Oxygen dissociation curves

Laura Armstrong & Joe Wolfensohn

Teachers

Laura Armstrong Joe Wolfensohn

Recall Questions

This topic requires prior knowledge of haemoglobin structure. You can test your knowledge on this below.

Describe the structure of haemoglobin.

Haemoglobin is a quaternary protein composed of four polypeptide chains, each containing a haem group, which contains an iron ion (Fe²⁺) that binds to oxygen.

What type of bonding holds oxygen to haemoglobin?

Oxygen binds to the iron ion (Fe²⁺) in the haem group via reversible binding, allowing oxygen to be transported and released as needed.

How does cooperative binding work in haemoglobin?

When one oxygen molecule binds to a haem group, the haemoglobin undergoes a conformational shift, making it easier for additional oxygen molecules to bind. This is known as cooperative binding.

Topic Explainer Video 1

Check out this @JoeDoesBiology video that explains oxygen dissociation curves or read the full notes below. Once you've gone through the whole note, try out the practice questions!

Topic Explainer Video 2

If you want to finish off the second part of @JoeDoesBiology's explanation of oxygen dissociation curves, check out this video and once you've gone through the whole note, try out the practice questions!

What the Oxygen Dissociation Curve Represents

  • The oxygen dissociation curve is a sigmoid (S-shaped) graph showing the percentage saturation of haemoglobin with oxygen at different partial pressures of oxygen (pO₂).

  • Partial pressure of oxygen is a similar measurement to the concentration of oxygen available.

  • It illustrates haemoglobin’s affinity for oxygen and how it changes in response to varying pO₂.

  • At high pO₂, haemoglobin is almost fully saturated with oxygen.

  • At low pO₂, oxygen is dissociated (unloaded) from haemoglobin to meet the metabolic demands.

Factors Affecting Oxygen Dissociation Curves

1. Different Types of Hemoglobin

  • Haemoglobin at High Altitudes: At altitude the air pressure is decreased so the pO₂ is reduced. Adaptations include increased haemoglobin concentration and a leftward shift in the dissociation curve to facilitate oxygen uptake in low oxygen environments. The shift to the left means the haemoglobin has a higher affinity for oxygen so a sufficient amount of oxygen associates with haemoglobin despite the lower pO₂.

  • For example horses and llamas are similar sizes but llamas have evolved to live at a higher altitude.

  • Their haemoglobin has a higher affinity for oxygen and so associates more readily with oxygen even at the lower pO₂ in the lungs of the llama.

  • Fetal Haemoglobin (HbF): The curve is also shifted to the left so fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin (HbA).

  • The fetal haemoglobin will associate more readily with oxygen even at the lower  pO₂ in the mothers placenta. This allows oxygen transfer from maternal to fetal blood.

  • Small Mammals (e.g., Mice): Smaller mammals have an increased SA:Vol so lose heat more easily and have a higher respiration rate to compensate. The curve is shifted to the right, indicating their haemoglobin has a lower affinity for oxygen. This allows oxygen to be unloaded more readily to the body cells to meet their higher metabolic demands.

2. The Bohr Effect (Bohr Shift)

    • When exercising the rate of aerobic respiration increases and demand for oxygen in the tissues increases. When the rate of aerobic respiration is high more CO₂ is produced.

    • The Bohr shift describes how haemoglobin’s affinity for oxygen decreases in the presence of high CO₂ concentrations or low pH (acidic conditions).

    • Why it happens: CO₂ (produced from aerobic respiration) dissolves in blood to form carbonic acid, which dissociates into H⁺ ions, lowering the pH.

    • Effect on haemoglobin: A decrease in pH causes a conformational change in haemoglobin that reduces its affinity for oxygen.

    • This means haemoglobin will unload or disassociate with oxygen more readily (e.g., in active tissues where respiration rate is high).

    • What it means for oxygen affinity: The oxygen dissociation curve shifts to the right, meaning at the same pO₂, haemoglobin unloads more oxygen.

Key Terms

  • Oxygen Dissociation Curve: Graph showing haemoglobin saturation at different oxygen partial pressures.

  • Affinity: How strongly haemoglobin binds to oxygen.

  • Association: When oxygen is bound (loaded) to haemoglobin

  • Bohr Effect: The shift in the oxygen dissociation curve due to changes in CO₂ and pH.

  • Cooperative Binding: Increased ease of oxygen binding after the first oxygen molecule attaches.

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Exam Tip

When asked about shifts in the dissociation curve, always relate it to haemoglobin’s oxygen affinity, the body’s oxygen demands and the association/dissociation to haemoglobin.

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Explain how the Bohr effect helps ensure an adequate supply of oxygen to tissues. (4 marks)

  • The Bohr effect causes a rightward shift in the dissociation curve in active tissues 

  • This is due to increased CO₂ and lower pH.

  • This reduces hemoglobin’s affinity for oxygen

  • This leads to greater oxygen dissociation at the respiring tissues.

Practice Question 1

Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!

Practice Question 2

If you want to try out another one, check this video out and see how you do!