Transport of CO2

• Aerobic respiration


  Glucose + Oxygen → Carbon Dioxide + Water + Energy
• Carbon dioxide has to be transported in the blood from actively respiring tissues to the lungs where it's breathed out
• Ways carbon dioxide is transported in the blood

  • Dissolved directly into the blood plasma (around 5%)
  • Forms a compound with haemoglobin molecules in the red blood cells (around 20%)
  • Transported as hydrogen carbonate ions in the blood plasma (remaining 75%)
• Carbamino haemoglobin


  Compound formed when carbon dioxide reacts with haemoglobin, a reversible reaction
• In respiring tissue, high level of carbon dioxide


  Carbamino haemoglobin forms
• In the lungs, low level of carbon dioxide

  Carbamino haemoglobin breaks down, releasing carbon dioxide
• Carbonic acid


  Compound formed when carbon dioxide reacts with water, a reversible reaction

  
  

  CO2 + H2O ⇌ H2CO3
• Carbonic anhydrase


  • Enzyme in red blood cells that speeds up the reaction between carbon dioxide and water to form carbonic acid
• Carbon dioxide diffuses into red blood cells

  Rapidly forms carbonic acid
• Carbonic acid forms


  -Dissociates into hydrogen carbonate ion and hydrogen ion

  -hydrogen carbonate ion has a negative charge. When the hydrogen carbonate ion diffuses out of the red blood cell, this creates a charge imbalance
• Chloride shift


  Negative chloride ion diffuses into the red blood cell to prevent charge imbalance as hydrogen carbonate ion diffuses out
• Haemoglobin acts as a buffer

  Binds to hydrogen ions to form haemoglobinic acid and prevent blood pH from falling acting as a buffer
• High level of carbon dioxide (e.g. in respiring tissue)


  Carbon dioxide forms carbonic acid and then hydrogen carbonate ions
• Low level of carbon dioxide (e.g. in the lungs)

  Hydrogen carbonate ions diffuse back into red blood cells, combine with hydrogen ions to reform carbonic acid
• Carbonic acid is broken down by carbonic anhydrase


  Forming carbon dioxide that diffuses out of the red blood cells and into the blood plasma
• Once in the blood plasma, the carbon dioxide can be exhaled from the lungs when the blood passes through the alveoli
• How CO2 forms a compound with haemoglobin

  1. Haemoglobin contains 4 polypeptide chains, in each of these polypeptides, the first amino acid has a free amino group.
  2. Each of the amino groups can react with a molecule of carbon dioxide.
  3. So, 1 molecule of haemoglobin reacts with four molecules of carbon dioxide.  
• Carbonic acid in RBC

  • Converting the carbon dioxide to carbonic acid, this ensures that the level of carbon dioxide in the red blood cell is low.  
  • Meaning there's a steep concentration gradient for carbon dioxide so there's a high rate of diffusion of carbon dioxide into the red blood cells.