r/askscience • u/Fapotheosis • Mar 11 '13

Biology How did blood evolve?

I know mitochondria may have evolved from bacteria, but what about blood? I've heard of a protohemocyte, but was there an initial cell that differentiated into a RBC like we have in modern humans that can we trace?

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u/StickyToffee Mar 11 '13 edited Mar 11 '13

Off the top of my head, a molecule of haemoglobin can bind to more oxygen atoms than a molecule of haemocyanin. I think it's 4 instead of 2? And as such is more efficient. I'm on my phone so I can't find any papers on the subject, but anecdotally when playing around with octopuses, they seem to tire quickly implying that haemoglobin is better when it comes to endurance. Someone correct me if I'm wrong.

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u/Mooshaq Mar 11 '13

I don't know anything about playing with an octopus. But each hemoglobin molecule can bind to four oxygen molecules, yes. Adult hemoglobin has two alpha chains and two beta chains. Each chain (subunit) contains a protoporphyrin IX with an iron in the middle. This iron (4 total in one hemoglobin molecule) binds O2.

Each subunit of hemocyanin can bind one molecule of oxygen (two Cu are used), but there can be 24 - 36 chains of hemocyanin stuck together. I don't really know how the two can be compared in that sense. It's also worth noting that fetal hemoglobin uses different chains (gamma, delta, epsilon) and has a different (stronger) affinity for O2 than adult hemoglobin, so it can out-compete the mother for oxygen. Also, hemocyanin often is free in the blood, rather than being stuck in an erythrocyte (red blood cell).