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OCR Biology A, the role of haemoglobin in transporting oxygen

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    • Thread Starter

    Someone help me to understand this. I cannot seem to get my head around it and no matter how much I go over it I just draw a blank every time I want to recall it.

    There are 4 things you need to know;
    -Haemoglobin is a pigment.
    -It is also a protein
    -It is carried in the blood stream in red blood cells
    -It is a receptor for oxygen (it carries it around)
    Hope that helps x

    I'm assuming you know a bit about Haemoglobin already (from the Biological molecules topic.)

    Haemoglobin can carry up to four oxygen molecules with it around the body. Each one binds to a Haem group (Fe2+ ion.)

    When talking about uptake of oxygen, you should think in terms of oxygen concentration, or as it ends up being called, partial pressure. There's also partial pressure of carbon dioxide that we consider, but I'll get to that.

    At different partial pressures of oxygen, Haemoglobin is less or more likely to take up oxygen (and is said to have a high or low affinity for oxygen.) This is best explained with a diagram (oxygen dissociation curve

    So, just concentrating on the central blue line there, you can see that as partial pressure of oxygen increases (pO2,) increases, so does the saturation of Haemoglobin (and by that logic, its affinity.) What's going on is something called positive cooperativity, where as one molecule of oxygen binds to a haem group, it changes the shape of the rest of the molecule to make it easier for more oxygen molecules to bind to the haemoglobin.

    In the lungs, where the partial pressure of oxygen is very high, it causes haemoglobin to become saturated with oxygen (ie. all four haem groups on a single haemoglobin are bound to oxygen molecules.) In the capillaries, the opposite occurs - the partial pressure of oxygen is much lower, so haemoglobin has a lower affinity for oxygen and thus releases it. Positive cooperativity also works backwards (as far as I remember) so when one oxygen dissociates from the haemoglobin, the molecule changes shape to make it easier to release the rest.

    The other thing going on in the capillaries is that there is carbon dioxide entering the bloodstream from the tissue fluid. This creates a high partial pressure of carbon dioxide, which prompts haemoglobin to have a lower affinity for oxygen (and encouraging oxygen to be released into cells by it.)

    Hope this was helpful.
    • Thread Starter

    This is really helpful thanks. Just a few questions:
    In terms of carbonic anhydrase, haemoglobonic acid and the chloride shift, how does this fit in? Thanks

    This all takes place in the capillaries (more towards the venous end I believe.) CO2 will diffuse into the capillaries from tissue fluid and then into the erythrocytes. Once inside, some of the CO2 binds to Haemoglobin (called carboaminohaemoglobin) and the rest reacts with water in the cytoplasm of the erythrocyte to form carbonic acid, which is catalysed by carbonic anhydrase. After this, the acid will dissociate into hydrogen carbonate (HCO3-) ions and hydrogen (H+) ions. The hydrogen carbonate ions move out of the erythrocyte and as a result, Chloride ions will move in so that the overall electric charge is maintained (this is the chloride shift.) Lastly, the hydrogen ions will combine with haemoglobin to form haemoglobinic acid.
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