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carbonic anhydrase

Discussion in 'MCAT Study Question Q&A' started by SKation, 05.17.14.

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  1. SKation

    SKation

    Joined:
    01.15.14
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    Pre-Medical
    Hey Guys,

    I am really confused about the bicarb ion and transportation of the co2 to the lungs. I understand that within the RBC we have carbonic anhydrase to form the bicarb ion but does the bicarb ion leave to the plasma to pick up co2 or where does the actual point of considering the bicarb ion needed for transportation of co2 come in?

    Thanks
     
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  3. Czarcasm

    Czarcasm Hakuna matata, no worries. 2+ Year Member

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    Bicarbonate is an important component of blood plasma (acts to buffer the blood and maintain a pH of 7.4). As you probably know, a byproduct of metabolism is CO2 expulsion. RBC's travelling to these tissues expel free oxygen (bound to hemoglobin), and at the same time, CO2, a nonpolar molecule, diffuses through the RBC and is eventually bound to carbonic anhydrase, an enzyme in RBC's. Like all enzymes and catalysts, it helps to achieve a steady equilibrium more quickly. In this case, we have CO2 + H2O (forming a very brief intermediate: H2CO3, which decomposes to): HCO3- and H+. Some of this H+ binds to hemoglobin (shifts to low affinity Hb) and helps facilitate the release of more oxygen into metabolically active tissues, down its partial pressure gradient (Bohr effect).

    The equilibrium can also be disrupted by other conditions as well. In the lungs, we exhale CO2 and therefore the capillaries at the lungs have a very low amount of CO2. At the same time though, binding oxygen (shifts to high affinity Hb) will cause bound H+ to be released. This will shift to produce more CO2, which is released into the air.

    So in summary, it facilitates the release of waste products (CO2),facilitates entry of essential nutrients (O2), and helps to buffer the blood at a constant pH (HCO3-).
     
    Last edited: 05.18.14
  4. mehc012

    mehc012 Big Damn Hero 2+ Year Member

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    Bicarb does not 'pick up CO2'. We consider bicarb because it is in equilibrium with CO2 as shown here:

    CO2 + H2O< -(1)-> H2CO3 <-(2)-> H+ and HCO3-

    The first reaction (1) is catalyzed by carbonic anhydrase. The second is the natural dissociation of carbonic acid, and equilibrium is dependent on the pKa. In the tissues, where pCO2 is high, the reaction is pushed to the right, forming bicarb (allowing more CO2 to dissolve in the blood and thus allowing us to transport a lot more). In the lungs, the dissolved CO2 leaves, pushing this equilibrium to the left, forming more CO2 which then leaves.

    Thus, we care about Bicarb levels because they are intimately related to the amount of CO2 dissolved in the blood (and the pH). We can also increase bicarbonate levels metabolically in order to shift the equilibrium of (2) to the left, decreasing H+ concentration and thus increasing pH.
     

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