The key difference between hemocyanin and hemoglobin is that hemocyanin is a copper-containing extracellular respiratory pigment present in some invertebrate blood while haemoglobin is an iron-containing intracellular respiratory pigment present in vertebrate blood.
In aerobic organisms, gas exchange occurs through the metalloproteins present in the blood. Therefore, hemocyanin and hemoglobin are two metalloproteins that facilitate gas exchange in invertebrate animals and vertebrate animals, respectively. Hemocyanin is a copper-containing respiratory pigment which is found suspended in the hemolymph of invertebrates. In contrast, hemoglobin is an iron-containing respiratory pigment that is bound to the red blood cells of vertebrates. The oxygenated form of hemocyanin is blue in colour. But, the oxygenated form of hemoglobin is bright red in colour.
