What are the major transport mechanisms for $C O_{2}$ ? Explain.

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Solution

Plasma and red blood cells transport carbon dioxide. This is because they are readily soluble in water.

(1) Through plasma: About 7% of $C O_{2}$ is carried in a dissolved state through the plasma. Carbon dioxide combines
with water and forms carbonic acid.

$C O_{2} + H_{O} \to H_{2} C O_{3}$

Since the process of forming carbonic acid is slow, only a small amount of carbon dioxide is carried this way

(2) Through RBCs: About 20- 25% of $C O_{2}$ is transported by the red blood cells as carbaminohaemoglobin. Carbon dioxide binds to the amino groups on the polypeptide chains of haemoglobin and forms a compound known as carbaminohaemoglobin.

(3) Through sodium bicarbonate: About 70 of carbon dioxide is transported as Sodium Bicarbonate. As $C O_{2}$ diffuses into the blood plasma, a large part of it combines with water to form carboic acid in the presence of the enzyme carbonic anhydrase. Carbonic anhydrase is a zinc enzyme that speeds up the formation of carbonic acid. This carbonic acid dissoclates into bicarbonate $H C O_{3}$ and hydrogen ions $(H^{+})$

$C O_{2} + H_{2} O C a r b o n i c a n h y d r a s e - ----------- \to H_{2} C O_{3}$

$H_{2} C O_{3} C a r b o n i c - ----- \to a n h y d r a s e H C O_{3}^{-} + H^{+}$

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