Okay so we need to know all about haemoglobin etc…
The haemoglobins are a group of chemically similar molecules found in many different organisms. Haemoglobin is a protein with a quaternary structure - each of the four polypeptides it contains is associated with a haem group which contains a ferrous ion which can combine with a single oxygen molecule (meaning a total of four oxygen molecules can be carried by a single haemoglobin molecule). It is efficient in loading oxygen (in one set of conditions) and unloading oxygen (in another set of conditions). It can do this because its affinity for oxygen changes depending on whether the haemoglobin molecule is at the exchange surface or respiring tissue. Haemoglobin with a high affinity for oxygen load it easily but unload it with difficulty. Haemoglobins with a low affinity for oxygen unload it easily but load it with difficulty. Haemoglobin can change it’s affinity because its shape changes in the presence of certain substances (e.g co2, which is abundant at the respiring tissue). For example, in the presence of carbon dioxide haemoglobin changes its shape so it binds more loosely to oxygen resulting in the haemoglobin releasing oxygen at the respiring tissue (where co2 concentration is high). This is known as the Bohr effect.
How actually does haemoglobin change it’s affinity? Here’s how…
- CO2 is constantly being removed at the gas exchange surface
- The pH is raised due to the low concentration of CO2 (co2 is acidic so less of it means a more alkaline pH, therefore a higher pH)
- The higher pH changes the shape of haemoglobin into one that enables it to load oxygen readily, increasing it’s affinity to oxygen meaning no oxygen is released whilst being transported in the blood (from the exchange surface to the respiring tissue)
- There is a high CO2 concentration at the respiring tissue as it is a waste product of respiration
- CO2 is acidic in solution so the pH if the blood within the respiring tissue is low
- This lower pH changes the shape of haemoglobin which reduces its affinity oxygen. This means that the haemoglobin releases its oxygen into the respiring tissues.
It is important to note that different organisms contain different types of haemoglobin (I mean like haemoglobin with different affinities or oxygen). Basically each species produces haemoglobin with a slightly different amino acid sequence meaning a slightly different highly specific tertiary and quaternary structure = different oxygen binding properties. E.g mountain goats will have haemoglobin that has a high affinity for oxygen because there’s not much oxygen all the way up there (a low partial pressure of oxygen) so it needs to bind to all that is can get its hands on.
Okay so now we’re getting on to oxygen dissociation curves which HIGHLY confuse me so please bear with me.
When haemoglobin is exposed to different partial pressures of oxygen (basically, different oxygen concentrations), it binds differently…
- The initial shape of the haemoglobin molecule makes it difficult for the first oxygen to bind to the first ferrous ion. This means at low oxygen concentrations only a small amount of oxygen binds to the haemoglobin molecules
- The binding of the first oxygen molecule changes the shape of the haemoglobin molecule and the second and third oxygen molecule can bind fairly easily. This means that just a small increase in partial pressure can cause the second oxygen molecule to bind - this is known as positive cooperation because the binding of the first makes the binding of the second easier etc. This steepens the gradient of the curve
- It is harder for the fourth oxygen molecule to bind simply because the majority fo binding sites are occupied so it is less likely that an oxygen molecule will find an empty site to bind. This causes the gradient of the curve to reduce once more and the graph flattens off.
Graphs differ for different affinities. A graph that is closer to the right shows a lower the affinity for oxygen as a larger increase in partial pressure of oxygen is needed to make another oxygen bind. A graph that is closer to the left shows a higher the affinity for oxygen as a smaller increase in partial pressure is needed to make another oxygen bind.
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