Oxygen dissociation curves
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
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
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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).
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Partial pressure of oxygen is a similar measurement to the concentration of oxygen available.
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It illustrates haemoglobin’s affinity for oxygen and how it changes in response to varying pO₂.
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At high pO₂, haemoglobin is almost fully saturated with oxygen.
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At low pO₂, oxygen is dissociated (unloaded) from haemoglobin to meet the metabolic demands.
Factors Affecting Oxygen Dissociation Curves
1. Different Types of Hemoglobin
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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.
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For example horses and llamas are similar sizes but llamas have evolved to live at a higher altitude.
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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.
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Fetal Haemoglobin (HbF): The curve is also shifted to the left so fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin (HbA).
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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.
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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)
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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.
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The Bohr shift describes how haemoglobin’s affinity for oxygen decreases in the presence of high CO₂ concentrations or low pH (acidic conditions).
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Why it happens: CO₂ (produced from aerobic respiration) dissolves in blood to form carbonic acid, which dissociates into H⁺ ions, lowering the pH.
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Effect on haemoglobin: A decrease in pH causes a conformational change in haemoglobin that reduces its affinity for oxygen.
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This means haemoglobin will unload or disassociate with oxygen more readily (e.g., in active tissues where respiration rate is high).
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What it means for oxygen affinity: The oxygen dissociation curve shifts to the right, meaning at the same pO₂, haemoglobin unloads more oxygen.
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Key Terms
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Oxygen Dissociation Curve: Graph showing haemoglobin saturation at different oxygen partial pressures.
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Affinity: How strongly haemoglobin binds to oxygen.
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Association: When oxygen is bound (loaded) to haemoglobin
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Bohr Effect: The shift in the oxygen dissociation curve due to changes in CO₂ and pH.
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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)
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The Bohr effect causes a rightward shift in the dissociation curve in active tissues
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This is due to increased CO₂ and lower pH.
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This reduces hemoglobin’s affinity for oxygen
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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!