Practice question

Hemocyanin serves as predominant extracellular respiratory pigment in many arthropods, notably crustaceans, xiphosurans and arachnids, characterized by copper atoms each coordinated by histidines that reversibly bind one oxygen molecule, turning blue when oxygenated. Hexameric or multi-hexameric molecules are dissolved freely in hemolymph, not packaged in cells, exhibiting cooperative oxygen binding albeit lower affinity than hemoglobin iron-porphyrin complex in vertebrates. Hemoglobin occasionally occurs in some aquatic insects and crustaceans, chlorocruorin in some annelids, myoglobin in muscle. Hemocyanin adaptation suits low and fluctuating aquatic oxygen levels, temperature sensitivity and high hemolymph pH.