Explain the oxygen dissociation curve and the role of Bohr shift.
(1) Oxygen is transported around the body in red blood cells. In the red blood cell it is bound to a protein called haemoglobin. Oxygen dissociation curves show the relationship between oxygen levels (as partial pressure) and amount of oxygen bound to haemoglobin in red blood cells (as % saturation). The oxygen dissociation curve has a positive gradient. This is due to diffusion [GCSE: movement from high to lower concentrations]. There is a low saturation of haemoglobin when oxygen levels are low (the haemoglobin releases O2 into the low ppO2 environment = hypoxic). There is a high saturation of haemoglobin when oxygen levels are high (haemoglobin binds O2 from high ppO2 environment). The oxygen dissociation curve is sigmoid. This is due to cooperative binding. As each O2 molecule binds, it alters the conformation of haemoglobin. This increases affinity for O2 i.e. makes subsequent binding easier (cooperative binding). This means haemoglobin will have a higher affinity for O2 in oxygen-rich areas (like the lung), promoting oxygen loading and vice versa. (2) Bohr shift describes how the oxygen dissociation curves relates to carbon dioxide [remember GCSE: respiration]. As carbon dioxide and/or acidity increases, the binding affinity of haemoglobin for O2 decreases. [Higher level: Carbon dioxide forms carbonic acid due to action of carbonic anhydrase in RBC. The increase in H+ ions bind oxyhaemoglobin to form HHb which decreases affinity due to an allosteric conformational effect.] This results in a Bohr shift, as can be seen on the graph. This is important as it makes supply of oxygen to respiring tissues more efficient. Cells with increased metabolism (respiration) release greater amounts of carbon dioxide. Hence there is lowest affinity of Hb for O2 in respiring tissues. [Effect of low oxgen levels meaning less cooperativity PLUS the effect of high carbon dioxide.] This means that oxygen is released to area of greatest need.
