Oxygen Dissociation Flashcards
Tell me about haemoglobin…
Haemoglobin (Hb): Hb is a molecule that transports oxygen. It has a quaternary structure (association of 4 polypeptides there are two types of polypeptide chains, 2 alpha and 2 beta chains). Each polypeptide chain is associated with a haem prosthetic group that contains a ferrous group(Fe++) and each can bind one oxygen molecule. So one Hb molecule can carry 4 oxygen molecules forming oxyhaemoglobin
What does an oxygen dissociation curve show?
The change in the affinity of Hb for oxygen at different pO2
Explain the differences in pO2 in the lungs and the tissues
At high pO2 the percentage saturation is very, around 95% and at low pO2 the percentage saturation is low.
So Hb coming from the lungs where pO2 is high is heavily saturated with oxygen. As it reaches the tissues where respiration is occurring at pO2 is low it is only about 50% saturated.
Thus Hb coming from the lungs is carrying a lot of oxygen, and as it reaches the tissues it releases this oxygen, which is then used for respiration
Why do we get an s shaped curve?
The S-shaped curve is explained by the behaviour of Hb. It shows cooperative bonding, its affinity for oxygen changes as the amount of oxygen bound changes. The addition of the first oxygen is difficult, but once bound, it changes the shape of the Hb molecules making it easier for the 2nd and 3rd to bind, it is harder for the fourth.
This behaviour is reflected in the curve. The curve is quite shallow up to around 2kPa, where only one oxygen molecule sis bound, then the curve rises steeply as the 2nd and 3rd oxygen bind, here small changes in pO2 causes large changes in % saturation.
Describe how Hb normally loads oxygen in the lungs and unloads oxygen in tissues.
Hb has a high affinity and loads O2 in high pO2
Oxygen diffuses into the cell
Attaches to the iron containing haem group
Becomes saturated with O2
Oxyhaemoglobin forms
Hb unloads oxygen in low pO2
This arises in actively respiring tissues which use O2
Carbon Dioxide shifts dissociation curve to right
Further reduces Hb affinity for O2
So more O2 unloaded and used in respiration
Why does a foetus need special haemoglobin?
Foetal haemoglobin has a higher affinity for oxygen at low partial pressures, so its oxygen dissociation curve is shifted up and left.
A developing foetus obtains its oxygen, not through its lungs, but from its mother’s blood in the placenta. So this different haemoglobin allows oxygen to diffuse from the mother’s blood to the foetus, and to be unloaded in the foetal tissues. Foetal haemoglobin is gradually replaced by adult haemoglobin during the first year after birth.
Lugworms live in the mud in estuaries and seashores. When the tide is out the lugworm stays in a burrow filled with sea water. But the oxygen concentration in this burrow can fall very low as the lugworm respires. How does the lugworm overcome this?
The lugworm has haemoglobin with a very high affinity for oxygen: its oxygen dissociation curve is shifted up and left. This allows the lugworm to obtain oxygen even when the pO2 is as low as 2kPa.
Mice lose heat very quickly due to their large surface area: volume ratio, so they have a high metabolic rate to generate more heat. Their tissues therefore have a constant demand for oxygen for respiration. How is their dissociation curve different to humans?
The oxygen dissociation curve for mouse haemoglobin is shifted down and right compared to humans, so plenty of oxygen is unloaded to all tissues all the time.
Myoglobin is a pigment found in muscles that are involved in sustained contractions. . It has a higher affinity for oxygen than Hb. Where will it’s dissciation curve be?
To the left of haemoglobin
What does Hb saturation depend on
pO2
When cells respire they produce CO2. This raises the pCO2. What effect does this have on the rate of O2 unloading? What is this called?
It increases shifting the curve to the right. This is callled the Bohr Shift
