Comment by georgecmu
1 year ago
For whatever reason, evolution decided those wavelengths should be overlapping. For example, M cones are most sensitive to 535 nm light, while L cones are most sensitive to 560 nm light. But M cones are still stimulated quite a lot by 560 nm light—around 80% of maximum.
The reason is simple: genes coding the long wave opsins (light-sensitive proteins) in these cones have diverged from copies of the same original gene. The evolution of this is very interesting.
Mammals in general have only two types of cones: presumably they lost full color vision in the age of dinosaurs since they were primarily small nocturnal animals or lived in habitats with very limited light (subterranean, piles of leaves, etc.) Primates are the notable exception, and have evolved the third type of cone, enabling trichromatic color vision, as a result of their fruitarian specialization and co-evolution with the tropical fruit trees (same as birds, actually).
So, what's interesting is that New World and Old World primates evolved this cone independently. In Old World primates the third cone resulted from a gene duplication event on the X chromosome, giving rise to two distinct (but pretty similar) opsin genes, with sensitivity peaks at very close wavelengths. As a note, because these genes sit on the X chromosome, colorblindness (defects in one or both of these genes) is much more likely to happen in males.
New World primates have a single polymorphic opsin gene on the X chromosome, with different alleles coding for different sensitivities. So, only some (heterozygous) females in these species typically have full trichromatic vision, while males and the unlucky homozygous females remain dichromatic.
Decent wikipedia article on the subject: https://en.wikipedia.org/wiki/Evolution_of_color_vision_in_p...
Types of opsins in vertebrates: https://en.wikipedia.org/wiki/Vertebrate_visual_opsin
This is only tangentially related, but I have always wondered why chlorophyll absorbs blue and red, but reflects green--green being sunlight's brightest component.
It's almost as if there was some evolutionary pressure towards being very visible in sunlight which is more important than evolving ways to collect as much sun energy as possible. When I guess at this I end up with something along the lines of reflected green being used as a signal to a neighboring plant: "I'm already here, grow in some other direction instead." There is some evidence that plants do this (https://en.wikipedia.org/wiki/Crown_shyness, https://onlinelibrary.wiley.com/doi/10.1111/1365-3040.ep1160...) but it's not clear that the need to do so is so strong that it would overshadow the drive to collect as much energy as possible.
Or perhaps there's something to do with the physics of absorbing light to drive a chemical reaction that makes it better to absorb at red and blue while passing on green (450nm and 680nm are not harmonics--so if this is the case it's more complex than which sorts of standing waves would fit in some chemical gap or other).
Chlorophyll a, which is the pigment that actually uses solar energy to split water, absorbs red light and violet light. Thus its color is blue-green, as it can be seen in some lichens that have only symbiotic cyanobacteria.
This is most likely a historical accident, with no special meaning.
Most algae and plants have auxiliary pigments, which absorb other parts of the solar spectrum and then transfer the energy to chlorophyll a.
The land plants and the green algae use mostly chlorophyll b as auxiliary pigment, which absorbs light in a blue band adjacent to the violet band of chlorophyll a, and in a red band that is distinct and adjacent to the red band of chlorophyll a.
Thus the addition of chlorophyll b increases considerably the amount of captured energy.
The algae that are dominant in oceans, e.g. diatoms and brown algae, have more auxiliary pigments, so that many are dark brown, even close to black.
Unlike for marine algae, for land plants, capturing more solar energy is not desirable, because they already have difficulties in avoiding overheating and excessive loss of water. So the pigments used by them are good enough for their needs.
I don't think the mystery goes away when you consider the other photosynthetic pigments. chlorophyll-a, chlorophyll-b, lutein, B-carotene, zeaxanthin, lycopene... they're all active between 450 nm and 550 nm. And then chlorophyll-a and chlorophyll-b have secondary activity between 650 nm and 700 nm.
The the lack of photosynthetic activity between 550 and 650 is still suspicious. I've learned from other commenters here that my assumptions about the gap corresponding with peak solar energy weren't on solid ground, but there is a gap.
Perhaps a different way to frame the question is: why do the chlorophyll pigments have two peaks, while the others appear to have only one? Perhaps they have an evolutionary past which involves absorbtion from a star besides sol?
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> green being sunlight's brightest component.
It actually peaks between magenta and blue: https://sunwindsolar.com/blog/solar-radiation-spectrum/
Green is only bright to us because of our cone sensitivities overlapping.
> It actually peaks between magenta and blue
No, it actually peaks wherever you want it to peak, depending on how you plot it: https://www.oceanopticsbook.info/view/light-and-radiometry/l...
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That graph shows sunlight in the upper atmosphere, but at sea level, the 400-450 nanometer blues are partially scattered out. The peak we see on the ground is broader, and centered more around 450-550 nanometers, a range that tends more towards teal or "Miami green". Wikipedia shows both spectra:
https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spe...
I think this is also why the sky appears to be a deeper, darker blue at higher altitudes.
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I may have been a bit lazy there and imagined the distribution as Gaussian despite having seen charts that indicate otherwise. I'm glad you pointed that out.
But the question remains... Why do plants reflect light so well at the frequency where my cone sensitivities overlap? Mere coincidence would be believable, but it seems to also hint at something about the relationship between myself and those plants.
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> something to do with the physics of absorbing light to drive a chemical reaction
Exactly that. Blue does two steps of the process, while red does only one. There's a cost for synthesizing all that machinery, so absorbing green would just be not worth it.
> 450nm and 680nm are not harmonics
In fact they're in 3:2 ratio with 1% margin. But they don't have to be. Take a look at fluorescence: it converts one wavelength to another, and they don't have to be multiples of each other. Once photon gets absorbed onto a chemical, the electronic structure of the molecule decides what will happen to it.
It could also be to prevent overstimulation; "maximize energy" is not really the goal. A lot of plants can die from too much Sun unless their other inputs are just right (plenty of water, etc.).
There was a proposed theory on this the spread of absorption created more stability in the power generation of plants over different conditions. This was supposed to be a more important factor than being able to absorb the peak and highest energy.
https://www.science.org/doi/10.1126/science.aba6630
I think this paper is what I was looking for, thanks. I may have to reread it after becoming more familiar with the chemical/physical nature of:
> Photoexcitation energy is rapidly transferred through an antenna network before reaching the reaction center
I didn't know that. With this in mind, perhaps a better formulation of my question is not:
> Why are plants green?
But instead:
> Why are green photosynthetic pigments more common than others?
Based on my read of this paper, the answer to that would be that a pigment which absorbed only a single narrow band of light would be prone to being either over or under powered most of the time. Absorbing red and blue, but not green, provides more opportunities to deliver constant power at the reaction center despite varying light conditions.
Here's a recent take: https://www.quantamagazine.org/why-are-plants-green-to-reduc...
TLDR: Plants are running an energy-harvesting system that can only respond so quickly to changes in light input. Making use of green would cause variance to be large enough that the gains would not offset the losses. So, avoid green and have lower variance --> higher energy capture on average.
> Plants are running an energy-harvesting system that can only respond so quickly to changes in light input.
That would be easy to test, I suppose.
In fact, perhaps we're already doing so by letting plants live in our offices with 60Hz flicker, and perhaps higher frequency flicker caused by LEDs and PWMs.
In short, I'm not buying this theory just yet.
This video talks about it as well, it includes an interview with the author
https://m.youtube.com/watch?v=TgGoW5AIKEY
Have you looked into band-gaps?
Also remember that these are random processes with selection pressure keeping those who survive to reproduce. Assigning a will to such processes makes them and the results harder to understand- imho.
Theres probably something more efficient at converting light into simple sugars.
There's also a chance that the primary photosynthesiizers on each happened to be purple for a while (purple earth) and the ancestors of plants absorbed red/blue and ignored green because they were getting leftovers. Also, even now, iirc the limiting step in oxygenic photosynthesis is by far rubisco's incorporation of CO2, so there's no immediately obvious fitness function that would be optimized by just increasing the efficiency of light harvesting.
I didn't see any mention of this so thought I would add:
This sounds like https://en.wikipedia.org/wiki/Retinal and https://en.wikipedia.org/wiki/Purple_Earth_hypothesis. Going through history, there have been times where the Earth has had oxygen spikes https://en.wikipedia.org/wiki/Geological_history_of_oxygen (Examples https://en.wikipedia.org/wiki/Great_Oxidation_Event or https://en.wikipedia.org/wiki/Neoproterozoic_oxygenation_eve...) Cool image showing how this process is unstable: https://en.wikipedia.org/wiki/Great_Oxidation_Event#/media/F...
You might be interested in the different photosynthesis cycles: https://en.wikipedia.org/wiki/C3_carbon_fixation https://en.wikipedia.org/wiki/C4_carbon_fixation https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism https://en.wikipedia.org/wiki/Alarm_photosynthesis - this one was only discovered in 2016!
Research into these may have profound impact on climate change.
This is fascinating, I’d never realized there is this seeming-paradox! Thanks for mentioning it
Maybe it was in response to an extinction level event that filtered sunlight for a long time, removing green but allowing primarily only blue or red.
It still doesn't explain the need to reflect green, though. They could have evolved to be black and absorb all energy.
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Red is nature's warning signal, and blue was already taken by the sky, so the only option left was green.
Just kidding of course, it is an interesting question.
Maybe it has something to do with keeping the albedo of the planet at a higher value.
This is a good biological explanation. The physical explanation is, if the sensitivities didn't overlap, our spectral sensitivity would not be continuous. There would be valleys of zero sensitivity between the cones, and a continuous wavelength sweep would result in us seeing black bands between colors.
Gray bands, or more realistically just desaturated bands. There'd still be sensitivity to light through rods (black and white), and even if the peaks of wavelength sensitivity were highly separated there would still be some cone response to wavelengths that didn't stimulate them strongly.
I'm pretty sure that line of the article didn't mean to imply that we don't know, or aren't sure, only that it goes beyond the scope of the article and isn't directly relevant to the topic at hand.
> So, only some (heterozygous) females in these species typically have full trichromatic vision
Wow that's wild how heterozygousity can be that helpful. Makes you wonder if there are other genes like that.
Some human females have functional tetrochromatic vision.
https://jov.arvojournals.org/article.aspx?articleid=2191517
No I meant like if there is some other gene where the two different variants are synergistic to each other.