It is my understanding that photosynthesis results in plants turning CO2 and the hydrogen from water into carbohydrates. This results in excess oxygen (I've been told from the water, though some [half in fact] must come from the CO2, no?) being released into the atmosphere as O2.

6 H2O + 6 CO2 = C6H12O6 + 6 O2

CO2 and O2 seem to balance more or less thanks to the biological processes of plant and animal life on the planet, but my question then is: what about the water? If plants have been soaking up water for hundreds of millions of years and converting it into O2, then what is preventing all the water on the planet form being used up? Is the ratio of total water to water used in photosynthesis simply extremely small - so much that plants could never use up all the earth's water in billions of years, or is there a natural process that balances this somehow by turning O2 back into H20? Or is it simply that when carbohydrates are used and converted back into energy that water is reformed/released?

You are correct, burning carbohydrate produces various combinations of C, CO and CO2, plus H2O. More importantly, the process of biological respiration takes sugar and effectively "burns" it in a series of controlled steps, but resulting in waste CO2, and water. The CO2 is literally breathed out, replenishing what the plants used to make the carbohydrates. The water is produced using oxygen gas as the final acceptor in the electron transport chain, so those molecules enter the population of other waters in our body, to be used like any other, or may be Incorporated directly into other molecules. Either way, it takes the place of one water molecule that would have been brought from the outside to fulfil the same function.

In essence, oxidative respiration is the biological mirror image of photosynthesis, in that each feeds the other in a beautiful meta-circle.

Last edited by John Steemson (20th May 2015 02:49:06)