7 At the molecular level, chlorocruorin closely resembles hemoglobin. In fact, its chemical composition diverges from hemoglobin in just one respect: Chlorocruorin contains an aldehyde group (-CHO) whereas hemoglobin has a vinyl group (-CH=CH2). Nevertheless, it is worth noting that chlorocruorin does not contain chlorine as its name may otherwise suggest. Here comes an interesting twist: Green blood does not solely rely on the presence of chlorocruorin. Like most vertebrates, green-blooded skinks from New Guinea use hemoglobin to carry oxygen. However, their blood and tissues are green [4]. This peculiar phenomenon is related to how these lizards recycle hemoglobin. In humans, the recycling of hemoglobin involves two steps, first by breaking the pigment down into a green chemical named biliverdin, then by converting biliverdin to a yellow compound called bilirubin. However, the lizards lack the ability to further metabolize biliverdin, leading to an accumulation of the green pigment in their blood [1, 2]. The color is so intense that it overshadows the natural red color of hemoglobin. Colorless Blood Perhaps the strangest “blood color” is having no color at all. The Antarctic icefishes stand out as one of the most unusual vertebrates as they lack any respiratory pigment [2, 3]. As a result, icefish blood is colorless with only blood plasma. Despite the absence of hemoglobin, scientists have discovered remnants of hemoglobin genes in icefish genomes, suggesting that these genes were lost during the course of evolution [5]. But the question remains: How do these Antarctic fishes survive without such an important oxygen carrier? Icefishes have developed several adaptations to compensate for that [2]. They have a larger blood volume than related fish species, and lead a relatively sedentary lifestyle, which helps reduce their oxygen demands. On the other hand, the cold water of the Southern Atlantic also promote icefish survival by maintaining higher concentrations of dissolved oxygen than warmer seas. Conclusion Now back to the same question: What is the color of blood? This time, you might be tempted to respond with a range of colors we have covered – red, blue, purple, green, and even colorless – and they are all right! For other creative answers that you may think of, why not? Scientific knowledge is tentative and subject to change when new evidence appears – so who knows, perhaps there are yet undiscovered ones waiting to join the evergrowing palette of blood colors! 1. Editor’s note: While most of the carbon dioxide is transported through blood plasma in the form of bicarbonate ions in humans (as mentioned in high school textbooks), 10% of the gas is actually carried by hemoglobin [6]. Hemoglobin (Heme B) Hemocyanin Chlorocruorin Hemerythrin
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