When I told people that I was going to start writing a weekly science blog for www.chapelboro.com, the reactions I got ranged from rampant enthusiasm to overwhelming skepticism. When I told the skeptics that I was planning to lead off with a two-part series on photosynthesis, their concerns, to say the least, were not assuaged. But if you want to really understand what’s happening in the world, with energy, with food, with the environment, or even evolution, you have to start with photosynthesis.
Your Mother the Plant
Here is your quick Common Science review of photosynthesis. Plants capture carbon dioxide from the air, combine it with water from the ground, and make glucose. In doing so, the plant is storing energy from the sun in the chemical bonds of the glucose. In addition to working a bit like a battery to store energy, the carbon-carbon bonds in the glucose are used as the building blocks from which other important hydrocarbons, like amino acids and proteins, can be built. The action of photosynthesis takes place on a molecule called chlorophyll, more specifically on a single atom of magnesium in the center of the chlorophyll.
To understand what happens on the magnesium, we first need to talk about adsorption. As I discussed in “The World’s Greatest Cheat Sheet,” chemical bonds are formed between atoms when two atoms have just the correct number of electrons to share with one another to form a stable molecule. Once formed, these chemical bonds are relatively strong and it is no longer possible to identify which electrons came from which atom. Atoms can also interact with each other in a less intimate way through a process called adsorption. In adsorption, atom A sticks to atom B, but both atom A and B retain their own electrons and the bond formed by adsorption is weaker than that of a chemical bond.
When atom A adsorbs on atom B, their close proximity results in changes in the behavior and energies associated with each atom’s electrons. This alteration of properties involved in adsorption is how catalysts work. Catalysts are substances that can speed up chemical reactions. Nearly all industrial chemicals use a catalyst in order to make the rate of production economically viable. I gave you a real world example of catalysis earlier in “Fun with Fritz and Carl” with the Haber-Bosch process to make ammonia by reacting nitrogen and hydrogen.
Chlorophyll is a catalyst. If you combine carbon dioxide and water without chlorophyll, and shine sunlight on them, in theory with enough time you would make some glucose. The uncatalyzed reactions are very, very slow because carbon dioxide, in its natural form, is very unreactive. When chlorophyll is present carbon dioxide adsorbs on the magnesium atom. This distorts the properties of the carbon dioxide sufficiently to make it much more accommodating to chemically reacting with the water to make glucose efficiently.
I could probably have written a relatively complete and informative blog regarding the fact that magnesium catalysis is the basis of all life on earth, and saved the rest of this blog for another entry. But there is another part of the story, something which can give you some insight into the early stages of the evolution of life. Who knew you’d find the secrets of life right here on www.chapelboro.com?
At the top of the page there are schematics of the chemical structures of both chlorophyll and hemoglobin. The similarity strikes you immediately. To me, when I was first presented with these structures, the sensation was not unlike when you first realize by looking at the world map that Africa and South America used to be connected. Just as looking at the world map can tell you about ancient history, so can looking at the chemical structures of chlorophyll and hemoglobin. These molecules are clearly related to one another. As I will explain, they are mother and daughter.
For those not familiar with the type of notation for chemical structures used in the schematic above, the corners of polygons are all carbon atoms. I’ll resist the temptation of a longer discussion of the structure. For the moment, all we need to focus on is that the structures of the two molecules above are nearly identical, with hemoglobin having iron instead of magnesium at its center. The hemoglobin in your body resides primarily in your red blood cells and is responsible for the transport of oxygen through your blood stream. The oxygen gets into your blood by entering your lungs and then hitching a ride on your hemoglobin by adsorbing to the iron atom. In this case, the adsorption provides just enough of an attraction to pull the oxygen along through your blood until it reaches a cell that needs it. After your hemoglobin drops off an oxygen molecule, it picks up a carbon dioxide molecule and transports it back to your lungs to be exhaled.
At the beginning of the blog I told you that photosynthesis would teach us about evolution. Here is your dramatically condensed, Common Science, history of the earth:
· The earth was formed 4.5 billion years ago,
· Single-celled life began 3.8 billion years ago;
· Over the course of the next 800 million years, those single celled organisms evolved sufficiently to make chlorophyll, which was the beginning of oxygen in our atmosphere;
· After about another 1.5 billion years, organisms which had been making chlorophyll for over a billion years learned to make a variant with an iron atom in the middle instead of a magnesium atom.
This had some remarkable consequences.
A defining feature of life is respiration. The advent of hemoglobin, with its ability to transport oxygen into the interior and an organism and bring carbon dioxide back out, opened up the possibility for multi-cellular animals to evolve. For organisms to become larger yet, there was a need for an inner support structure, bones, giving rise to vertebrates like us.
If we trace our evolutionary history backward, somewhere there is our mother, a plant which started making a version of chlorophyll with an iron atom instead of a magnesium atom creating daughters who could now transport oxygen. Then all we needed to do was wait another couple of billion years and here we are.
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