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Photosynthesis: Turning CO2 into O2 – or maybe not.

28 Sep
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It’s so simple, right?

“The evolution of photosynthesis remade the Archaean Earth. Before photosynthesis, the air and oceans were anoxic. Now the air is a biological construction, a fifth of which is free molecular oxygen”  – Bendall et al. 2008cIt’s easy to mistakenly think that photosynthesis turns CO2 into O2, people have been doing it for years. In fact, you’d even be remiss not to initially think that it’s the case – it is, after all, a simple conclusion to make and William of Ockham tells us to always start with the simplest ideas.

How could we do this experiment now?

We could  use radiolabeled Oxygen in our CO2 and then look for that same radioactive O2 being produced as a waste from the plant. But if that experiment were done, we’d quickly see that this wasn’t the case. As we will see below, this experiment was eventually what was done and instead of labeled CO2 being produced, the leaves of the plant becoming radio labeled, while only ‘cold’ CO2 was being released. Vexing!

One complication in addressing this idea comes from the very notion of air as being something to begin with. So, what is air? – and what happens (to air) during photosynthesis?

The Dutch scientist and physician, Jan Baptista van Helmont (1579-1644), did some early experiments to understand the nature of photosynthesis. His experiment was to determine where the mass of the plant came from. He suspected that it would be from the soil it was growing in, and did a very simple experiment that refuted this hypothesis. He reasoned that if the mass of the plant came from the soil, then it was a simple conversion that he could observe happening over time as soil was depleted resulting in an equal growth in mass of the plant. His experiment used a potted willow tree planted in 200 lbs of soil. In five years, his 5 lb sprig grew to 169 lbs, using only 2 oz. of soil.

Clearly the mass was coming from somewhere else. Knowing that he watered his tree regularly, he speculated that this was the source of the tree’s growing mass.

Helmont’s experiment did nothing to answer the question directly, but it does introduce a new player into the mix: Water… H2O. There’s Oxygen in water too – another possibility?

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What could possibly have killed this mouse?

In 1771 Joseph Priestley came onto the scene with experiments examining the nature of air as something more than just “nothing.” He noticed that a flame tainted the air with a kind of pollutant that was not amenable to animal life. He called this pollutant, phlogiston. Phlogiston could be produced by burning a candle in a closed container until the candle put itself out. Then, any animal (he used a mouse), that was put in this phlogistated air would quickly die. Yet a sprig of mint could counter this effect and somehow clean up the phlogistated air.

What do we know now?

1. Air is not just ‘nothing.’

2. Air quality (composition) is affected by certain biologic and abiologic processes.

a. Candle flames pollute the air with something toxic to animals (at least mice)

b. A mint sprig is sufficient to neutralize or eliminate this pollutant

Another Dutchman, Ingenhousz determined that de-phlogistation by plants occurs only in the light and required he green parts of plants to accomplish this.

(Much of the above material can be found in the excellent History of Research Page)

How to observe these gasses more easily? Perhaps under water, where gas will appear as bubbles.

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A simple experimental setup to measure photosynthesis

“When a sprig [of Elodea] is placed upside down in a dilute solution of NaHCO3 (which serves as a source of CO2) and illuminated with a flood lamp, oxygen bubbles are soon given off from the cut portion of the stem. ” -from a History of Photosynthesis. Using this device (pictured below) as a readout, F.F. Blackman measured gas production under various conditions by observing the production of bubbles under a number of conditions.

Data from such an experiment looks like this:

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The data

From these data, Blackman concluded that photosynthesis occurred in several stages, the first was a ‘light-limited’ stage that hastened with increasing light intensity, the second did not increase with increasing light intensity and required the work of enzymes (accounting for the effect of heat speeding up the reaction).

The Dutch scientist, van Niel  first suggested the idea of Oxygen gas coming from H2O based on his observations of purple sulfur bacteria converting H2S to S2 and assuming a parallel reaction was occurring in green plants.

CO2 + 2H2S → (CH2O) + H2O + 2S             (observed in purple sulfur bacteria)

CO2 + 2H2O → (CH2O) + H2O + O2             (predicted in green plants)

The final proof of this did not come until Ruben and Kamen were able to use an isotope of Oxygen to trace its route through photosynthesis.

Using algae, given ‘heavy’ oxygen in the form of either water or carbon dioxide, it was found that the isotope given in H2O was invariably that found in the resulting O2. Their experimental procedure is outlined in the diagram as two parallel experiments:

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% 18O FOUND IN
H2O CO2 O2
START 0.85 0.20
FINISH 0.85 0.61* 0.86
START 0.20 0.68
FINISH 0.20 0.57 0.20

So, what we should be saying is not that plants turn carbon dioxide into oxygen, but that plants turn carbon dioxide into sugar, which is precisely why van Helmont was confused by a 169 lb. tree growing from only 2 oz. of soil. He probably never would have believed that all that tree was actually built out of thin air.

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Posted by on September 28, 2013 in Uncategorized

 

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