By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

Photosynthesis Part I: Oxygen Gets All the Press

By Jeff Danner Posted April 25, 2011 at 12:53 am

Any discussion of petroleum starts with photosynthesis.  Actually, pretty much any discussion of anything should start with photosynthesis.  No photosynthesis, no us, no Chapelboro.

Somewhere in your high school chemistry class (actually not long ago someone told me that you can now graduate from high school without taking chemistry, but I am repressing that thought) your teacher started a lesson on photosynthesis.  Not long into the lesson he/she wrote the following on the board:

6 CO2 + 6 H2O  à C6H12O6  + 6 O2

At this point what could have been a fascinating lesson on the most important process on earth probably began to go south.  Perhaps your teacher paused at this point to use photosynthesis as an opportunity to review the most dreaded topic in high school chemistry, balancing equations.

To the extent that the impact of photosynthesis was discussed the emphasis was almost certainly on the conversion of carbon dioxide to oxygen.  This is, of course, an important topic.  Around 3 billion years ago the earth’s atmosphere lacked oxygen.  At about this time, some unknown precocious microorganism had just the right mutation and evolved into the first photosynthetic organism, cyanobacteria.  

Let me digress for a second here.  Usually at this point in a conversation, say over Thanksgiving dinner, I’ve just got my momentum going on the topic I am looking to address and someone stops me to ask a question starting with “But how do they know ……”.  In this conversation the question would be “But how do they know it was 3 billion years ago?” What I learned from these questions over the years is that if you are going to tell a story about science it needs to be fully integrated with explanations of the measurement and data analysis techniques.  Otherwise the no one pays attention to or believes the story.

We know it was 3 billion years ago from radio carbon dating. So here is a brief explanation of radio carbon dating.  What defines an atom is the number of protons it has.  The nuclei of nearly all carbon atoms have 6 protons and 6 neutrons.  To get the mass of an atom you add the number of protons and neutrons in the nucleus.  Therefore, your average carbon atom has an atomic mass of 12 (carbon-12).  A very small percentage carbon atoms have 2 extra neutrons, so they have a mass of 14 atomic mass units (carbon-14). (I suspect some of you are asking “but why do some of the carbon atoms have 2 extra protons?  For now I need you to just accept it. If I go off on a tangent on this tangent we’ll never get to the end of this blog entry).  As long as a plant is alive it keeps photosynthesizing more carbon dioxide from the air and some of this carbon dioxide is made from Carbon-14. Once the plant dies it stops adding new carbon-14.  The carbon-14 in it is not stable (this is another potential tangent we can’t address at this point, maybe I can get to it in the comment section later) and degrades over time.   Therefore, by measuring the ratio of carbon-12 to carbon-14 you can calculate how long ago the plant died since the longer it has been dead, the less carbon-14 that remains.  You can use this same method to figure out how long ago an animal died from testing the level of carbon-14 in its remains since all animals either eat plants or animals which eat plants.

OK, so you find an old rock that contains organic matter from really old bacteria and also contains metal oxides (which are created when parts of rock react with oxygen in the air).  Test the remains of the bacteria for how much carbon-14 is left, and you can figure out when the bacteria died and therefore how old the rock is.  Based on these types of measurements, we know that rocks which are about three billion years old started to have oxygen containing compounds which tells us that this is when photosynthesis started (make sense?).

Over the next billion years, those cyanobacteria and their evolutionary progeny generated enough oxygen to support the evolution of animals and to reach the level of ~21% of the atmosphere that exists today.  (The reason that oxygen reached and leveled out an equilibrium level of 21% is an equally fascinating question beyond the scope of this blog entry.)

At this point, your first lecture on photosynthesis likely came to an end and your teacher was likely feeling happy and satisfied. The story of how the world came to be filled with oxygen is a compelling one and can hold the attention of all by the most cynical of students.  For our series on petroleum, the next lecture, the one about glucose, is the important one.  We’ll get to glucose next week.

Now, those are my thoughts. What are yours? Comment below!”

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