Plasma & Carbon Dioxide (Cambridge (CIE) AS Biology): Revision Note

Plasma & carbon dioxide

• Waste carbon dioxide produced during respiration diffuses from the tissues into the blood 

• This waste carbon dioxide is transported around the body in different ways:

  • Around 5 % of carbon dioxide dissolves directly in the blood plasma

  • Around 10 % of carbon dioxide is bound to haemoglobin as carbaminohaemoglobin

  • Around 85 % of carbon dioxide is found in the blood plasma in the form of hydrogen carbonate ions (HCO₃^-)

Carbon dioxide in the plasma

• Carbon dioxide released as a waste product from respiring cells diffuses into the cytoplasm of red blood cells

• Inside red blood cells, carbon dioxide combines with water to form carbonic acid (H₂CO₃)

CO₂ + H₂O  ⇌  H₂CO₃

• Red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between carbon dioxide and water

  • Without carbonic anhydrase this reaction proceeds very slowly

• The plasma contains very little carbonic anhydrase hence H₂CO₃ forms more slowly in plasma than in the cytoplasm of red blood cells

• Carbonic acid dissociates readily into hydrogen ions (H⁺) and hydrogen carbonate ions (HCO₃^-)

H₂CO₃  ⇌  HCO₃^– + H⁺

• Hydrogen ions can combine with haemoglobin, forming haemoglobinic acid and preventing the H⁺ ions from lowering the pH of the red blood cell

  • Haemoglobin is said to act as a buffer in this situation

• The hydrogen carbonate ions diffuse out of the red blood cells into the plasma to be transported in solution