Comment by __MatrixMan__
1 year ago
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?
Most organic pigments have 2 or more absorption bands, but this is not always apparent for humans, because the second band is located in near ultraviolet or in near infrared.
Like I have said, in marine algae you can find a variety of colors, as already seen in their names, e.g. red algae, yellow algae, brown algae. Most of them have strong absorption for green light, due to the auxiliary pigments that they happen to use.
Only the green algae and their descendants, the land plants, do not absorb green light and as a consequence they are competitive with the other algae only in places with abundant light, i.e. in very shallow waters or on dry land, where they can get all the energy that they can use without damage, so they do not need better coverage of the solar spectrum.
The energy of the captured photons does not matter much, because even the red photons have enough energy.
The captured photons are not used for "photosynthesis", which is a misnomer whose origin lies in a time when the mechanism of "photosynthesis" was not known.
In oxygenic phototrophs, a part of the captured energy is used to oxidize some manganese atoms so strongly that they can oxidize the oxygen from water, converting it into free dioxygen. The hydrogen from water is bound into a reduced organic substance (NADPH), which will be used later as a reducing agent (without needing light) to make carbohydrates from carbon dioxide.
The rest of the energy of the captured photons is used to pump ions through the membrane of the chloroplasts. The energy stored in an ion gradient will be used later to power all organic syntheses.
The ionic pumps could work even with infrared photons, as they do in phototrophic bacteria that live under algae, so they have modified chlorophylls whose red absorption band is shifted into infrared, away from the red band that is removed from solar light by the algae sitting above. However, those bacteria cannot split water, the energy of infrared photons is too low for that (splitting water needs around 1.25 eV, while red photons have over 1.5 eV).
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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...
From the link, what appears to be the crux of the issue:
> Figure 1 shows plots of [solar] energy irradiance as functions of both wavelength and frequency. The red line is the solar irradiance at the top of the atmosphere. The green curve in the left panel is the corresponding blackbody irradiance for a temperature of 5782 K, reduced by the distance of the Earth from the Sun
> The peak of the blackbody irradiance spectrum is at 501 nm for a temperature of 5782 K. This corresponds to a frequency of ν = c/λ = 5.98 ⋅ 10¹⁴ Hz.
> The right panel of the plot shows the same solar data plotted as a function of frequency, along with the corresponding blackbody spectrum
> Note that when plotted as a function of frequency, the solar and blackbody spectra have their maxima near 3.40 ⋅ 10¹⁴ Hz, which is not the frequency corresponding to the maximum when plotted as a function of wavelength in the left panel. Indeed, 3.40 ⋅ 10¹⁴ Hz corresponds to a wavelength of λ = c/ν = 880 nm.
> In other words, when plotted as a function of wavelength, the solar irradiance is a maximum near 500 nm, in the visible, whereas the maximum is at 880 nm, in the near infrared, when the spectrum is plotted as a function of frequency.
[emphasis original]
> This occurs because the relationship between wavelength and frequency is not linear, so that a unit wavelength interval corresponds to a different size of frequency interval for each wavelength: ∣dν∣ = ∣c / λ² dλ∣.
However, it continues:
> Figure 2 shows the solar photon irradiance [number of photons per second, as opposed to number of joules delivered per second] and the corresponding blackbody spectra
> Now the maximum is at 635 nm when plotted as a function of wavelength and at 1563 nm, in the short-wave infrared, when plotted as a function of frequency. The maxima are at still different locations if the spectra are plotted as a function of wavenumber.
This time the emphasis is mine. This contradicts the explanation given above, that the curve peaks in different places along different scales because those scales are nonlinearly related. The relationship between wavenumber and frequency is linear. Why does that lead to a different plotted peak?
Are we measuring wavenumber somewhere within the atmosphere (where?), rather than in a vacuum? That would mean that different frequencies of light had different velocities, complicating the relationship between frequency and wavenumber. But it would also be a strangely artifactual way to represent solar irradiance. What's so special about wherever it is that we standardized wavenumber measurements?
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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.
The chart I linked shows both upper atmosphere and sea level irradiance, for what it's worth. Though it's possible the chart simply isn't the most accurate. I only did casual research.
Doing more research, looking at [1] (the data source for that Wikipedia image), it looks like the peak is between 480 nm (cyan) and 540 nm (lime green) for "global tilt" and 480 nm to 580 nm (yellow) for "direct + circumsolar" (I have no idea what the difference is between "global tilt" and "direct + circumsolar").
Interesting. It seems like the chart in my original comment isn't as precise as I believed.
[1]: https://www.nrel.gov/grid/solar-resource/spectra-am1.5
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.
You have a kind of cone with maximum sensitivity in green, because that is what plants reflect, not the other way around.
You have another kind of cone with maximum sensitivity in blue, because that is what the clear sky provides.
So a mammal looking upwards (or forward for a tree-dwelling animal) can distinguish easily leaves from sky.
Monkeys and us have another kind of cone with maximum sensitivity in red for 2 reasons, blood is red and many mature fruits have reddish colors (including orange and purple).
The redness of blood matters not only for noticing wounds, but also for detecting emotions on the faces of mates or adversaries (due to increased blood flow; for some monkeys the emotions may be seen not only on faces, but also on other hairless areas, e.g. buttocks).
We often hear about the relationship between perception of red and berries (food). Perhaps for survival in very dry areas: green == plants == water?
> 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.
Well I think the question then becomes can they use that much energy without cooking themselves alive? Maybe the entire spectrum is simply too much incoming energy to dissipate effectively and blue and red absorption are either more efficient or is easier to accomplish than green.
One, you're adding intent to a process that has no intent.
It's not reflected- it's just not used (and thus absorbed). If the plant has more energy that it needs already to produce offspring... what pressures exist to produce more?
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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.