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Haemoglobin: an S-shaped curve and everything it explains questions
Haemoglobin as a conjugated globular protein, cooperative binding and the sigmoid dissociation curve, reading loading and unloading from the curve, the Bohr shift and its value during exercise, comparing foetal, llama and small-mammal haemoglobins, and the transport of carbon dioxide including the chloride shift.
7 original questions · 23 marks · the haemoglobin: an s-shaped curve and everything it explains notes · Animal transport and cardiovascular biology
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Explain why the oxygen dissociation curve of haemoglobin is S-shaped rather than a straight line, and explain why that shape suits the job haemoglobin does.
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- B1 the first oxygen molecule binds with difficulty, because the folded molecule presents its binding sites awkwardly, so the curve is shallow at low partial pressures
- B1 binding changes the quaternary structure so the remaining sites are easier to reach, and the second and third oxygens bind more readily, which is cooperative binding and makes the curve steep
- B1 the curve flattens at the top because only one site is left to find and there is almost no spare capacity above about 90 per cent saturation
- A1 at the alveoli haemoglobin sits on the plateau and loads reliably to about 97 per cent, while in respiring tissue it sits on the steep part where a small fall in partial pressure releases a great deal of oxygen
Explain how the Bohr shift increases the oxygen delivered to an exercising muscle, and explain why the same shift costs almost nothing at the lungs.
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- B1 the muscle produces more carbon dioxide, which forms carbonic acid in the blood and releases hydrogen ions, lowering the pH
- B1 hydrogen ions bind to haemoglobin and alter its tertiary and quaternary structure, so it holds oxygen less tightly and the curve shifts to the right
- B1 at the partial pressure found in respiring tissue the curve is steep, so a sideways shift drops the saturation a long way and much more oxygen is unloaded
- A1 at the lungs the partial pressure is around 12 kPa, where the shifted curve has already flattened off and is still over 90 per cent saturated, and blowing off carbon dioxide raises the pH and moves the curve back anyway
Describe how carbon dioxide produced by a respiring tissue is carried in the blood, including the chloride shift.
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- B1 carbon dioxide diffuses into the plasma and on into the red blood cells, with about 5 per cent staying dissolved and about 10 per cent binding to haemoglobin as carbaminohaemoglobin
- B1 carbonic anhydrase catalyses its reaction with water to form carbonic acid, which dissociates into hydrogen ions and hydrogencarbonate ions
- B1 hydrogencarbonate ions diffuse out into the plasma, where about 85 per cent of the carbon dioxide is carried, while the hydrogen ions are taken up by haemoglobin as haemoglobinic acid, which buffers them
- A1 chloride ions move into the red blood cell from the plasma to replace the negative charge lost, keeping the cell electrically neutral, which is the chloride shift
Blood arriving at a hard-working muscle is 97 per cent saturated with oxygen. Inside the muscle the partial pressure of oxygen falls to 2 kPa, where the dissociation curve gives a saturation of about 17 per cent. Given that 100 cm³ of fully saturated blood carries 20 cm³ of oxygen, calculate the volume of oxygen released to the muscle per 100 cm³ of blood.
Compare the position of the foetal haemoglobin dissociation curve with that of adult haemoglobin, and compare what each one is adapted to do.
A lugworm lives in a burrow in mud, where the partial pressure of oxygen is low. Suggest where its oxygen dissociation curve lies relative to the human adult curve, and suggest why.
Name the prosthetic group carried by each polypeptide chain of haemoglobin, and state how many molecules of oxygen one haemoglobin molecule carries.
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