Respiratory Answer

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1a) On graph paper
1b) At molecular level, Haemoglobin is a protein made of four subunits bound together. Each of the subunits consists of a molecular group known as ‘heme’ and a polypeptide attached to the ‘heme’. The ‘heme’ group contains one atom of iron (Fe²+) and it is this iron that the oxygen molecule combines itself to. As there are four of the iron molecules in the haemoglobin this shows that a maximum of four oxygen molecules can be carried by the haemoglobin at anytime. For simplicity the equation of the reaction of haemoglobin and oxygen is written as a single polypeptide ‘heme’ chain reacting with oxygen: O2 + Hb ↔ HbO2 . The reactions of the four subunits occur in a consecutive manner, with each subunit assisting the progress of the next. As a result when one subunit binds to oxygen, the binding sites of the other subunits are more exposed to oxygen which increases the affinity for oxygen. 1c) Cooperativity is a key feature of the haemoglobin when exposed to oxygen; this is shown when the globin units of deoxyhaemoglobin are tightly held by electrostatic bonds in a conformation with a relatively low affinity for oxygen. The binding of oxygen to a heme molecule breaks some of the electrostatic bonds between the globin subunits, leading to a conformation change that leaves the remaining oxygen binding sites more exposed, thus the binding of one oxygen molecule to deoxyhaemoglobin increase the affinity of the remaining sites on the same haemoglobin molecule. 2a) As carbon dioxide is released from metabolically active tissues it enters blood capillaries; some of this CO2 gets dissolved in the plasma and some enter the erythrocytes which contain the enzyme carbonic anhydrase. This enzyme allows CO2 to combine with water to form H2CO3 (carbonic acid). H2CO3 dissociates to form H+ and HCO3 . The H+ lowers the blood pH. As the haemoglobin molecule changes it molecular shape when there are changes to the pH of its surroundings to a conformation...
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