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Scientists Aim To Improve Photosynthesis
vasanth writes "Two new initiatives at the University of Cambridge aim to address the growing demand on the Earth's resources for food and fuel by improving the process of photosynthesis. Four transatlantic research teams – two of which include academics from Cambridge's Department of Plant Sciences – will explore ways to overcome limitations in photosynthesis which could then lead to ways of significantly increasing the yield of important crops for food production or sustainable bioenergy. Despite the fact that photosynthesis is the basis of energy capture from the sun in plants, algae and other organisms, it has some fundamental limitations. There are trade-offs in nature which mean that photosynthesis is not as efficient as it could be – for many important crops such as wheat, barley, potatoes and sugar beet, the theoretical maximum is only 5%, depending on how it is measured. There is scope to improve it for processes useful to us, for example increasing the amount of food crop or energy biomass a plant can produce from the same amount of sunlight."
Disclaimer: My group is collaborating with one of the guys from the FA.
It is not so simple as you think. But I see the same type of misunderstanding in many people in the field (especially the kind that is good at getting grants and bad at doing science, there are many of them...).
Leaves are pretty well designed (I mean that not in the intelligent way), and being green is one of them. The pigments that absorb the majority of the light (chlorophylls) have absorption peaks in the blue and red, and the absorption for green is indeed quite low. However, if you look at the total absorption spectrum of a whole leaf you will see the dip in the green is only 10-20% for most leaves. It is even less if you consider the whole canopy. Nevertheless our eyes pick up this small difference so that leaves look green.
The problem with having black leaves (i.e. absorbing all light, some seeweeds do that) is that you get too much energy in the upper most layer of your leaf (a leaf is several hundred micrometers thick), giving you plenty of energy, but other things (enzyme capacity, CO2 levels, etc) become limiting. Thus, this absorbed energy is wasted, or even starts to damage things (lots of electrons flying around is not always a good thing).
Thus, the green "window" allows part of the light to travel into deeper layers of the leaf, which is also often more porous, resulting in more scattering (longer pathlength, thus increasing chance of absorption) of the light. In this way, the green light drives much of the photosynthesis in the lower part of a leaf. Spreading out the light energy over several 100 micrometers makes the leaves much more efficient, but this would not work if the pigments absorbed green light equally well.
That is not to say that nothing in the pigments can be optimized. Crops are often large stands of genetically identical organisms. We want to optimize the growth of the whole group. This is different from what might have been selected for by evolution (in a mixed canopy, a good survival strategy is to overshadow your competitors, i.e. become tall and allocate more pigments to the top). Big increases in grain yields were realized by breeding for shorter plants (little stem, mostly leaves). This would not work in nature because if one genotype starts to cheat (become bigger), the others will be starved of light. A similar gain might be possible by optimizing pigment allocation to allow a better distribution of the light (most plants still put too much in the top).
The photosystem is pretty good at capturing photons! It's after that initial step that the tough bits of chemistry come in.
First up, whenever you capture energy, you will heat up, hence plants have to manage that, and they do that with radiating out the excess energy captured (it is a lot!). There are a bunch of publications on reducing chlorophyll quantity in algae, which led to an improvement in photosynthetic efficiency and a drop in the excess energy radiated out (I think it was in the red region).
After that, the captured energy is used to split water, generating rather damaging radicals/ions (I forget which one) in the process - one changes your pH, the other causes redox stress. Either way, both are bad. The photo-system can't take a lot of that either, hence there are a ridiculous number of processes to effectively convert the split water back to water! (Refer Dynamics of Photosynthesis - Annual Review by Eberhard et al... Hah, luckily remember one paper from my thesis work!)
It doesn't get much better by the time the Hydrogen ion travels across the membranes, creating the much wanted NADPH, and some ATPs in the process. Now, depending on the chemicals wanted by the cells, the ratio of NADPH/ATP need to be tweaked, losing some energy there too.
And then come in the enzymes which start to use this simple energy to climb up the rather hard entropy ladder to create ordered polymers from the ridiculously simple water and carbon dioxide. Not the easiest of tasks in my opinion... and my un-calculated and un-verified bias is that the free energy change needed to accomplish this must be pretty high. Thermodynamics didn't like me very much... Nonetheless, the often abused number of 5% or 10% or 1% (yes, you can find all of these numbers in literature) photosynthetic efficiency means little as people always compare sunlight received to the calorific value of the biomass, completely missing out all the effort it took to build up that complexity against the rather real forces of disorder. Burning it is a complete waste!
Which is why my money (when I will have money!) will be on chemically simpler fuels - higher efficiencies are possible. But unfortunately, none of our alternatives to biomass have the self-replicating chemistry awesomeness of Biology. Hence it's not very cheap to manufacture and maintain. Plants kind of grow... You don't have to do much. Except, of course, if you're a corn ethanol producer, where you're doing too much! ;)
So basically, the problems are not at the pigments... The quantum yield of photon capture is near 100%. The complexity is after that. It's a mix of matching rates of various processes along the way, and losing energy working against entropy. And we're not even *close* to figuring out this system. Long shot.
Sayash