The oxygen dissociation curve of haemoglobin is S-shaped because

  1. binding of the first O₂ makes it easier for further O₂ to bind
  2. haemoglobin has only one binding site
  3. oxygen dissolves in plasma
  4. CO₂ competes for haem
Answer

A) binding of the first O₂ makes it easier for further O₂ to bind

Cooperative binding. AS & A Level Biology · Topic 8 Transport in Mammals · 8.2 Transport of oxygen and carbon dioxide

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