Why is green light not useful for photosynthesis?

  • #1
Plant leaves only appear green because they absorb the red and blue wavelengths of light and reflect green away. Is there an intrinsic and innate reason for this that relates to the physics of light? Maybe green wavelengths are inefficient at storing energy in chemical bonds? Have plants simply just not evolved to make use of green light yet? Maybe environmental stressors and natural selection required plants to stay blended in with all other flora? P.s I appreciate this question encompasses a variety of scientific disciplines and not just physics. If the thread needs to be flagged as unrelated to physics I fully understand. Thank you for your time everyone!
 

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  • #2
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Wikipedia has quite a good answer: Photosynthetically active radiation
The relevant frequencies are basically the entire spectrum of visible light. It is the Chlorophyll as the most active substance that makes them look green.
Chlorophylls absorb light most strongly in the blue portion of the electromagnetic spectrum as well as the red portion. Conversely, it is a poor absorber of green and near-green portions of the spectrum. Hence chlorophyll-containing tissues appear green because green light, diffusively reflected by structures like cell walls, is less absorbed.
 
  • #3
hutchphd
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My statement of the question is why are leaves not dark gray?
These are two kinds of algae: red and green. If you mix them they are gray. I believe the red algae lived at the surface of the ocean and lower layers developed the chlorophyl pigment to soak up the remaining red and blue light after the red algae sucked up the green. Maybe they grew roots to stay on the bottom and are the progenitors of trees.
 
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Thank you for replying guys I've learned something new from both of your replies so thank you for enlightening me!
 
  • #5
Wikipedia has quite a good answer: Photosynthetically active radiation
The relevant frequencies are basically the entire spectrum of visible light. It is the Chlorophyll as the most active substance that makes them look green.
Is it simply a product of the cell walls intrinsic light permeability that causes green light to reflect? Or is green light just intrinsically useless as a form of energy?

It just seems strange to me that plants can photosynthesize with short and long wavelengths but not green which is in the middle. The analogy im thinking of is that sprinters arent trained for marathons and marathon runner's aren't trained for sprinting but if you asked them both to jog they wouldn't have any issues.

I appreciate my analogy is probably completely misguided so i apologise in advance for taking up your time.
 
  • #6
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It just seems strange to me that plants can photosynthesize with short and long wavelengths but not green which is in the middle.
The graphic shows that green is also used. I don't know why chlorophyll reflects green. Why are sulfur yellow and bromine brown?
 
  • #7
pinball1970
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Is it simply a product of the cell walls intrinsic light permeability that causes green light to reflect? Or is green light just intrinsically useless as a form of energy?

It just seems strange to me that plants can photosynthesize with short and long wavelengths but not green which is in the middle. The analogy im thinking of is that sprinters arent trained for marathons and marathon runner's aren't trained for sprinting but if you asked them both to jog they wouldn't have any issues.

I appreciate my analogy is probably completely misguided so i apologise in advance for taking up your time.
If you look into how colour comes about you will see it is just about what is absorbed and what is reflected in terms of wavelengths. That depends on the molecule. 'Green' in terms of the the colour we see will not be just a peak at 550nm if you measure it you will see a spread of different wavelengths. In fact I will stick a leaf in the spectro next week and post it here. Edit @fresh already posted a link but I will still do the measurement.
 
  • #8
TeethWhitener
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Plant leaves only appear green because they absorb the red and blue wavelengths of light and reflect green away. Is there an intrinsic and innate reason for this that relates to the physics of light? Maybe green wavelengths are inefficient at storing energy in chemical bonds? Have plants simply just not evolved to make use of green light yet? Maybe environmental stressors and natural selection required plants to stay blended in with all other flora? P.s I appreciate this question encompasses a variety of scientific disciplines and not just physics. If the thread needs to be flagged as unrelated to physics I fully understand. Thank you for your time everyone!

My statement of the question is why are leaves not dark gray?
These are two kinds of algae: red and green. If you mix them they are gray. I believe the red algae lived at the surface of the ocean and lower layers developed the chlorophyl pigment to soak up the remaining red and blue light after the red algae sucked up the green. Maybe they grew roots to stay on the bottom and are the progenitors of trees.
This is how I’m interpreting the question as well. It’s an open scientific question as to why plants are green and not black to maximize photosynthetic efficiency. Some plants are black, or at least very dark purple/red, but they clearly don't dominate the ecosystem. There are at least a few different theories for why most plants are green, and googling "why aren't plants black" gives a good flavor of them.

Do keep in mind that green plants absorb green light--just not as well as red or blue light. One of the more compelling theories to me is the notion that plants are partially transparent in the green wavelengths (the peak intensity of the solar spectrum) so that all parts of the plant can be irradiated and participate in photosynthesis. If plants were black, only the very top layer of the plant would absorb light, and the sugars photosynthesized there would have to be transported to the rest of the plant, which would cost energy. Since the plants allow some light to penetrate through them, this is less of a problem.

In evolutionary terms, this might also be helpful: early photosynthesizers that were dark would be limited to a surface layer in the ocean, but green photosynthesizers could grow in multiple layers, overwhelming the dark photosynthesizers by sheer numbers.
 
  • #10
pinball1970
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This is how I’m interpreting the question as well. It’s an open scientific question as to why plants are green and not black to maximize photosynthetic efficiency. Some plants are black, or at least very dark purple/red, but they clearly don't dominate the ecosystem. There are at least a few different theories for why most plants are green, and googling "why aren't plants black" gives a good flavor of them.

Do keep in mind that green plants absorb green light--just not as well as red or blue light. One of the more compelling theories to me is the notion that plants are partially transparent in the green wavelengths (the peak intensity of the solar spectrum) so that all parts of the plant can be irradiated and participate in photosynthesis. If plants were black, only the very top layer of the plant would absorb light, and the sugars photosynthesized there would have to be transported to the rest of the plant, which would cost energy. Since the plants allow some light to penetrate through them, this is less of a problem.

In evolutionary terms, this might also be helpful: early photosynthesizers that were dark would be limited to a surface layer in the ocean, but green photosynthesizers could grow in multiple layers, overwhelming the dark photosynthesizers by sheer numbers.
As promised, a spider leaf measurement showing spectral reflectance data BETWEEN 400-750nm
1632917883525.png

This does not tell us much about the chemistry just that there are molecules there that absorb blue and reflect green and some yellow and red.

A quick google shows Chlorophyll degrades to a compound that no longer has Mg at the centre of the molecule. Soils that lack Mg /Fe can lead a degradation of chlorophyll.

I did not realize the similarities with haemoglobin chemistry (or forgot) if you read the links.

Chromophores can have a metal complex (dyes) Chlorophyll looks similar.

The chloroplasts is where the biochemistry is happening and has a high concentration of chlorophyll.

https://en.wikipedia.org/wiki/Chloroplast

The chloroplast / mitochondria evolution story is interesting if you deep dive but that will be off topic.
 

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