# Killies Import > Planted Tanks >  Photosynthesis

## TyroneGenade

In response to a question about photosynthesis...

*What is photosynthesis* 

Photosynthesis is a 2 part process whereby plants trap energy from light in the form of simple sugars they can use to burn for energy or build new cells. 

The 2 parts are termed the _Light_ and the _Dark_ reactions for reasons that will be explained below. 


During the Light reactions the two types of chlorophyll molecules (a and b) will trap light energy of around 460 and 680 nm (type a) and around 480 and 640 nm (type b). The light energy is used to convert ADP to ATP (the energy currency of the cell) and NADPH (the energy bank of the cell) along with 1 molecule of O2. These reactions need light to proceed as well as a demand for NADPH. 

During the Dark reactions the the ATP and NADPH is used to incorporate CO2 into the five carbon sugar ribulose with production of 2 phosphoglycerate (a 3 carbon sugar). 

The 1st law of thermodynamics states that matter and energy cannot be created nor destroyed. Hence everything has to balance. 6 CO2 are used along with 18 ATP and 12 NADPH to produce 12 phosphoglycerate that are used to produce glucose. As long as there is NADPH, CO2 will be fixed by the plant. For every 2 NADPH used 1 O2 is produced. This means that in fixing 6 CO2, 6 O2 are produced by the plant from 12 water molecules and 2 photons (packages of light). 

Bottom line: the plant releases O2 and trapps CO2. Energy is stored in the form of sugar. 

*What happens in the dark* 

Without light the dark reaction will continue until all the NADPH is exhausted. The plant will also switch towards burning sugar to produce ATP. So in the absence of light the plant will now produce CO2. 

What happens with this CO2? 

In water CO2 is a rare commodity. The plant does not want to get rid of it. This is a problem faced by dessert and ant/artic plants too. Any plant that cannot open its stroma to allow CO2 in (and so allow water out) has to find a way to trap CO2 when it can. 

Plants have at least two means to do this: C4 photosynthesis and malic acid metabolism (CAM). In both cases the CO2 is fixed in either oxaloacetate or malate with the use of energy. Any and all CO2 that enters or is formed in the cell is fixed as oxaloacetate or malate. It is not known which (if either) method aquatic plants employ but it is well accepted that they do not release the CO2 they produce. (See here  for more information. )

So during the dark they will become net O2 users but will not release any CO2 into the environment. 

It is important to note that not all plant practice CAM or C4 metabolism. Trees, shrubs etc... all practice C3 metabolism where CO2 is directly incorporated nor stored in tissue as oxaloacetate nor malate. These plants will release CO2 at night as they are not limited by a low CO2 and/or water environment. It is normally this mode of photosynthesis that is harped on about in school textbooks.

It is also important to realise that not all aquarium plants practice these CO2 saving measures. Those that don't normally do not thrive in aquaria. I am however, unable to come up with a single example... Even the difficult plants of the Lobeliaceae family practice CAM photosynthesis!

*What happens when CO2 runs out* 

When the CO2 runs out there is no longer any need for NADPH. There is always a need for ATP however. While they cannot use the light to make NADPH (because there is no NADP+ to make it from) they are still gathering light energy and this energy, is now instead of producing O2, producing free radicals which is bad news for the plant. 

The plant will, in response to the lack of need for light, tilt its leaves such that as little light falls on the leaves as possible. In this state they will not produce O2 but will still produce small amounts of ATP. No growing will occur, and in fact a lot of energy will be wasted as well as CO2. 

Leaving the lights on all the time will not make your plants grow better. Plants need a period of darkness to replenish their ADP/NADP+ stores and fix the CO2 either as sugar or malic acid which can be used to make sugar. 

*Some useful resources* 

A lot more goes on than this in the plant during photosynthesis. There is also a lot of variation between different types of plants as to what exactly happens. Additional information (as well as illustration to the above) can be found at these sites: 

Botany online - The Internet Hypertextbook 

MIT Biology Hypertextbook

P.S. this article has been edited to claritify certain points and institute a few corrections.

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## gary35111

Thanks Tyrone! This is all I need. *Pat on the shoulder* You sure good Biochemist.  :Laughing:  

So, as long as my fishes do not come to the surface to gasp for air, make sure I do not run out of CO2 and let my plants "rest" in the dark, everything should be fine. Right?

Btw, the aeration which I provide for my fishes and tank come in the form of a sponge filter. The sponge filter is attached to an air pump for it to work 24 hours. Will providing of aeration and CO2 simultaneously cause either one of the element to be reduced? My CO2 is not the cylinder type but 'liquid-fermentation-process' one (Nutrafin CO2).

Gary.

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## stormhawk

Use a timer to make the pump start operation once the tank lights are off. This is to ensure maximum use of the CO2 during daylight hours and to stave off a pH swing at night when the plants aren't using CO2.

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## TyroneGenade

You should also check to make sure you water is properly buffered. What are your day/night pH values? Your KH? and GH?

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## whuntley

There must be a typo in the first article, above.





> ...chlorophyll molecules will trap light energy from light of wave lengths larger than 700 nm (blue light) and less than 680 nm (red light).


700nm is red light and 680 is indistinguishable from 700 nm (680 is also quite red). It is only 20 nm away!

Did you mean 400 and 680?

Chlorophyl traps very little energy from IR and longer wavelengths, such as those longer than 700nm.

Wright

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## TyroneGenade

Yes, Wright. Thanks for pointing that out. I've edited the post rather than add addendum here.

Regards

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## TyroneGenade

This just in from my old Botany Prof:




> Aquatic CAM was first described in Isoetes but overnight acid
> accumulation has been detected in about 160 aquatic and marine species.
> Many of these are seaweeds or freshwater algae and most show very low
> acidification.. The species with substantial overnight acidification are the ones you mention (I'm no sure about Lobeliaceae). There is some doubt whether plants with mild acidification are CAMming. See JE Keeley (1996), Aquatic CAM photosynthesis (In K Winter and JAC Smith, Crassulacean Acid Metabolism, Ecological Studies Vol 14, pp
> 281-295)
> 
> Regards to the fish - If the plants are CAMmming then one would not get
> acidosis. One would get low O2 though. I doubt there is much CAMming going on in an aquarium. In any case it would not deplete CO2 levels that much. I think both CO2 accumulates and O2 is depleted at night, but not to levels that could kill fish - especiually if there is aeration at the same time and the water is mixing. However, it is easy to measure. Get hold of an oxygen electrode and pH meter and measure.


Seems many plants use CAM metabolism to fix CO2 at night and then switch back to C3 metabolism during the day. I'm told by Roger Miller that he read somewhere that aquatic plants trap most of their CO2 in their lacunae (gas filled spaces) and so in likelihood do not add measurably to aquatic CO2 content during the night.

Bottom line: increase aeration at night to prevent O2 deprivation.

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## cS

> Seems many plants use CAM metabolism to fix CO2 at night and then switch back to C3 metabolism during the day.


So you're saying that these plants have their stomata open all the time, day and night? Both CAM and C3 mechanisms require for the stomata to be opened. If a plant pratices both as claimed, then it would have these pores opened 24/7. If so, then it seems really counter-productive (or extremely efficient). If you practice C3 metabolism, then why practice CAM as well? Evolutionary baggage? I am under the impression that CAM plants have issues with opening their stomata during the day, so that is why they choose to have them opened at night. If so, then would it not make sense to have CO2 on through the night as well; instead of the often heard "turn off your CO2 at night"? Increasing aeration at night drives down CO2 content, making it even worse for these CAM plants who depend on night time to suck up CO2. After all, it is precisely high O2 and low CO2 that led to the evolutionary emergence of C4 and CAM metabolism.

A lot of aquatic plants are emergent in nature and are not from arid regions of the world. Why then would CAM metabolism even being practiced by them? I am confused by this evolutionary adaptation among these plants that aren't exposed to extremely hot arid conditions.

Confused.  :Think:

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## TyroneGenade

CAM isn't so much a mechanism to save water as much as it is to save CO2. In the desert or artic plants can't open the stomata during the day because they would dessicate and die. (In the artic and antartic it is very cold and so there is low humidity, low humidity more evaporation.) These means they also can't take in CO2 during the day, so they open them at night when the air is perhaps a bit more moist and evaporation not such a problem. The important thing here to note is that the real problem is CO2 supply and not water.

The reason why the CAM and C4 routes are used is because the enzyme involved in C3 photosynthesis (Rubisco) is wasteful and under low CO2 conditions it will oxidize rather than reduce the phosphoglycerate and trap the CO2 as sugar. This process is light energy driven which is why leaving the fishtank lights on all day is counter productive for many plants.

In water there is a lot less CO2 than in the air (about 18 vs 360 ppm if memory serves). So to adapt the plants trap CO2 whenever they can. And as Rubisco and C4 metabolism needs light to work the only method available is CAM or something like it.

During the day there is no need to reply on CAM and the plants can continue with C3 which is energeticly more efficient (it would appear).

At least this is what I make of it...

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