To understand the colour of the blood we need to know the blood proteins. Today for many animals, the blood protein of choice is globin. A globin molecule has a special prong on it that binds to an atom of iron, which in turn is surrounded by a doughnut-shape molecule called heme. And on the opposite side of the doughnut, a molecule of oxygen can bind to the iron. The basic protein structure that cradles this heme doughnut is called the globin fold. And this fold is so distinct and so good at holding onto and releasing oxygen, that it's been used in many different forms, by many different organisms to do a variety of jobs over the aeons. Today, in many animals, including you, blood carries oxygen around the body with the help of a protein called haemoglobin. This is why your blood is red because of iron. Blood inside the body is darker but if you get a cut and bleed the blood would become bright red because of oxygen.
Haemoglobin is not the only one. The horseshoe crab has blood proteins called hemocyanin. Hemocyanin has copper rather than iron. This is why horseshoe crabs have blue blood because copper turns greenish-blue when it's oxidized. Hemocyanin and Haemoglobin are the most common oxygen-carrying blood proteins found in animals today, and they're the ones we know the most about. But other animals have different blood proteins.
Many species of marine worms and brachiopods, for instance, use a totally different blood protein hemerythrin. It uses irons to transport oxygen, but it doesn't have doughnut-shaped heme. Because of this, the blood in those animals turns a bright violet when it's oxygenated. And like hemocyanin, this protein is less efficient, but it's also simpler so simple, in fact, that it's thought to have been used by the very earliest single-celled organisms. Blood can also be green. Some animals like certain species of lizards have a lime green pigment in their blood called biliverdin which is produced when haemoglobin is broken down and having a lot of this stuff might actually make their blood more resistant to disease. And other animals have even lost their blood proteins entirely like the Icefish which lives off the coast of Antarctica. Its blood is a clear white because unlike other fish it doesn't have any haemoglobin or other proteins at all. That might be because having blood cells would cause its blood to clot too easily in such cold temperatures or maybe it was just a genetic accident. But even without blood proteins, the Icefish gets along by having a low metabolism and living in oxygen-rich waters.
To understand the colour of the blood we need to know the blood proteins. Today for many animals, the blood protein of choice is globin. A globin molecule has a special prong on it that binds to an atom of iron, which in turn is surrounded by a doughnut-shape molecule called heme. And on the opposite side of the doughnut, a molecule of oxygen can bind to the iron. The basic protein structure that cradles this heme doughnut is called the globin fold. And this fold is so distinct and so good at holding onto and releasing oxygen, that it's been used in many different forms, by many different organisms to do a variety of jobs over the aeons. Today, in many animals, including you, blood carries oxygen around the body with the help of a protein called haemoglobin. This is why your blood is red because of iron. Blood inside the body is darker but if you get a cut and bleed the blood would become bright red because of oxygen. 
Haemoglobin is not the only one. The horseshoe crab has blood proteins called hemocyanin. Hemocyanin has copper rather than iron. This is why horseshoe crabs have blue blood because copper turns greenish-blue when it's oxidized. Hemocyanin and Haemoglobin are the most common oxygen-carrying blood proteins found in animals today, and they're the ones we know the most about. But other animals have different blood proteins.
Many species of marine worms and brachiopods, for instance, use a totally different blood protein hemerythrin. It uses irons to transport oxygen, but it doesn't have doughnut-shaped heme. Because of this, the blood in those animals turns a bright violet when it's oxygenated. And like hemocyanin, this protein is less efficient, but it's also simpler so simple, in fact, that it's thought to have been used by the very earliest single-celled organisms. Blood can also be green. Some animals like certain species of lizards have a lime green pigment in their blood called biliverdin which is produced when haemoglobin is broken down and having a lot of this stuff might actually make their blood more resistant to disease. And other animals have even lost their blood proteins entirely like the Icefish which lives off the coast of Antarctica. Its blood is a clear white because unlike other fish it doesn't have any haemoglobin or other proteins at all. That might be because having blood cells would cause its blood to clot too easily in such cold temperatures or maybe it was just a genetic accident. But even without blood proteins, the Icefish gets along by having a low metabolism and living in oxygen-rich waters.
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