-Haemoglobin has a quaternary structure. A quaternary struct

Haemoglobin

Structure

-Haemoglobin has a quaternary structure. A quaternary structure of a protein is more than one polypeptide joined together.

Haemoglobin's specific quaternary structure is:

-4 polypeptide chains.

Haemoglobin can hold oxygen because associated with these polypeptide chains is 4 haem groups/units, the are ferrous ions, Fe2+. Each ferrous ion can hold an O2 molecule- therefore, 4 oxygen molecules can be carried by one haemoglobin- one for each haem unit.

At gas exchange surface

Four/more than one polypeptide chains.

At the gas exchange surface, the partial pressure of oxygen is high and the partial pressure of CO2 is low, therefore the affinity of haemoglobin for oxygen is high. Having a high affinity means that oxygen is taken up more easily and released less readily-essentially you are oxygenating, deoxygenated blood that is passed through the lungs. As oxygen is being taken up, we call this loading or association.

Oxygen Dissociation

So picture this, at the gas exchange surface oxygen concentration is high/oxygen partial pressure is high and partial pressure of CO2 is low= haemoglobin has a higher affinity. The oxygen dissociation curve shifts to the left, as more oxygen is being taken up easily so it is become saturated at a faster rate.

Different types

Although our haemoglobin can change affinity's due to different partial pressures of CO2/O2. There is also different types in general that will relate to the metabolic rate/activeness or environment of an animal.

-Its a basic concept to grasp. Llamas live in the high tops of the mountains, their oxygen concentration is low/limited, therefore there partial pressure of oxygen is low. So to make sure, they take up as much oxygen as possible they need to take oxygen up more easily, this means they will have a ........... affinity.

-If an animal is extremely active, it uses a lot of energy and performs a lot of aerobic respiration, as oxygen is being used in respiration it will need to replace this used oxygen. It will need more oxygen to be provided to the tissues, therefore it will release oxygen more easily, this means it will have a............. affinity.

At the tissues

Why the S-shape?

At the respiring tissues/muscle cells, the partial pressure of oxygen is low and the partial pressure of CO2 is high, this CO2 lowers the pH of the blood as when dissolved in water is forms carbonic acid. Therefore, the lower the pH, the higher the partial pressure of CO2, the lower the partial pressure of oxygen, the lower the affinity for oxygen. This means oxygen is released more easily, and taken up less readily, in order to provide oxygen for the tissues. As you are releasing oxygen, this is known as unloading or dissociation,

We have discussed how the graph shifts and why, but why the S shape? Well...

At the bottom of the graph the percentage saturation of the haemoglobin with oxygen is low, this is because haemoglobin doesn't absorb the oxygen evenly when exposed to different partial pressures. Therefore, the four polypeptides are closely united. But, when the first molecule of oxygen has been absorbed, the haemoglobin changes it shape to a high affinity for oxygen therefore the other molecules can be absorbed quickly/easily. It plateaus or flattens at the top of the graph as the haemoglobin has become saturated it has absorbed 4 O2 molecules.

Oxygen Dissociation

So picture this, at the respiring tissues oxygen concentration is low/oxygen partial pressure is low and partial pressure of CO2 is high = haemoglobin has a lower affinity. The oxygen dissociation curve shifts to the right, as more oxygen is being released for aerobic respiration reactions so it is becoming saturated with oxygen at a slower rate.