Haemoglobin Oxygen Dissociation Curve/Bohr Shift Exam Questions - How to understand the Sigmoid Curve

Pack of questions From OCR A, OCR B and Eduqas. 

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The key to understanding dissociation curves is firstly to understand the concept of partial pressure and what would make it change. And to understand cooperative binding.

Partial pressure is the pressure exerted by one gas in a mixture.

You can increase the partial pressure of Oxygen by either having greater air pressure or a larger proportion of the air being Oxygen.

Therefore when you ascend Everest the percentage of Oxygen in the air does not fall - but the partial pressure in your alveoli does. 

Additionally, remember that Oxygen is the final electron acceptor in oxidative phosphorylation on the inner mitochondrial membrane, where it becomes water.

The harder a tissue works - the more ATP will be produced from aerobic respiration, therefore the lower the partial pressure of Oxygen in the tissues. This lowered partial pressure lowers the affinity of Hameoglobin for Oxygen therefore more dissociates.

Respiration produces Carbon Dioxide by decarboxylation of pyruvate in the link reaction and citrate (etc) in Krebs cycle. This diffuses (from mitochondrial matrix) into the plasma and hence into the cytoplasm of the red blood cell - where it is turned into carbonic acid (catalysed by carbonic anhydrase).

Greater respiration produces more Carbon Dioxide, hence more H ions, this lowers the affinity of Haemoglobin for Oxygen, so more Oxygen dissociates. This is the Bohr effect, whose consequence is that tissues with the most respiration receive more Oxygen.

Cooperative binding - when an Oxygen binds to a haemoglobin (remember 2 alpha chains and 2 beta chains and 4 Hame groups so can carry 4 Oxygen molecules in total), then the whole haemoglobin molecule changes in shape slightly and its affinity for Oxygen increases - hence the sigmoid shape of the curve - not a straight line.

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This from Wikipedia

A hemoglobin molecule can bind up to four oxygen molecules in a reversible way.

The shape of the curve results from the interaction of bound oxygen molecules with incoming molecules. The binding of the first molecule is difficult. However, this facilitates the binding of the second, third and fourth, this is due to the induced conformational change in the structure of the haemoglobin molecule induced by the binding of an oxygen molecule.

In its most simple form, the oxyhemoglobin dissociation curve describes the relation between the partial pressure of oxygen (x axis) and the oxygen saturation (y axis). Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape.