4.3 - Transport of Gases in the Blood Flashcards
What is the structure of Haemoglobin?
a large globular protein with a quaternary structure
made up of 4 polypeptide chains (globin part) ( 2 alpha and 2 beta chains)
Each chain has a haem group so 4 haem groups (prosthetic group, doesn’t have amino acids but is part of protein)
Haem group contains iron ions and gives Hb it’s red colour
Haem is necessary as the iron ion can attract and hold an oxygen molecule as it is said to have a high affinity for O2
Haem allows O2 to reversibly bind (load and unload)
As each haem group can hold 1 oxygen molecule, each Hb can carry 4 oxygen molecules (8 oxygen atoms)
What is the function of Haemoglobin?
is responsible for binding with oxygen in the lungs and transporting the oxygen to the tissue to be used in aerobic metabolic pathways
The existence of iron (II) ion in the prosthetic haem group allows oxygen to reversibly bind + has high affinity for O2
oxygen loads onto Hb in the lungs (alveoli) where there’s a high pO2
HbO8 is formed and this is a reversible reaction
Oxygen released (unloaded) where it needed respiring tissues where pO2 is low
Why is O2 not carried around the body in the plasma?
As oxygen is not very soluble in water and Hb is, oxygen can be carried more efficiently around the body when bound to the Hb
Hb’s affinity for oxygen
Hb usually has a high affinity for oxygen
however, affinity for oxygen varies depending on it’s condition
One condition is ‘Partial Pressure of oxygen’ (pO2) = the concentration of oxygen
oxygen loads onto Hb to form HbO8 in high pO2 like in alveolus
HbO8 unloads its oxygen where there’s low pO2 like in respiring tissue as they use up O2
Oxygen Dissociation Curves for Adult Hb
in a sigmoid shape (s-shape) due to cooperative binding
Due to the shape of the haemoglobin, it is difficult for the first oxygen molecules to bind to the haemoglobin. This means that binding of the first oxygen occurs slowly explaining the relatively shallow curve at the bottom left corner of the graph.
After the first oxygen molecule binds to Hb, it alters the conformation of Hb making subsequent binding easier, allowing the Hb to be saturated quicker. This explains the steeper part of the curve in the middle of the graph
As the Hb molecule approaches saturation it takes longer for the fourth O2 molecule to bind due to the shortage of remaining binding sites which explains the leveling off of the curve in the top right corner of the graph.
What is cooperative binding?
when the first O2 molecule binds with the Hb, alters the conformation of the Hb which makes subsequent binding easier and allows Hb to be saturated
What is Foetal Haemoglobin?
has a slightly different composition than adult Hb. Foetal has 2 gamma and 2 alpha chains
it has a higher affinity for oxygen (so the oxygen dissociation graph for foetal Hb shifts to the left)
foetal Hb is almost completely gone by 6 months postnatally
Why is foetal Hb needed?
since it has a higher affinity for oxygen than adult Hb
this is vital as it allows the foetus to load oxygen from its mother’s blood at the placenta
at the placenta, there is low pO2 as some of the oxygen is used up by the mother’s body so the oxygen unloads at the placenta at a low pO2
to ensure the foetus has enough oxygen to survive, its Hb has to have a higher affinity for oxygen so it takes up enough oxygen
If its Hb had the same affinity for oxygen as adult Hb its blood wouldn’t be saturated enough
There is a counter-current exchange to deliver O2 from the mother to the foetus, so the maternal and foetal blood runs in opposite directions.
Explain why the foetal haemoglobin
curve is to the left of the adult haemoglobin curve?
1) at the placenta there is low pO2
2) Adult HbO8 will unload O2 in a low pO2 (placenta)
3) Foetal Hb has a higher affinity for oxygen
4) Foetal Hb is still able to load some oxygen at lower pO2 (placenta), unlike adult Hb who can’t load as much O2 at lower pO2
What is the structure of myoglobin?
a globular protein
consisted of a single polypeptide chain
consists of alpha-helices in their secondary structure
has only one haem group meaning it can only bind 1 oxygen molecule
found in muscle tissue
What is the function of myoglobin?
it stores oxygen in muscles for use during periods of high demand
binds oxygen strongly and releases it only at very low oxygen levels
Myoglobin’s affinity for O2
Has a higher O2 affinity than Hb so when blood reaches muscle tissue, O2 is transferred from HbO8 to myoglobin
What is the oxygen dissociation graph for myoglobin?
lies to the extreme left in a hyperbolic shape as there is no cooperative binding
Explain why deep-diving mammals have very high concentrations of myoglobin in muscles?
Allows it to store lots of oxygen in muscles so they can dive for longer amounts of time as more O2 = allows respiration = more muscle contraction
Similarities between myoglobin and haemoglobin
both are globular proteins that are water-soluble and function in oxygen transport or storage
Both contain a haem group which includes an iron (II) ion capable of binding to oxygen
both play roles in oxygen management
1. myoglobin: stored oxygen in muscle tissues
2. haemoglobin: transports oxygen from lungs to tissues
both primarily consist of alpha-helices in their secondary structure
Differences between myoglobin and haemoglobin
Hb consists of 4 polypeptide chains whereas myoglobin only has one
Hb can bind up to 4 oxygen molecules, whereas myoglobin can only bind 1 oxygen molecule
Hb: Oxygen affinity increases when one oxygen binds as the oxygen binding is cooperative, myoglobin has no cooperative binding as it is a monomer
Hb is found in red blood cells (erythrocytes), myoglobin is found in muscle tissue
What is the Bohr Effect?
1) where tissues, like muscles, are contracting and so respiring more = more CO2
2) CO2 combines with water inside of red blood cells producing carbonic acid. . This reaction is catalysed by the enzyme carbonic anhydrase (reversible reaction and reaction is reversed when the blood reaches the capillary network surrounding the alveoli in the lungs)
3) This acid immediately dissociates to produce H+ and HCO3-
4) HCO3- ions diffuse out of red blood cells into plasma. As a result of all these negative ions leaving the red blood cells, the red blood cells develop a positive charge. To maintain electroneutrality, chlorine ions diffuse into the red blood cells from the plasma. This is called the ‘chlorine shift’
5) H+ ions compete for spaces taken up by O2 inside the red blood cell and cause the HbO8 to dissociate and release some of the oxygen it’s carrying. This is exactly what’s required if CO2 is in the blood as it means cells are respiring and in need of oxygen.
6) When CO2 is present, H+ ions displace the oxygen attached to Hb to produce haemoglobinic acid (HHb). This prevents the H+ ions collecting and causing a fall in pH and the blood becoming acidic. Therefore, Hb acts as a buffer.
7) While this is happening oxyhaemoglobin unloads O2 and this O2 is released into blood plasma
When does the Bohr effect occur?
When the pCO2 high….
Hb is less efficient at binding with oxygen because H+ is complete in the place of oxygen
Hb is much more efficient at releasing O2 dissociation because muscles have high O2 demand
What is the hr effect due to?
Due to the presence of CO2 but due to the presence of H+
CO2 is transported in three main ways to the lungs..
1) 5% is dissolved directly into the plasma
2) 10% is combined with Hb to form the compound carbaminohemoglobin
3) 85% is transported in the form of hydrogen carbonate ions in the plasma
