SCIENCE: photosythesis and orchids

[Myxamatosis]

My primary knowledge that I can contribute to this forum is in mid level biological explanations. So in that light here is how orchids do their thing with the sun, no pun intended.

PHOTOSYNTHESIS
We all know photo synthesis looks something like
CO2 + H2O+ sunlight -> O2
or 'plants breathe carbon dioxide
How this is done is a little involved but the generalities will explain a lot of things you already know.

There is a very wonderful enzyme called RUBISCO (shortened name). RUBISCO's function is to assist in the carbon fixation process. Carbon fixing is taking the C in CO2 and making it into sugar (C6 H12 O6). RUBISCO helps with some rather fancy chemical reaction in what is called the Calvin Cycle which helps make sugar (fix carbon). RUBISCO only does this when there is more carbon dioxide than oxygen.

If there is more oxygen the RUBISCO will start a process call photorespiration. photorespiration is what we do, if we used light (O2 -> CO2). the problem with this is that plants are not good at it. Frankly, they are horrible at it. When this happens they are left with just a wee bit of usable energy and several acids that are more or less useless, and toxic in high concentrations. check the footnote for more about this deal.

PhotoSYTHESIS good, photoRESPIRATION bad.

Plants have developed interesting ways to deal with this respiration.

PHOTOSYNTHESIS C3, C4, and CAM
-this is where things might get a little hairy, wikipedia explains this well-
there are three ways to take care of buisness, C3, C4, and CAM. C3 and C4 are more common than CAM (orchids are CAM). Three things need to happen for this to work, we need the light to be made into usable energy, we have to store the energy somewhere, and then use it.
Catching a Photon
photons are little packets of light, this is where our energy comes from. Light is caught in the grana (stacks of chloroplasts). the process of catching the light is complicated, all we need to know is that the light is caught and turned into energy called ATP and ATP+Pi. ATP or Adenosine Triphosphate, and ATP+Pi Adenosine Triphosphate with a inorganic phosphate group. Unless you are a chemist that should mean nothing, the way to think of them as cash cards.**
ATP + Pi is a cash card that is 'loaded' so you can 'buy' things with it
ATP is just the plastic, if you can get more energy you can make it into ATP+Pi to buy things.
Once we have our ATP+Pi we send it to the Calvin Cycle. In the cycle we give up our ATP+Pi and get ATP and sugar. do this a few more times and you end up with a big pile of sugar, you connect them all together and you get a starch. Starches are very stable and are very effective at storing sugar over long periods of time, this can be thought of as a bank.

C3 Think grass and corn
Now things get easier.
All of those steps happen in one cell in a quick and 'simple' manner. The problem is if you have too much photosynthesis at once then you have alot of O2 build up, which causes photorespiration, which is bad news bears. The plant has 'vents' called Stomata. the stomata allow for O2 and CO2 to be moved in our out, but they also let out water. so in the heat of day when the most photosynthesis is happening the plant cant open the gates because it will dry out. So it photoresperates. In the end everything works out fine except the plant then goes on clean up detail to get ride of all the acids, which takes energy.

C4 not most plants
C4 is a wonderful thing! it is slightly less efficient but we cut out this photorespiration buisness. photon capture occurs in the outer layer of cells and ATP+Pi is made while O2 and CO2 build up in the cell. The ATP+Pi and CO2 move to the second cell, which is behind the first. oxygen can not move to the second cell because of the complicated thing called active transportation. the calvin cycle occurs in the second cell. the oxygen byproduct is then dealt with with fancy chemistry. the stomata open at night or while shaded for gas exchange but the CO2 never builds up. Once again, we have sugar, the plant wins.

CAM Orchids, Cacti, & Pineapples (the ones you are reading this for)
Crassulacean Acid Metabolism is just like C4 except is is done in one cell. the process is divided into day and night cycles.

DAY
photons are absorbed and rather than going to the Calven cycle, the energy is stored in fancy chemicals. the stomata are closed at all times during the day.*** By nightfall we have lots of high energy chemicals.

NIGHT
the stomata open wide and gases exchange freely while those 'fancy chemicals' are broken back down to ATP+Pi and go through the Calvin Cycle and you know the rest. Because the stomata are open photorespiration effectively does not occur. The fancy chemicals are 're-primed' for the morning and we wind up with sugar, all is well. This takes a lot more energy, but it works!

What this means for my orchids
Ever wonder why your orchids like the cool night temperatures? This is because it is 'easier' for the stomata to open wide and they lose less water due to evaporation. families like Masdev's from the cloud forests with no roots and nearly no psuedobulbs survive because they suck up water from the air at night. orchids grow so much more slowly because the storing chemicals takes so much energy. The more you learn about where a species is from can help explain how the plant is doing its thing.

So now you know, doesn't really help with the care of the plants, but the more you think about it the more you can see it. these cycles also tell us a lot about pH, but that is another story.

SM

FINAL NOTE
I typed all of this in one sitting, with out my notebook that this info is in. sorry for any technical slips & typos.

FOOTNOTES
*the evolution of RUBISCO's duel action p.syth & p.Resp is extremely debated and some point to this molecule as a sign of the division between the plantae and animalia kingdoms
**you can not spend 'half an ATP', it is an all or nothing process because the energy comes from the separation of the Pi from the ATP+Pi.
***for our purposes they are closed

[patticake]

Thank you, Myxamatosis!! You are the photosynthesis guru! That's putting your college education to work! I was sort of fuzzy on the very basics of photosynthesis, but you just fine-tuned it to a high definition picture! Wowsers! Now I have to go off to work with my head spinning! (I hope there won't be a test on this later...)

[hera]

Thanks for the education professor. This needs to be a sticky.