I understand that at least some of the water that enters the plant is split and “new” water is made from the resulting oxygen molecules combining with the hydrogen molecules from the carbon dioxide taken in through the leaves and dismantled. My question is: is all the water that enters the plant split? Or does some of it remain whole and help move the nutrients through the plant? What percent of the water that enters a plant is split?
Additionally, I read that a lot of water is lost through the leaves (on a hot day in the desert a cottonwood can lose 100 gallons per hour!). Is this water “new” water, i.e., water that has been split and reconstituted?
I am not a botanist but my guess is that this would vary with the plant. Certainly, a good portion of water would not be used for photosynthesis and would travel through the plant carrying nutrients with it. Some of the water exiting the plant would be water produced by cellular respiration.
A lot of the answer would also depend on the time of day. There is more transpiration and photosynthesis at midday while photosynthesis shuts down at night.
While water is split by plants, algae, cyanobacteria, and photoplankton, it is not made back into water – what would be the point of going through all that effort, just to arrive back at water where you started? None of the water in a plant is “new water”.
Nor can plants “manufacture” water … otherwise, I wouldn’t have to water my garden, and all manner of plants could grow in the desert.
Instead, the hydrogen is used to make sugars, which are used for energy and plant growth, and the oxygen is released into the atmosphere.
I do not know how much of the water in the plant is split, but I suspect it would be a small percentage. In addition to being split, water is used to transport various substances within the plant, to mechanically strengthen the plant through cell pressure (turgor), and to cool the plant through evapotranspiration. As you point out, cooling can require many gallons.
The amount split, on the other hand, can be estimated by looking at the plant’s weight gain. This is orders of magnitude smaller than the amount of water used for cooling alone.
I am most definitely not a chemist, biochemist or a botanist but I am fairly sure that as far as the OP is concerned any water that results from condensation reactions within plants, such as polysaccharide synthesis, is “new water”. Not every hydrogen and oxygen atom that is in it came from split water or at least water split in that particular plant.
So the question, as it stands, seems valid and I am waiting eagerly for somebody who knows this stuff to arrive!
That “0.86 to 6.05 moles per square metre per day” translates to 15.5 to 109 milliliters of water split per day per square meter.
Transpiration rates are considerably higher. Wikipedia conveniently gives a nicely reffed number:
Assuming all the CO[sub]2[/sub] (6 moles, 264 g) goes to dry weight, that square meter of plants would transpire between 53 and 264 Liters of water a day. Even if you claim only 1% of fixed CO[sub]2[/sub] goes to dry weight (lots of respiration), you’ll transpire half a liter of water for each 100 mls that gets split by photosynthesis.
Short answer: A minimum of 5 times as much water gets transpired through a plant as gets split by photosynthesuis.
Well, some of this is a little over my head, but do we know that the water that is transpired is NOT “new” (recently split) water? Is it possible that all the water that comes into a plant is split, then some of it goes to respiration, some to glucose, and some to new water?
I understand that 12 molecules of water and 6 molecules of carbon dioxide are all split and reconstituted into one molecule of glucose, 6 molecules of oxygen and 6 new molecules of water. I gather that’s prettty basic info. The question remains, what percent of the water that enters the plant is split.
Yeah, we do know, by measuring the rate of NADPH formation from water splitting. That rate is consistent with a tightly couple photosystem.
Cycling through all the water while allowing most of the captured energy to get away via a back reaction would be inefficient.