Biology / Science

Blue-blooded

Here’s summat I’d like to know more about: some (all?) insects have blood not based on haemoglobin. What’s the difference? How does it work? Are there even more different blood chemistries out there that I haven’t heard about? (Could maybe make multiple posts if that is a lot.)

I thought I’d start by answering the easiest question! And then I immediately got into the weeds. So here we go…

It turns out that arthropods (like spiders, crustaceans and insects) don’t actually have blood in the same sense as we do; it’s different enough to justify giving it a different name. Instead of blood, arthropods have an analogous substance called “haemolymph”. Though that immediately sounds like the probably-familiar vertebrate blood protein, haemoglobin, the oxygen-carrying component of haemolymph is actually based on copper, rather than iron as the heme unit of haemoglobin is. This protein is called “haemocyanin”.

However! It’s not a simple one-for-one substitution. In fact, insects don’t use their circulatory system in quite the same way as we do. When we think about our blood, we often think in terms of its oxygen-carrying properties — our whole bodies are designed to carry our blood through the lungs (picking up oxygen) and then disseminate their oxygen throughout our tissues, after all. But in most arthropods, the haemolymph doesn’t actually carry oxygen. How they do get oxygen is a whole other blog post, but in summary, it’s a system which relies on the fact that insects are typically small, so the exchange of gases doesn’t need to be as efficient.

Larger arthropods do need an oxygen-carrying molecule, though, and this is carried in the haemolymph like vertebrate blood cells. The main difference (other than the chemical makeup, which I’ll get to in a second) is that haemoglobin is typically bound to a blood cell, but haemocyanin is free-floating in the haemolymph. Haemoglobin is made up of many linked subunits, but the part that binds oxygen is the heme group.

As you can see in the diagram, the iron (Fe) is bound in the middle of that ring, and one heme group like this can bind one oxygen. The structure here is really important: it’s no good for the heme group to be able to bind oxygen if it can’t let go of it, so the bond that forms between iron and an oxygen molecule is reversible: a covalent bond between one of the oxygen atoms in the O2 molecule and the iron ion in the centre of the ring. When there’s no oxygen molecule to bind, the heme subunit weakly binds a molecule of water instead. (Carbon dioxide, the waste product of respiration, is not carried by heme but by another part of haemoglobin.)

Haemocyanin is similar in structure, but the part that binds oxygen is formed from copper. The principle is basically the same, even though it’s not a one-for-one exchange between copper and iron: the subunit shown below is binding an oxygen molecule with the copper in a reversible way.

In any case, haemoglobin is usually four times as efficient as haemocyanin at carrying oxygen, which is why it’s so incredibly common and used in almost all vertebrates (about which, more next week after I’ve had a chance to bone up on it) but at low temperatures haemocyanin outperforms haemoglobin.

Getting any deeper into the chemistry is a bit beyond me without some serious study, but I think that demonstrates how it works in principle!

Next week, I’ll hopefully be able to tell you about how icefish do it.

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