confused on this kaplan question

[RySerr21]

If the pH of blood were to increase to 7.6, what would be the likey outcome?

a) increase in carbonic anhydrase activity
b) decrease in carbonic anhydrase activity
c) an increase in the rate of CO2 exhalation
d) a decrease in the rate of CO2 exhalation


I put A but kaplan says it is D. My thinking was that if you incresae CA activity, you will be increasing the reaction C02 + H20 -- H + HCO3. By doing this, you are increasing the number of protons and bringing the pH back to its normal value. But then wouldnt you also be technically decreasing the rate of CO2 exhalation b/c the equation is shifting to the right, making less CO2 available to exhale.

basically, i think that A and D are saying the same thing in different ways. can someone help me explain why A is wrong?

---

Members don't see this ad.

 

[BloodySurgeon]

carbonic anhydrase is only a catalyst, it would not change the Keq but will lower the activation energy to make the forward and reverse reaction faster. A decrease in the rate of CO2 exhalation, however, means more CO2 which will shift the reaction to the right and lower the pH in blood.

 

[TawMus]

If the pH of blood were to increase to 7.6, what would be the likey outcome?

a) increase in carbonic anhydrase activity
b) decrease in carbonic anhydrase activity
c) an increase in the rate of CO2 exhalation
d) a decrease in the rate of CO2 exhalation


I put A but kaplan says it is D. My thinking was that if you incresae CA activity, you will be increasing the reaction C02 + H20 -- H + HCO3. By doing this, you are increasing the number of protons and bringing the pH back to its normal value. But then wouldnt you also be technically decreasing the rate of CO2 exhalation b/c the equation is shifting to the right, making less CO2 available to exhale.

basically, i think that A and D are saying the same thing in different ways. can someone help me explain why A is wrong?

The way I approached this question was to first ask myself what is the content of the blood at 7.6? Low quantities of Co2 right? (High lvls of CO2 in blood results in acidic pH) So if there is low quantity of CO2 in blood, then less CO2 will be exhaled.

 

[RySerr21]

The way I approached this question was to first ask myself what is the content of the blood at 7.6? Low quantities of Co2 right? (High lvls of CO2 in blood results in acidic pH) So if there is low quantity of CO2 in blood, then less CO2 will be exhaled.

hmmmm. the first thing i thought was "the pH is 7.6, the body is going to want to decrease it back to normal of 7.4. how do you do this? increase the number of protons."

then i started looking at the answers and increasing the activity of CA would increase the number of protons. it would also give low levels of CO2 because more of CO2 is in the form of H and HCO3. i just dont see how the two answers differ. they are saying the same thing in a different way.

 

[Chemist0157]

There's an equilibrium between

H2CO3 <==> H2O + CO2,

right?

If you increase the activity of CA, both reactions will occur faster but with no net change. Since we want more acid, we need to shift the reaction to the left, and we can do that by increasing [CO2] (decreasing exhalation).

 

[RySerr21]

There's an equilibrium between

H2CO3 <==> H2O + CO2,

right?

If you increase the activity of CA, both reactions will occur faster but with no net change. Since we want more acid, we need to shift the reaction to the left, and we can do that by increasing [CO2] (decreasing exhalation).

Can you say that? its not at equilibrium so why would there be no net change?. If you increase the activity of CA, then the forward reaction will proceed more readily.

i understand your second point and if i would have thought about it like that i would have put D....thanks!!

 

[BTC]

You are going against le chatelier's principle.

H+ + HCO3- <====> H2CO3 <===> H2O + CO2

a reduction of H+ concentration will only affect the dissociation of bicarb, not the carbonic anhydrase activity. Since reagent concentrations remain the same, carbonic anhydrase activity will remain the same. Your body, however, will recognize the increased pH and hold in more CO2 to move the equilibrium more towards the left.

 

[rocuronium]

Also, as the H+ concentration decreases, there is less loading of CO2 onto hemoglobin at the tissues, so less gets delivered to the lungs for exhalation.

 

[tncekm]

Can you say that? its not at equilibrium so why would there be no net change?.

Yes, you can say that. Catalysts only effect the rate (kinetics) not the thermodynamics. So, adding a catalyst (or increasing catalytic activity) will increase the rate at which the reaction will occur both forward AND backward, but the ratio of the products to reactants will remain the same, so the blood pH would remain the same.