Haemoglobin

Structure of haemoglobin molecules

Primary structure, consisting of four polypeptide chains.
Secondary structure, in which each of these polypeptide chains is coiled into a helix.
Tertiary structure, in which each polypeptide chain is folded into a precise shape - an important factor in its ability to carry oxygen.
Quaternary structure, in which all four polypeptides are linked together to form an almost spherical molecule. Each polypeptide is linked to a haem group.

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The role of haemoglobin is to transport oxygen.
To be effict at transporting oxygen, haemoglobin must:
- readily associate with oxygen at the surface where gas exchange takes place
- readily disociate with oxygen at those tissues requiring it
Haemoglobin changes its shape in the presence of certain substances, such as carbon dioxide
In the presence of CO2, the new shape of the haemoglobin molecule binds more loosely to oxygen, meaning it releases its oxygen

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The process by which haemoglobin combines with oxygen is called loading, or associating.
This takes place in the lungs.
The process by which haemoglobin releases its oxygen is called unloading, or dissociating
This takes place in tissues.

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The further to the left the curve is, the greater the affinity the haemoglobin has for oxygen
- meaning it takes up oxygen readily but releases it less easily
The further to the right the curve is, the lower the affinity the haemoglobin has for oxygen
- meaning it releases oxygen readily but takes it up less easily

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The greater the concentration of carbon dioxide, the more readily the haemoglobin releases its oxygen (the Bohr effect).
At the gas-exchange surface (the lungs), the level of CO2 is low because it diffuses across the exchange surface and is expelled from the organism.
- The affinity of haemoglobin for oxygen is increased
- Oxygen is more readily loaded by haemoglobin
- Reduced CO2 levels has shifted the dissociation curve to the left
In rapidly respiring tissues (e.g. muscles), the level of CO2 is high
- The affinity of haemoglobin for oxygen is decreased
- Oxygen is readily unloaded from the haemoglobin into the muscle cells
- Increased CO2 levels has shifted the dissociation curve to the right

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At the gas-exchange surface, CO2 is constantly being removed.
The pH is raised due to the low level of carbon dioxide.
The higher pH changes the shape of the haemoglobin into one that enables it to load oxygen more readily.
This shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood to the tissues.

In the tissues, CO2 is produced 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 respiring tissues

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Mice are small animals and therefore have a large surface area to volume ratio.
As a result, they tend to lose heat rapidly when the environmental temperature is lower than their body temperature.
To compensate for this, they have a high metabolic rate that generates heat and helps to maintain their normal body temperature.

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Flights in birds and swimming in fish are both energy-demanding processes.
The muscles that move a bird's wings are powerful and require a lot of oxygen to allow them to respire at a sufficient rate to keep the body airborne.
Flight muscles have a very high metabolic rate and, during flight, much of the blood pumped by the heart goes to these muscles.
While birds use a great deal of energy opposing gravity in a medium that gives little support, fish have a different problem.
They expend considerable energy swimming through a medium that is very dense and therefore difficult to move through.

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