|MadSci Network: Botany|
Well, this may sound like a technicality, but in the strictest sense, science cannot answer questions of why things are the way they are. It can only tell us how they could have become that way based on our knowledge of natural processes, such as evolution in the case of biology. And even though we may think of organisms as being perfectly crafted to fill their own particular ecological niche, almost none of them truly are. One of my professors compared the process of the evolution to trying to improve the design of your car while you're driving it down the road. If particularly bad design flaws are incorporated into a system that is vital for your survival, you can't just scrap that system while you work on developing something better. And also, because natural selection, the driving force behind evolution, only occurs on systems where improvements increase survival rates, an organism also can't improve a new part without relying on it. So, many biological systems have serious "design" flaws from an engineering perspective. Photosynthesis is one of them. However, the major "flaw" in photosynthesis is not in harvesting light. Plants are actually very good at that. And the small window in the green wavelengths where they don't absorb quite as well is actually fairly shallow. Plants can absorb and use green light to do photosynthesis, just not as well as they can red and blue light. Let's look at some absorbance spectra, and I'll show you what I mean(1): So in this experiment light of particular wavelengths was shined onto a few materials, and then the amount that got through was measured. The x-axis is the wavelength of light in nanometers (10-9 meters), and the y-axis is the amount of light absorbed by the material being observed. The absorbance is defined as A=log T0/T, where T is the amount of light transmitted and T0 is the initial amount of light before passing into the sample. An absorbance of 1.0 means that 90% of the light was absorbed, A=0.5 means about 2/3 was absorbed, and A=0.0 means that it all got through. There are two important traces here. One is at the top, and has data points represented as open boxes. It is from a thin, single leaf. The other important one is at the bottom and has open circles. It is the absorbtion spectrum for chlorophyll that has been extracted from a leaf. As you can see, although chlorophyll by itself doesnít absorb very well in the green wavelengths (the low trough on the graph), the whole leaf has A > 0.5, so it absorbs over 2/3 of the incident light in the green wavelength. Also, we must consider that most plants are part of a "canopy", where light first encounters tall plants, and then what isnít absorbed by the tallest plants goes down to the next level, and so on. So, eventually almost all of even the green light is absorbed. The wavelengths scanned here pretty much cover the visible spectrum. Wavelengths shorter than this carry too much energy and can be actually harmful, so the plant is better off reflecting them. While longer ones do not have enough energy to be of much use chemically, so absorbance of them would lead to excessive heating. But even more importantly, as I said earlier light capture isnít really where the major "design flaw" of photosynthesis is found. It is in carbon uptake, and one enzyme in particular is the culprit: ribulose 1,5 bis-phosphate carboxylase, also known as rubisco. This is the enzyme that takes a carbon dioxide molecule and adds it to the end of a short sugar molecule, called ribulose-1,5-bisphosphate. This enzyme is ancient, and almost certainly first appeared billions of years ago when photosynthesis was relatively new, and carbon dioxide made up a large percentage of the atmosphere, and oxygen a very small part. It is very slow, for example: while some enzymes may process over a million substrate molecules a second per active site, bloated rubisco can only process 2 or 3 per second per active site. And not only is it slow, it is also horribly non-discriminating in its substrates. It can react with oxygen instead of carbon dioxide in a reaction that is not just useless, but also very wasteful. It slows down carbon dioxide uptake through competitive inhibition, and it causes the plant to have to go to great lengths and actually spend energy and give off a carbon dioxide molecule to recycle the sugar molecule consumed in the process. And while oxygen makes up about 21% of the gas in our atmosphere, carbon dioxide makes up much less than a tenth of 1%. Because of the horrible inefficiency of rubisco, and the low amounts of carbon dioxide in the atmosphere, the plant has to make lots of rubisco to be able to fulfill its carbon needs. As a matter of fact, it is probably the most abundant protein on earth. In a typical plant, it may actually contain something like 50% of all the nitrogen in the whole plant. So the plant must balance its needs for carbon dioxide with its ability to take up nitrogen. And to further complicate things, the same pores that the plant relies on to "breathe in" carbon dioxide also let a lot of water evaporate out of the plant. Some plants actually lose 1500 molecules of water for every one carbon dioxide that they take up(2). So the plant must also balance its carbon needs with the water that is available in its immediate environment. So it is a wonder to me that plants can grow at all sometimes. Some plants have found ways to somewhat get around the limitations of rubisco and a low carbon dioxide atmosphere, but none of them have found a way to totally replace the enzyme. And neither have genetic engineers, but some of them are working on improving it in crop plants. Well, I donít want to make this reply too long. I hope Iíve at least partially answered your question. If you or your students are interested in learning more about photosynthesis, here are some web sites to check out: Hereís the home page of the lab of Dr. Tony Crofts, where I am working on my Ph.D. thesis: http://ahab.life.uiuc.edu/home.h tml We work mainly on the bc1 complex, a major component of the electron transport chain in mitochondria, bacteria, and some kinds of photosynthetic bacteria. But there are links to pages about other components of photosynthesis as well. Hereís a page with structural and mechanistic information about Rubisco, but I didnít see any information on photorespiration (oxygen uptake by rubisco, and the pathways used to account for it): http://www.rrz.uni-hamburg.de/biologie/ialb/lehre/molbio/1rxo/e1rxoe.h tm Hereís a page on carbon fixation: http://www.epsilon-assoc.com/~jkimball/BiologyPages/C/CalvinCycle.html a> Hereís a previous Mad Scientist question on photorespiration: htt p://www.madsci.org/posts/archives/may96/828679089.Bt.q.html Hereís how one group of plants has worked around the photorespiration problem: http ://www.marietta.edu/~spilatrs/biol103/photolab/c4photo.html Also, the light reactions send their electrons to the dark reactions of photosynthesis. So, in high light conditions the dark may reactions get backed up due to the inefficiency of rubisco and low carbon dioxide, and the light reactions can actually find themselves absorbing too much energy from light and generating an excess of electrons. They must find some way to get rid of this excess energy other than photochemistry. This is called non-photochemical quenching, and hereís a link to a paper on it by Dr. Crofts and Dr. Christine Yerkes: http://ahab.life.uiuc.edu/lhcii .html 1. The figure was copied from notes that Dr. Don Ort gave out in the Biophysics 332 Photosynthesis class on 2/21/2000. 2. Also from notes given out by Dr. Ort, but on 3/1/2000.
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