Photosynthesis - The Amazing Process

The Process of Photosynthesis is truly amazing!

Say you get a dozen ice cubes, and also a small block of dry ice. Say Presto and those two very simple materials can change into any plant, whether a bush or a tree or a rose or a tomato plant.

You might want to think of this experiment in even simpler terms. A simple glassful of water and a room or tank which contains carbon dioxide. (Remember to say Presto!)

How is this possible? Water is a very simple chemical, just two atoms of hydrogen attached to one atom of oxygen, which we call H2O. Carbon dioxide is an equally simple chemical, just two atoms of oxygen attached to one atom of carbon, which we call C O2.

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Solar energy is very important to enable the process of Photosynthesis to create spectacularly complicated carbohydrate molecules. Actually, the process always creates a molecule called Glucose first, which contains 24 atoms, in a molecule we describe as C6H12O6. Glucose molecules can rearrange themselves into creating all the other molecules in all plants and animals.

Chemical Analysis of Photosynthesis

It took scientists a long time to figure this all out, but it is now known that the actual chemical reaction that happens is

(6) H2O + (6) CO2 + sunlight energy gives C6H12O6 + (6) O2.

It's really an interesting process, taking generic water (usually from the ground) and carbon dioxide (from the air) and energy from sunlight and creating a complex hydrocarbon molecule (called glucose) and releasing oxygen in the process. Essentially all living things exist because of this single chemical reaction, because we all use that glucose as the energy source for nearly all biological functions. (As the glucose is broken down, back into water and carbon dioxide inside us, for example, energy that had been earlier captured from sunlight is released to drive needed reactions in each living thing.

More complicated.

graphic from John W Kimball

The chemical bond strengths between atoms are: O-O 58 kcal/mole; (usually) C-O 93.5 kcal/mole; O-H 110 Kcal/mole. It is possible to then analyze the equation of the chemical reaction above. An H2O molecule has two O-H bonds, and there are six molecules involved, so the first term in the photosynthesis equation involves 110 * 2 * 6 or 1320 Kcal/mole of binding energy in those molecules. We have a similar situation for the CO2, except that those bonds are double (covalent) bonds, so we have 93.5 * 2 * 2 * 6 or 2244 Kcal/mole. So on the right side, we have a total binding energy of all the molecules of (1320 + 2244 or) 3564 Kcal/mole. On the left side, we have that complex molecule, where we can add up all the separate binding energies to get 2182 Kcal/mole for the binding energy of that molecule; and the O2 has a covalent bonding so we have 58 * 2 * 6 or 696 Kcal/mole. This gives a left hand side total of (2182 + 696 or) 2878 Kcal/mole. The difference of 686 Kcal/mole is the "free energy" which solar energy has to provide to drive the chemical reaction of photosynthesis. Or, similarly, it is the amount of energy that gets released when the bonds are broken in the other direction in an animal's respiration or decomposition process.

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This presentation was first placed on the Internet in April 2012.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago