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Science Post of the Day

In my opinion, about 98% of the problems students (and society in general) have with science is how it’s presented.  Science is not a pile of unrelated facts that require memorization, but a way of thinking about the physical world and how it is all interrelated that makes sense.

Case in point.  I have given the following explanation to many students over the years and have always gotten a reaction along the lines of WHY THE FUCK DIDN’T ANYONE JUST SAY THAT?

Combustion and photosynthesis are the exact same thing, just in reverse.

First a clarification of terms.  The word “combustion” calls to mind fire, but it has a much more general meaning chemically.  It is the oxidation of a carbon-containing compound resulting in energy being given off.  This is the same basic reaction whether it’s your car burning gasoline, a fire burning logs, or your body burning that bagel you just ate.  Photosynthesis, well everybody knows what that is, mostly.

Photosynthesis requires energy INPUT, in the form of UV radiation.  Plants take that energy, water, and carbon dioxide, and use it to grow and generate carbohydrates (bark, stems, leaves, fruits, etc) and in the process give off oxygen.

Combustion GENERATES energy.  It takes those carbohydrates (starches, sugars), adds in oxygen, and generates energy and carbon dioxide and water.  That is how we get back the energy that the plants used to make the carbohydrates.

So the reaction looks like this:

WATER + ENERGY + CO2 <—->  CARBON COMPOUNDS + OXYGEN

If you read this reaction left to right, that’s photosynthesis.  If you read it right to left, it’s combustion.  Note that the energy bit is on the left.  That means for the reaction to go to the right, you have to ADD energy (solar energy for photosynthesis).  When the reaction goes to the left, energy is GIVEN OFF (in the form of heat of a fire, or energy your body uses to function).

Our bodies run on solar power, using plants as a carrier.  Plants take solar energy and convert it into a form that we can consume, directly or indirectly.  Fossil fuels come from the decomposition of animals, who ate plant materials, which…yeah.  

So that time you were learning about photosynthesis in biology class, and that other totally different time you were learning combustion in chemistry class, guess what?  Same thing.  But nobody every SAYS that.  And therein lies the problem.

4 October 2012 ·

• Tags
• science
• combustion
• photosynthesis