so here we have a red blood cell and our red blood cells are specialized to carry oxygen so they contain lots of a molecule called hemoglobin hemoglobin as we can see here is a protein made up of four subunits and this means it is got a called ternary structure so it's a really large protein and each of these polypeptide chains contains a heme group each han group contains an iron ion and that's our Fe 2 plus and that's a part of the hemoglobin that's going to bind the oxygen when oxygen binds to hemoglobin which we often call loading oxyhemoglobin is formed and this reaction is reversible when oxygen unloads we talked about the loading and unloading of oxygen from hemoglobin in relation to our lungs and our tissues so in our lungs we have a high concentration of oxygen and in our tissues we have a low concentration of oxygen so in the lungs at the alveoli oxygen has entered the capillaries and it's going to load onto hemoglobin this is because hemoglobin has a high affinity for oxygen due to the high concentration of oxygen or partial pressure of oxygen affinity for oxygen just means the tendency a molecule has to bind with oxygen so by the time this has got round to the tissues where oxygen is being used up for respiration hemoglobin is going to have a lower affinity for oxygen because of the low concentration of oxygen this means oxygen readily unloads from hemoglobin for use in respiration you need to be able to understand this in relation to the oxyhemoglobin dissociation curve so what we can see on our x-axis is the partial pressure of oxygen and that's basically just another way of saying concentration of oxygen on our y-axis we can see the percentage saturation of hemoglobin with oxygen so how much oxygen is on that hemoglobin so I'm just going to draw on some units on the x-axis but don't worry too much about these so we would imagine that there would be a positive correlation between how much oxygen there is and how much of this is bound to hemoglobin but as you can see this isn't directly proportional and the curve looks a bit like an S shape and I'll come on to why that is in a second so we need to pay close attention to two parts of this graph where our partial pressure of oxygen is high that's going to show us what the percentage saturation of hemoglobin is like in the lungs and where the partial pressure of oxygen is lower that's going to show us what our saturation of hemoglobin is like in tissues so in the lungs as you would expect hemoglobin is saturated with oxygen whereas in the tissues hemoglobin is less saturated with oxygen that's because hemoglobin has a high affinity for oxygen in the lungs and therefore oxygen loads whereas in the tissues hemoglobin has low affinity for oxygen so oxygen unloads as I already explained so now going back to why the graph is s-shaped so we can see at really low partial pressures of oxygen as we increase the oxygen the percentage saturation of hemoglobin doesn't increase that much but once we get a little bit higher the percentage saturation oxygen increases really quickly for a small increase in oxygen and this is due to the cooperative nature of oxygen binding so after the first oxygen molecule binds the shape of hemoglobin changes in a way that makes it easier for the second and third oxygen molecules to bind to meaning hemoglobin has a higher affinity for oxygen so on the graph the gradient gets steeper the rate of increase in percentage saturation increases as the oxygen further increases but after hemoglobin starts to become more saturated it gets harder for further molecules to bind to and this is why plateaus so now let's take a look at when this curve can shift so carbon dioxide causes the curve to shift to the right and this is called the Bohr effect this is because when we increase the carbon dioxide in our blood for example when we're exercising we lower the pH of the blood this reduces the affinity of hemoglobin for oxygen and that's because hemoglobin changes shape making it harder for molecules to bind so overall this is beneficial because it increases the amount of oxygen that's being unloaded from hemoglobin at the tissues and this oxygen is going to be used in respiration and quite often in the exam you will be given a graph and you won't be asked to use the graph to explain this effect so what you'll have to do is you have to find the concentration of oxygen or roughly the concentration of oxygen there's going to be in the tissues and find the corresponding percentage saturation of hemoglobin for each of these curves and that will show you that a particular partial pressure of oxygen the percentage saturation of hemoglobin will be lower so finally you need to understand that hemoglobin is a molecule that's found in lots of different organisms however its structure may differ slightly between these organisms depending on the environment that they're adapted to live in this is because hemoglobin is a protein made of amino acid and when we change these amino acids hemoglobin can have a different structure this is because its primary structure changes resulting in at folding in a slightly different way this can change its shape and the affinity there has for oxygen so some organisms oxyhemoglobin dissociation curves may have shifted to the left while others may have shifted to the right and in order to interpret this we need to pick an advantage whether that's in the lungs or in the tissues so when the curve shifted to the left that means hemoglobin has a higher affinity for oxygen and means that it loads more readily in the lungs at a lower oxygen concentration and this is our main advantage we don't need to worry about the effect on the tissues this is particularly useful for organisms in low oxygen environments because they can load more oxygen in the lungs an example of this would be fetal hemoglobin fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin this is important because by the time the mother's blood reaches the placenta is oxygen saturation has decreased and this is because some has been used up by the mother's body therefore fetal hemoglobin needs to be better at absorbing oxygen than its mother's hemoglobin so the fetus can still get oxygen from its mother's blood across the placenta in organisms whose curve has shifted to the right this means that they have a lower affinity for oxygen this time we don't need to worry about the effect of this in the lungs it's going to be beneficial for our tissues so we can see from our graph there the same partial pressure of oxygen in the tissue our hemoglobin again has a lower saturation of oxygen and this means that more has been unloaded to the tissues similar to the Bohr effect this is important in organized that need more oxygen in their tissues for example ones with a high metabolic rate which may be small or active so these are the main two types of ways which organisms can be adapted to their environment by having different types of hemoglobin this is really important because it enables organisms to survive better in their environments