Haemoglobin

• Red blood cells contain haemoglobin which enables them to carry far more oxygen than if it was just dissolved in their cytoplasm
• Haemoglobin is a protein consisting of four polypeptide chains, each containing a haem group including an iron ion, which allows them to transport four molecules of haemoglobin
• The concentration of oxygen in the air or dissolved in a solution is referred to as the partial pressure of oxygen
• As each oxygen binds to a haemoglobin, the protein change shape, meaning the haemoglobin molecule has a different affinity for oxygen depending on how many oxygens have bound previously
• Erythrocytes (red blood cells) have a biconcave shape which gives them a large surface area for diffusion of oxygen. They are flat and thin which creates a short diffusion pathway
• The haemoglobins are a group of chemically similar molecules that are found in many different organisms
• Oxygen is transported by binding to haemoglobin in red blood cells
• Haemoglobin is a large, globular protein with a quaternary structure made up of four polypeptide chains. Each chain/subunit is a globin protein (two α–globins and two β–globins) and each has a prosthetic (a permanent non-protein part of a protein) haem group containing an iron ion (Fe2+)
• a prosthetic group is a permanent non-protein part of a protein
• Four oxygen molecules can bind to the four haem groups to form oxyhaemoglobin
• The four globin subunits in haemoglobin are held together by disulphide bonds and arranged so that their hydrophobic R groups are facing inwards, helping to preserve the three-dimensional spherical shape, and the hydrophilic R groups are facing outwards, helping to maintain solubility
• When oxygen binds to haemoglobin, oxyhaemoglobin is formed

  4O2 + Hb <-> Hb4O2
• The binding of the first oxygen molecule results in a conformational change in the structure of the haemoglobin molecule, making it easier for each successive oxygen molecule to bind; this is cooperative binding
• Affinity means how ‘sticky’ haemoglobin is – the higher its affinity, the stronger it binds
• Haemoglobin binds to oxygen with high affinity in the lungs, where there is a high partial pressure of oxygen. This causes haemoglobin to pick up oxygen in the lungs
• Haemoglobin binds with low affinity at respiring tissues, which there is a low partial pressure of oxygen. This causes oxyhaemoglobin to deposit oxygen at the tissues so they can use it for aerobic respiration
• Haemoglobin is seen as saturated when it contains four oxygen molecules (all binding sites are taken up)
• A dissociation curve has a sigmoid, or S-shaped, because binding of the first oxygen molecule causes oxygen to change shape and uncover the other oxygen binding sites. This makes it easier for further oxygen molecules to bind (cooperative binding). Once three oxygen molecules are bound, it is harder for the final one to bind because there is only one binding site available
• In the blood, CO2 combines with water to form carbonic acid. This acid then lowers the pH of the blood. The change in pH effects haemoglobin (since it is a protein) and causes it to change slightly, leading to the release of oxygen
• when the partial pressure of carbon dioxide in the blood is high, haemoglobin’s affinity for oxygen is reduced
• means in respiring tissues, where CO2 is being produced as a waste product of respiration, haemoglobin will give up oxygen more readily
• A high carbon dioxide concentration shifts the dissociation curve to the right. This is referred to as the Bohr Effect
• Organisms that live in low-oxygen environments (e.g. waterlogged soils) have a dissociation curve that lies to the left – this means that haemoglobin is good at binding to oxygen when partial pressures are low
• Organisms that are especially active (e.g. birds of prey) have a dissociation curve that lies to the right – this means that haemoglobin is good at releasing oxygen at higher partial pressures so that their cells can use it to respire
• Foetal haemoglobin's dissociation curve is shifted to the left to allow the foetus to obtain oxygen from the mother's blood
• At respiring cells the oxygen leaves (dissociates)
• The addition and removal of oxygen can also be described as loading and unloading of oxygen
• The partial pressure is a better measure of O2 levels instead of concentration because it gives an idea of how much O2 is actually in the air you are breathing, rather than a percentage constant
• 100% oxygen saturation means every haemoglobin is carrying four oxygens
• The colour of blood is related to the prosthetic group in haemoglobin
• Animals adapted for diving have a dissociation curve shifted to the left so their haemoglobin holds onto oxygen and acts as an oxygen store