What is the role of haemoglobin in transport??


The main function of Hb is to carry and transport oxygen to various tissues. The binding of oxygen to Hb is cooperative binding. The binding and release of oxygen from Hb in the lungs and tissues respectively is due to the transition between low oxygen affinity T state (Tense) and high oxygen affinity R state (Relaxed).

Transport of oxygen

The affinity of oxygen to Hb is affected by pH, 2,3 BPG (2,3-Bisphosphoglyceric acid). Low pH, high BPG and CO2 present in tissues favour T-state and oxygen are released, whereas R-state is favoured in the alveoli due to high pH, low COand BPG concentration, which leads to the binding of oxygen to Hb.

Binding of oxygen is also regulated by the partial pressure of oxygen. In the lungs where pO2 is high, oxygen binds with Hb and in tissues, where pO2 is low, oxygen is released.

Every 100 ml of oxygenated blood carries 5 ml of O2 to the tissues.

Binding of the first oxygen molecule to the heme unit of one subunit of the deoxyhaemoglobin (T-state) causes conformational changes leading to an increase in the affinity, thereby the second molecule binds more rapidly. The binding of the fourth molecule occurs, when it is already in the R state. The binding of oxygen to Hb shows a sigmoid curve.

This type of binding is known as allosteric binding, where binding at one site affects the affinities of the remaining binding sites.


Transport of Carbon dioxide

Around 20-25% of CO2 is transported bound to haemoglobin as carbaminohaemoglobin. In tissues where pCO2 is more and pO2 is less, binding of carbon dioxide is favoured and in the alveoli dissociation of carbaminohaemoglobin takes place due to high pO2 and low pCO2. Rest of the CO2 is transported as bicarbonate, which is facilitated by an enzyme called carbonic anhydrase.

Every 100 ml of deoxygenated blood carries 4 ml of CO2 to the alveoli.

Haemoglobin also transports nitric oxide bound to the globin protein. It binds to the thiol groups present in the globin chains.

Carbon monoxide can also bind to haemoglobin and forms the carboxyhaemoglobin complex. Haemoglobin has 250 times higher affinity for carbon monoxide than oxygen. So even the slightest concentration of CO can affect the binding of oxygen. So, inspiring air rich in CO can cause headache, nausea or even unconsciousness. It can block 20% of active binding sites of oxygen in heavy smokers.

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Haemoglobin is found in red blood cells which has the principal function of transferring oxygen from the lungs to the needy tissues of the body. Haemoglobin can bind to oxygen and gaseous nitric oxide. This binding has a very important role in transporting oxygen.
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