Final answer:
If carbonic anhydrase activity decreased, the plasma levels of carbon dioxide (PCO₂) would increase due to a slower conversion of carbon dioxide to bicarbonate, causing a decrease in blood pH.
Step-by-step explanation:
If carbonic anhydrase activity decreased, particular changes would occur in the plasma. Normally, carbonic anhydrase catalyzes the reaction CO₂ + H₂O → H₂CO₃, which quickly converts carbon dioxide and water into carbonic acid (H₂CO₃). This carbonic acid then dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺).
When the activity of carbonic anhydrase is reduced, the reaction slows down, which means less carbon dioxide is converted into carbonic acid and less bicarbonate and hydrogen ions are produced. Consequently, carbon dioxide (PCO₂) buildup occurs in the plasma because it is not being converted to bicarbonate as effectively.
Since carbon dioxide is acidic when dissolved in blood, an increase in PCO₂ leads to an increase in the acidity of the blood, or a decrease in pH. This is a reflection of the Le Chatelier's principle, suggesting that the equilibrium of the chemical reaction shifts in response to a change in concentration of reactants or products. Therefore, if carbonic anhydrase activity decreased, pH would decrease, not increase. This would mean option (c) PCO₂ is the factor that would increase in the plasma.