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PART - 2 - Unit V : CH - 14 Breathing and Exchange of Gases — BIOLOGY STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 ScienceBIOLOGYPART - 2 - Unit V : CH - 14 Breathing and Exchange of Gases5 Marks
Question
What are the major transport mechanisms for CO₂? Explain.
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Answer

Transport of CO2 : After cellular respiration, molecules of CO₂ and H₂O are finally formed. This $CO _2$ is also transported more easily from the cells to the lungs through blood, because it is 20 times more soluble in water than $O _2$.Blood has the capacity to carry 50 to 60 percent carbon dioxide according to its volume, but in humans only 100 ml of blood transports 4 ml of $CO _2$.
Transport of $CO _2$ is done in the following three ways :
1. In the form of Carbamino Compounds or in combined form with haemoglobin: About $23 \%$ of $CO _2$ is transported in the form of Carbamino Haemoglobin as a result of combination of NH group of haemoglobin or in the form of Carbamino protein as a result of combination with plasma proteins (because as a result of oxidation of proteins-NH, amino group and COOH forms carboxyl group).
$R - NH _2+ CO _2 \longrightarrow R - NHCOOH$
$\quad$ (Carbamino Protein Group)
$HbNH _2+ CO _2 \longrightarrow \begin{array}{l} HbNHCOOH \\ \quad \text { (Carbamino haemoglobin) }\end{array}$
2. In the form of bicarbonate: $70 \%$ of $CO _2$ in the body is transported in the form of bicarbonate. Most of the $CO _2$ diffuses from the plasma to the red blood cells and combines with the water in the cytoplasm of the red blood cells to form carbonic acid. Due to the presence of the enzyme carbonic anhydrase in red blood cells, the rate of formation of carbonic acid increases 5000 times as compared to plasma. Due to this, $70 \% CO _2$ of the blood enters the red blood cells and forms carbonic acid. This acid quickly breaks down into bicarbonate and hydrogen ions.
$CO _2+ H _2 O \rightleftharpoons H _2 CO _3$ (carbonic acid)
Most of the H+ gets conjugated with haemoglobin, due to this the pH of the blood remains stable at 7.4 because haemoglobin is an effective acid-base buffer and due to the bicarbonate ion $\left( HCO _3^{-}\right)$ being highly diffusible, it diffuses from the red blood cells. After passing through, they come into plasma.
At this time, it is necessary to maintain the normal pH and electrical neutrality of the blood, therefore, to compensate for the number of bicarbonate ions coming from the red blood cells into the plasma, the same number of chlorides $\left( Cl ^{-}\right)$ reach the cytoplasm of the red blood cells. This process is called Hamburger's Phenomenon or Chloride Shift. Bicarbonate ions combine with sodium and potassium ions present in the blood plasma to form sodium and potassium bicarbonate respectively.
$\begin{array}{l} Na ^{+}+ HCO _3^{-} \longrightarrow NaHCO _3 \\ K^{+}+ HCO _3^{-} \longrightarrow KHCO _3\end{array}$
These bicarbonates reach the respiratory level through blood plasma and their reactions take place in the reverse direction due to which CO₂ is released and goes into the lungs.
3. Dissolved in plasma or in the form of physical solution: CO₂ is 20 times more soluble than oxygen in plasma. CO₂ reacts with water in blood plasma to form carbonic acid. In this condition only $7 \%$ of $CO _2$ is transported.
$CO _2+ H _2 O \longrightarrow H _2 CO _3$ (carbonic acid)

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