SEx Education: It's Blood, Jim, But Not As We Know It

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Science Explained

It's Blood, Jim, But Not As We Know It

While discussing evolutionary advantages elsewhere, Son of Roj Blake popped into SEx to ask this question:

My blood is red. It is this colour (as I understand it) because the chemistry of blood is based on the oxidation of iron.

All blood I am aware of in the natural world is red. Is blood chemistry based on iron a universal feature of life on earth (life with a cardio-pulmonary system, that is)? Or are there any organisms with a blood chemistry based on, say, copper?1

Unusually for a question on the SEx forum, this one turned out to have a fairly straightforward answer. First to supply it was Potholer:

Horseshoe crabs have copper-based blood.

And anyone who can remember playing with copper sulphate in school chemistry lessons won't be surprised to learn that copper-based blood is blue. So, it appears that the answer to SoRB's question is a simple 'yes'. But what else can the SExperts tell us on this subject? Potholer provided this snippet about horseshoe crab blood:

It also has various medical uses2

Indeed, it does. Horseshoe crab blood is the source of the compound Limulus amebocyte lysate3, used to test medicines for bacterial contamination. In the past, horseshoe crab shells have been used to make agents that promote blood clotting, and as a basis for absorbable sutures.

Horseshoe crabs aren't the only animals with copper-based blood. Certain less-exotic creatures also share this peculiarity, with Pedro unearthing a couple of handy links showing that some species of snail and lobster also have blue blood. As Gnomon explained:

Their branch of the family split from ours so long ago that we hadn't even evolved blood at the time. So we and the molluscs developed it independently and differently. We have red blood based on iron (haemoglobin). They have blue blood based on copper (cyanoglobin).

Digging Deeper

Having answered the original question far too easily, the SExperts next pondered why iron-based blood should be more common. 2legs and Mu Beta teamed up to provide a comprehensive answer:

a) It's abundant, therefore has a high probability of incorporating itself somehow into organic life

b) It has a variety of oxidation states, which make it particularly flexible in terms of complex formation

— Mu Beta
I'd hazard a guess it's something to do with the binding potential of the iron in haemoglobin, with oxygen being suitable so it can bind it, but not so strong it won't release it again in the tissues
— 2legs

To finish, Orcus brought the conversation back to where it began: evolution.

Whilst the identity of the metal might change between species, both haemoglobin and myoglobin4, cyanoglobin, chlorophyll and vitamin B12 all contain a unit that is more or less the same in all of these proteins. It's called the 'haem' in the first three enzymes; chlorophyll is very similar and the corrin ring from B12 is close.

Do not assume that there is much difference going down here - one could imagine a common ancestor for all these with little leap of faith. There are a lot of destructive, oxidising enzymes in the liver called cytochromes (because they are coloured) that also contain haem-like structures. The metal might change but the organic molecule that binds it remains almost identical.


This article was based on a conversation at the SEx forum - where science is explained.
Why not pop over with your own questions? The pick of the bunch will feature in The Post's next issue.

SEx Education Archive Archive

Danny B.


Back Issue Page

1And can we get the references to Mr Spock out of the way as quickly as possible, this is a serious question...2Good for us, but not necessarily fortunate for them.3The blood is extracted without harming the animals, which are returned to the sea after the procedure.4The protein in cells that accepts oxygen from the haemoglobin in blood.

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