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First exams 2025

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Haemoglobin & Oxygen (HL) (HL IB Biology)

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Foetal & Adult Haemoglobin

  • Haemoglobin is the molecule responsible for binding oxygen in our blood
    • They are globular proteins found in abundance in red blood cells
    • Each haemoglobin molecule consists of four polypeptide subunits
    • At the centre of each subunit is an iron-containing haem group with which oxygen combines
      • Each haem group can bind to one oxygen molecule
      • That means that each molecule of haemoglobin can transport four oxygen molecules
  • Oxygen is one of the gases found in air and each of these gases exerts a pressure
    • The pressure of each gas in a mixture of gases is called its partial pressure
    • The symbol for partial pressure is p, therefore the partial pressure of oxygen can be denoted as pO2
  • Due to the shape of the haemoglobin molecule it is difficult for the first oxygen molecule to bind to its haem group
  • However, after the first oxygen molecule binds, the haemoglobin protein changes shape, or conformation, making it easier for the next oxygen molecules to bind
    • This is known as cooperative binding
  • The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen
    • In areas where there are high partial pressures of oxygen (such as the alveoli of the lungs), the affinity of haemoglobin for oxygen is high
      • This means haemoglobin and oxygen will bind easily
    • In areas where there are low partial pressures of oxygen (such as respiring muscle cells), the affinity of haemoglobin for oxygen is low
      • This means haemoglobin and oxygen will dissociate easily from each other
    • This ensures that haemoglobin can easily bind to oxygen in the lung capillaries to transport it to and then release it near respiring cells that require oxygen 

Foetal haemoglobin

  • The haemoglobin of a developing foetus has a higher affinity for oxygen than adult haemoglobin
  • This is vital as it allows a foetus to obtain oxygen from its mother's blood at the placenta
    • Foetal haemoglobin can bind to oxygen at low pO2
    • At this low pO2 the mother's haemoglobin is dissociating with oxygen
  • We can represent the percentage saturation of haemoglobin at different partial pressures of oxygen as a graph
    • This is called the oxygen dissociation curve
  • On a dissociation curve, the curve for foetal haemoglobin shifts to the left of that for adult haemoglobin
    • This means that at any given partial pressure of oxygen, foetal haemoglobin has a higher percentage saturation than adult haemoglobin
  • After birth, a baby begins to produce adult haemoglobin which gradually replaces foetal haemoglobin
    • This is important for the easy release of oxygen in the respiring tissues of a more metabolically active individual

Foetal and adult haemoglobin

Foetal haemoglobin has a higher affinity for oxygen; its oxygen dissociation curve therefore lies further to the left than the curve of adult haemoglobin

  • Haemoglobin has the ability to change shape, or conformation, once oxygen binds to it due to cooperative binding
    • Proteins like this are known as allosteric proteins as they can exist in multiple conformations
  • Carbon dioxide is an allosteric inhibitor of haemoglobin
    • This means that when it binds to haemoglobin, it is more difficult for oxygen to bind to haemoglobin as the protein cannot change its conformation
    • This lowers the affinity of haemoglobin for oxygen
  • Carbon dioxide has less of an allosteric effect on foetal haemoglobin
    • This enables foetal haemoglobin to have a higher affinity for oxygen even if carbon dioxide is bound to it

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.