Why is blood red? It contains heme, a molecule very similar to chlorophyll... but with iron instead of magnesium at its heart!
Like chlorophyll, heme is a porphyrin. That means it's made of four pentagons of carbon and nitrogen joined together by bridges of carbon. In chlorophyll, almost all the carbons connected by alternating single and double bonds. In heme, they all are.
These alternating single and double bonds let electrons roam around in a smeared-out, quantum way... and when light hits the molecule, it's like a kid jumping on a trampoline! It makes these smeared-out electrons vibrate back and forth. But unlike the kid, the light gets absorbed if it's vibrating at the right frequency: it transfers all its energy to the molecule.
Chemists have a really sexy way of talking. Listen to how they say what I just said:
Heme and chlorophyll are porphyrins. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). Porphyrins are aromatic. That is, they obey Hückel's rule for aromaticity, possessing 4n+2 π electrons delocalized over the macrocycle. Thus porphyrins are highly conjugated systems. As a consequence, they typically have intense absorption bands in the visible region.
Nothing about kids jumping on trampolines...
But I've got some questions. It's important that chlorophyll absorbs light - that's how plants get energy. It doesn't seem important that heme absorbs light. What heme does is grab oxygen, carry it to where it needs to go, and then let it go. So:
Puzzle 1: is it just a 'coincidence' that blood is red, or is its color somehow a necessary aspect of its biological function?
Puzzle 2: did heme evolve from chlorophyll? Did chlorophyll evolve from heme? Did they both evolve from a common molecule? Or in each case did nature arrive separately at the idea of using a porphyrin with a metal ion in the middle?
I can give you some hints for Puzzle 2. Hemoglobin and hemoglobin-like molecules are also found in many invertebrates, fungi, and plants. These molecules can carry oxygen, or transport and regulate other things such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide ions.
There are also porphyrins containing other metals! The blood of crabs contains copper instead of iron:
Other porphyrins contain zinc. In fact, if you get lead poisoning, your body has trouble making heme and starts making a very similar compound with zinc at its center:
For more, check out:
The molecule shown here is heme b, the most common form in humans. Heme a is a lot like chlorophyll a, which I showed you earlier - they both have a long chain of carbons attached to the porphyrin:
"Data smashing is based on a new way to compare data streams. The process involves two steps. First, the data streams are algorithmically 'smashed' to 'annihilate' the information in each other. Then, the process measures what information remained after the collision. The more information remained, the less likely the streams originated in the same source."
They say it works on electro-encephalograph (EEG) patterns pertaining to epileptic seizures, detection of anomalous cardiac activity from heart sound recordings, and classification of astronomical objects from raw photometry.
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