7.2 - Transport of oxygen by haemoglobin

Question 1

what is the graph that shows the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen known as

Answer 1

the oxygen dissociation curve

Question 2

explain why the oxygen dissociation curve has an initially very shallow gradient for the curve

Answer 2

• the shape of HB makes it difficult for the 1st O2 molecule to bind to 1 of the sites on its 4 polypeptide subunits because they are closely united.
• Therefore at low oxygen concentrations, little oxygen binds to HB.
• Therefore the gradient of the curve is initially shallow

Question 3

explain why the oxygen dissociation curve starts to steepen

Answer 3

• the binding of the 1st O2 molecule changes the quaternary structure of the HB molecule, causing it to change shape
• this makes it easier for other subunits to bind to an oxygen molecule
• therefore it takes a smaller increase in the partial pressure of oxygen to bind to the second oxygen molecule than it did to bind the first one
• this is known as positive cooperativity because binding of the 1st molecule make binding of the 2nd easier and so on
• so the gradient of the curve steepens

Question 4

explain why the oxygen dissociation curve starts to flatten off

Answer 4

• after the binding of the third molecule, it becomes harder due to probability
• with the majority of the binding sites occupied, it is less likely that a single oxygen molecule will find an empty site to bind to
• therefore the gradient reduces and the graph flattens off

Question 5

if a oxygen dissociation curve is further to left, what does it suggest

Answer 5

• there is a greater affinity of HB for oxygen
• i.e. loads oxygen readily, but unloads it less easily
  suggesting this is due to:
• higher altitude
• aquatic environment

Question 6

if a oxygen dissociation curve is further to the right, what does it suggest

Answer 6

• there is a lower affinity of HB for O2
• i.e. loads oxygen less readily, but unloads it more easily
  suggesting this is due to:
• high metabolic rate (smaller organisms have a higher metabolic rate)
• increased temp (which leads to an increased blood pressure)

Question 7

what happens to HB’s affinity in the presence of CO2

Answer 7

reduced

Question 8

how does the behaviour of haemoglobin change at the gas-exchange surface (e.g. lungs)

Answer 8

• the conc of CO2 is low because it diffuse across the exchange surface and is excreted from the organism
• the affinity of haemoglobin for oxygen is increased, which, coupled with the high conc of oxygen in the lungs, means that oxygen is readily loaded by haemoglobin
• the reduced CO2 concentration has shifted the oxygen dissociation curve to the left

Question 9

how does the behaviour of haemoglobin change in rapidly respiring tissues (e.g. muscles)

Answer 9

• the conc of CO2 is high
• the affinity of HB for O2 is reduced, which, coupled with the low concentration of O2 in the muscles, means that oxygen is readily unloaded from the haemoglobin into muscle cells
• this increased CO2 conc has shifted the oxygen dissociation curve to the right

Question 10

Describe the loading, transport and unloading of oxygen at the exchange surface, in context of CO2

Answer 10

• CO2 is being constantly removed
• the pH is slightly raised due to the low concentration of CO2
• the higher pH changes the shape of haemoglobin into one that enables it to load oxygen readily
• the shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood towards tissues

Question 11

Describe the loading, transport and unloading of oxygen in the tissues, in context of CO2

Answer 11

• CO2 is being respired by respiring cells
• CO2 is acidic in solution, so the pH of the blood within the tissues is lowered
• the lower pH changes the shape of haemoglobin into one with a lower affinity for oxygen
• Haemoglobin releases its oxygen into the respiring tissues

Question 12

Do most haemoglobin become completed saturated normally when passing through the lungs

Answer 12

• no
• at atmospheric pressure, the overall saturation is around 97%

Question 13

How might species of animals that have adapted to live in environments with lower partial pressure of oxygen be adapted

Answer 13

• their HB has a higher affinity for oxygen

Question 14

Describe the conditions for lugworms and how it affects the affinity for their HB

Answer 14

• they’re not very active
• spends most of its life in U-shaped burrows, most of the time the sea will cover these burrows and circulate inside
• oxygen will diffuse into the lugworm’s blood from the water and use HB to transport it to tissues
  However, when the tide goes out…
• can’t circulate a fresh supply —> water contains progressively less oxygen, because the lugworm uses it up
• therefore their HB has a high affinity for oxygen (oxygen disassociation curve shifted to left) so that the HB is fully loaded with what little oxygen there is left

Question 15

Describe the conditions for a llama and how it affects the affinity for their HB

Answer 15

• lives at high altitudes
• the atmospheric pressure is lower and so partial pressure of oxygen is lower
• therefore difficult to load haemoglobin with oxygen
• therefore their HB has a higher affinity so that the HB is fully loaded with what little oxygen there is

Question 16

Explain the Bohr effect

Answer 16

• Changes in the oxygen dissociation curve as a result of carbon dioxide levels are known as the Bohr effect
• When the partial pressure of carbon dioxide in the blood is high, haemoglobin’s affinity for oxygen is reduced
• This is the case in respiring tissues, where cells are producing carbon dioxide as a waste product of respiration
• This occurs because CO2 lowers the pH of the blood because the CO2 combines with water to form carbonic acid
• Carbonic acid dissociates into hydrogen carbonate ions and hydrogen ions
• Hydrogen ions bind to haemoglobon, causing the release of oxygen
• This is a helpful change because it means that haemoglobin gives up its oxygen more readily in the respiring tissues where it is needed

Question 17

Describe the effects of living at high altitudes on HB

Answer 17

• The partial pressure of oxygen in the air is lower at higher altitudes
• Species living at high altitudes have haemoglobin that is adapted to these conditions, e.g. Llamas have haemoglobin that binds much more readily to oxygen
• This is beneficial as it allows them to obtain a sufficient level of oxygen saturation in their blood when the partial pressure of oxygen (pO2) in the air is low

Question 18

Describe Foetal Haemoglobin

Answer 18

• 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
• Fetal haemoglobin can bind to oxygen at low pO2
• At this low pO2 the mother’s haemoglobin is dissociating with oxygen
• On a dissociation curve graph, the curve for foetal heamoglobin 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

Question 19

Describe the effect of being a smaller organism on HB affinity

Answer 19

• smaller organisms = higher metabolic rate
• HB therefore has a lower affinity for O2 so that it can be unloaded more easily at respiring tissues more rapidly
• the oxygen dissociation curve would therefore be shifted to the left
• this is important for body temp regulation as well, due to their larger SA:Vol resulting in significant heat loss. Because respiration produces heat, having this higher metabolic rate is therefore able to help regulate this heat loss