of oxygen where it is needed most. Highly respiring tissues have more CO2 and heat, prompting a shift of haemoglobin saturation to the right relative to the partial pressure of O2. This means that haemoglobin does not load itself with oxygen, instead releasing its oxygen into the respiring tissue.

As the oxygen dissociation curve shifts to the right, more oxygen is released from haemoglobin. The Bohr effect provides a good feedback loop that maximises the efficiency of haemoglobin delivering oxygen to tissues at the right time.

Ok byeeeeeeee

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