Quinten. Thanks for that. I remember reading Goldsmiths theory back in the late 80's/early 90's (showing my age now ) and for me it conjured images of life trying to overcome the problem of light only being on a certain point of earth for limited hours of the day. I was of the opinion that the need to overcome this limitation was the driver for life developing motor functions.
I have to say I was less struck by his assertion of the halobacterium causing algae to be green, because algae would not have had heat issues, just light issues, and there would surely always be some opportunity to absorb blue and red light so they would have favoured black.
Also halobacterium would have more reason to be black, it couldn't store energy so would need to make use of all available energy and again, I don't see them having heat issues.
Why would they absorb blue AND red then?
Cis or trans? That's what matters.
in desending wavelength absorbtion, Carotenoids, Chlorophyll b and Chlorophyll a are the major light aborbing pigments in the plant and form part of the light absorbing complex.
In this complex energy is absorbed by all pigments, but not all pigments have the capasity for photosynthesis. This meens that lots of non-photosynthesizing pigments surround a active photo-center.
Energy is caried over from one pigment to another by inductive resonance till it reaches the photo-center. For this to happen there needs to be a decline in energy levels from example Carotenoids to Chlorophyll b and Chlorophyll a. This funnels the energy to the reaction center where it drives photosynthesis.
If there was a pigment that absorbed green light the gradient of energy between pigments would by less and energy could travel back to Carotenoids or travel lateraly away from the reacion center.
More than this the plants ability to adjust to high light intensities by synthesis of antosianin would be destroyed. As antosianin absorbs green light reducing green light scattering.
As a side note, shade plants do absorb green light by use of their long intercelluler spaces deflecting green light till some of the enery is lost and it can be absorbed.
If we forget about photosythesis for the moment we can look at light senceing molecules such as Phototropin (Blue light), Chryptochrome (Blue and UV-A) and Fitochrome (Red and far Red). Resposible for Phototropism, photomorfogenesis, circadian rhythm, sensitivity to IAA and stomata opening/closeing.
The functioning of these molecules depend on the large gap between red and blue light, if green light was absorbed it would influance the activities of these molecules and plant responses.
Don't forget that any wavelength above 680nm can not be used to drive photosinthesis, as the photosistems work at 680nm or 700nm.
Blue, yellow and green light reduces their energy levels in the plant by floresence and chemical heat dissipation to be able to drive photosynthesis. If to much energy must be dissipated reactive oxigen radicals can form which damages DNA and RNA as well as lots of other prosseses in the plant.
It is logical then that plants do not wish for green light to be absorbed
Six pages of discussion and I didn't spot one alternative solution: the light of our sun has not always conformed to its current radiation spectrum.
If our skies were purple, ie high red/ blue (+ UV) then not only did plants benefit.
The eyes of humans, animals and birds are mainly peak sensitive in the 550nm range (green light). So eyes are naturally adapted to give a more equal energy spread, taking advantage of what was (is) most plentiful.
I think Nature appears contradictory, actually!
1) Why be inefficient and provide our eyes with too much sensitivity in the Green zone?
2) Why do plant leaves reflect the highest energy part of the current sun's spectrum?
3) UV is not particularly abundant, and yet birds, reptiles and insects are fitted with UV sensors?
4) Plants make big use of UV to attract insects, why not use a higher energy colour?
Thanks for supporting my theory - obviously Nature took advantage of the time when Blue and Red were the most abundant wavelengths in the sky. And it still works, even in our yellow sun era.
so leaves are really "clear"? please email me at [e-mail address] im very interested thanx
Last edited by JackBean on Tue Sep 27, 2011 5:50 am, edited 1 time in total.
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I think I have read/seen a very interesting answer to this.
Firstly, we have to understand endosymbiotoc theory: the organelles responsible for photosynthesis of plants, chloroplasts are captured cyanobacteria: cyanobacteria/chloroplasts are green.
Cyanobateria evolved a long time ago, but they were not the first photosynthetic organisms.
Purple bacteria existed already, they absorb green light and reflect blue and red(that's why they are purple). They don't use H2O to create O2, they use different elements, most commonly H2S to create S. They do not like oxygen and die is exposed to too much of it.
The early earth had an atmosphere without oxygen: purple bacteria were free to roam whereever they wanted (now they are kindof rare) and dominated the seas.
Purple bacteria evolved to use the most abundant photons (yellow-green), they had(have) the solution that was optimal... cyanobacteria evolved later, in a world dominated by purple bacteria... the cyanobacteria had to live from the light 'left over': the blue and green. This was optimal for them because the green light was not available (purple bacteria used it all). And that's why they developed pigments that absorb blue and red light. After cyanobacteria appeared, their color was fixed in evolution, in the same way as other solutions that made sense once but no longer do are fixed (air and food in same opening is a bad solution, but lungfishes evolved lungs by 'swallowing' air...today it works, so no change): today green leaves get the job done, so why change it (even if it could be better). After cyanobacteria changed the atmosphere (without them (in ocean and in plants) there would be no oxygen in the air) purple bacteria lost their dominating status...
Purple bacteria are older than cyanobacteria according to molecular geneticist, and there are papers that claim the chemical markers of purple bacteria are widespread in ancient sediments. This is not generally accepted (yet), though.
The problem with this argument is that today, the position is reversed: purple (sulphur) bacteria live in deeper, oxygenstarved water, where they live from the light that is not absorbed by the green organisms higher in the watercolumn.
So all in all I am not sure that this is true, reading more about it as we speak, but it sounds rather convincing to me.
The heat argument doesnt make sense to me: why are pinetrees in siberia not black? We don't see color differences in plants between hot and cool places. If heat was so important, tropical plants should have different colors than arctic plants.
The argument that there's more than enough light so it is not limiting makes no sense either: plants have lots of tricks to grow tall and block the light of competitors, and many seedlings in the forest don't make it for lack of light. Every photon is precious in the struggle for life.
The scattering/sharing argument is just not the way biology works. Evolution creates very selfish beings. I don't think ants or humans are an exception, actually... communism failed to work for the reasons of selfish evolutionary biology (but that's another discussion).
The argument of other-spectrum sun is interesting, but at first makes no sense either: if the sun used to be bluer, why do plants also use red light... if the sun used to be redder, why do they use blue?
BUT if the yellow-green light was filtered away by gasses in atmosphere... that's close to the purple bacteria argument... the atmosphere used to be different as well, perhaps back then yellow-green light was absorbed by atmosphere before it reached the ground (purple skies). hmmm, should read on what was composition of atmosphere...
After all, it might also be chemical restrictions:
the water-oxygen reaction is taken for a reason: oxygen is a far better electron receptor than atoms used by alternative photosynthetic organisms: more energy is released using oxygen than when using sulphur or any other possibility. Perhaps there simply is no pigment possible that uses oxygen and green light. This is not very satisfing and convincing to me, though.
So after all, I think the case for purple bacteria world domination is pretty good.
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