Questions on haemoglobin and oxygen dissociation curves

Question 1

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.

Accepted Answer

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

Question 2

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.

Accepted Answer

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

Question 3

Describe how carbon dioxide produced by a respiring tissue is carried in the blood, including the chloride shift.

Accepted Answer

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

Question 4

Compare the saturation of haemoglobin at the two carbon dioxide concentrations at a partial pressure of oxygen of 4 kPa, and compare what this means for oxygen delivery to a respiring muscle.

Accepted Answer

B1 at 0.5 kPa carbon dioxide the haemoglobin is about 60 per cent saturated, and at 1.5 kPa about 40 per cent B1 so about 20 per cent more of the haemoglobin has unloaded its oxygen at the higher carbon dioxide concentration, at the same partial pressure of oxygen B1 carbon dioxide lowers the affinity of haemoglobin for oxygen, which is the Bohr shift, and shifts the whole curve to the right A1 a respiring muscle produces carbon dioxide, so it generates the conditions that make haemoglobin release more oxygen exactly where the oxygen is needed

Question 5

A mouse has a very high metabolic rate for its size and a heart rate of around 600 beats a minute. Its oxygen dissociation curve lies to the right of the human adult curve. Suggest why a curve shifted to the right suits a mouse's tissues, and suggest why the mouse is not disadvantaged by this at its lungs.

Accepted Answer

B1 a curve to the right means lower affinity for oxygen, so oxygen dissociates from haemoglobin more readily at any given partial pressure B1 the mouse's very high metabolic rate demands rapid oxygen delivery to its tissues, and lower affinity haemoglobin releases oxygen faster where it is needed B1 saturation at the lungs is only slightly reduced by the rightward shift, because the partial pressure of oxygen in the air the mouse breathes is the same as for any other mammal at sea level A1 the constraint that matters for a small, fast-metabolising animal is the rate of unloading in the tissues, not the saturation reached at the lungs, so the mouse is not disadvantaged there

Question 6

At rest, a muscle takes up about a third of the oxygen in the blood passing through it; during sprinting, the same muscle can take up four fifths or more. Explain what causes this increase.

Accepted Answer

B1 sprinting muscle respires much faster than resting muscle, so the partial pressure of oxygen within it falls much lower, taking the muscle further down the steep part of the dissociation curve B1 the faster respiration also produces far more carbon dioxide, which lowers the pH of the blood in the muscle and shifts the dissociation curve to the right, the Bohr shift B1 a lower partial pressure and a rightward shift both reduce the saturation the blood reaches in the muscle A1 both effects act in the same direction and both are driven by the muscle's own increased activity, so together they release a far higher fraction of the oxygen in the blood passing through than at rest

Question 7

Blood arriving at a resting muscle is 97 per cent saturated with oxygen, and blood leaving it is 62 per cent saturated. 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, and calculate the volume of oxygen released per litre of blood.

Accepted Answer

M1 the difference in saturation is 97 − 62 = 35 per cent of full capacity M1 volume released per 100 cm³ = 0.35 × 20 A1 7.0 cm³ of oxygen released per 100 cm³ of blood A1 70 cm³ (0.070 dm³) of oxygen released per litre (1000 cm³) of blood

Question 8

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.

Accepted Answer

M1 the fall in saturation is 97 − 17 = 80 per cent of full capacity M1 multiply that fraction by the volume carried at full saturation: 0.80 × 20 A1 16 cm³ of oxygen per 100 cm³ of blood, keeping the percentage and the volume distinct

Question 9

Compare the position of the fetal haemoglobin dissociation curve with that of adult haemoglobin, and compare what each one is adapted to do.

Accepted Answer

B1 the fetal curve lies to the left of the adult curve, so at any given partial pressure fetal haemoglobin is more highly saturated than adult haemoglobin B1 fetal haemoglobin therefore has the higher affinity for oxygen and adult haemoglobin the lower A1 at the low partial pressure available at the placenta the fetal blood can take oxygen from the maternal blood, whereas adult haemoglobin is adapted to load at the lungs and unload readily in respiring tissue

Question 10

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.

Accepted Answer

B1 its curve lies to the left of the human adult curve B1 a curve further to the left means a higher affinity for oxygen, so the pigment is more highly saturated at any given partial pressure A1 the only oxygen available in the burrow is at a low partial pressure, so a higher affinity is needed if the pigment is to load properly there

Question 11

Give the partial pressure of oxygen at which each haemoglobin is 50 per cent saturated, and give the difference between them.

Accepted Answer

B1 adult haemoglobin is 50 per cent saturated at about 3.5 kPa, accepting 3.4 to 3.6 B1 fetal haemoglobin is 50 per cent saturated at about 2.5 kPa, accepting 2.4 to 2.6 A1 a difference of about 1.0 kPa, consistent with the two values given

Question 12

Explain how the difference between the two curves allows a fetus to take up oxygen from its mother's blood in the placenta.

Accepted Answer

B1 the fetal curve lies above the adult curve at every partial pressure, so fetal haemoglobin has the higher affinity for oxygen B1 at the low partial pressure in the placenta (any value on the steep part of the curves, such as 4 kPa), fetal haemoglobin is far more saturated than adult haemoglobin at the same pressure A1 so oxygen unloaded by the mother's haemoglobin is taken up by the fetal haemoglobin, and the fetus can saturate its blood at a partial pressure at which the mother's is already unloading

Question 13

A llama living at 4000 m has an oxygen dissociation curve lying to the left of the human adult curve. Explain why this higher affinity suits an animal living at high altitude.

Accepted Answer

B1 at 4000 m the partial pressure of oxygen in the air is only about two thirds of its value at sea level B1 a curve to the left means a higher affinity for oxygen, so haemoglobin reaches a high saturation at a lower partial pressure than the human adult curve would A1 this restores a high saturation at the lungs despite the lower partial pressure of oxygen available in the air at altitude, whereas a haemoglobin with the human adult's affinity would be poorly saturated there

Question 14

At the lungs, the chemical reactions that carried carbon dioxide in the blood run in reverse. Explain what happens to hydrogencarbonate ions and hydrogen ions there, and explain why this allows carbon dioxide to be breathed out.

Accepted Answer

B1 at the lungs the partial pressure of carbon dioxide is low, so hydrogencarbonate ions move back into the red blood cells from the plasma B1 they recombine with hydrogen ions released from haemoglobin, and carbonic anhydrase, which catalyses the reaction in both directions, converts them back into carbon dioxide and water A1 the regenerated carbon dioxide diffuses out of the blood into the alveoli, down its concentration gradient, and is breathed out

Question 15

Name the prosthetic group carried by each polypeptide chain of haemoglobin, and state how many molecules of oxygen one haemoglobin molecule carries.

Accepted Answer

A1 the haem group, each holding one iron(II) ion A1 four molecules of oxygen in total