Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why are plants green?

  1. Jul 20, 2011 #1
    Surely black would be better.
  2. jcsd
  3. Jul 20, 2011 #2
    Yes black would definitely better. It certainly makes sense for plants to absorb the entire spectrum of white light that is avilable. And so does absorbing infrared, ultraviolet, and whatever EM radiation that is available. Yet that is not the case. May be that is because there are no molecules, which are efficient enough to capture green light or other wavelengths and produce energy that gives the plant positive returns for its investment in making those molecules.

    Indeed there are organisms which do not use chlorophyll as the major light absorbing pigment. A prime example is red algae, which absorb longer wavelengths since they have more penetrating power through larger depths in the sea.
  4. Jul 20, 2011 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    Quoting http://science-at-home.org/kid-question-why-is-grass-green/" [Broken];
    So in summary; plants are green because chlorophyll is. There is most likely something about chlorophyll that is advantageous though it is not currently known why. Personally I agree with John Berman;

    EDIT: I'm not sure if this has been explored (I'll look it up when I have time) but being black would obviously cause a rise in temperature. This could disrupt the catalytic activity of the enzymes involved in the photosynthetic process, perhaps the need for a more efficient (and probably complex) thermoregulatory process makes anything other than chlorophyll unlikely to evolve as it would not be competitive against the simpler green chlorophyll unless it also evolved this thermoregulation at the same time.
    Last edited by a moderator: May 5, 2017
  5. Jul 20, 2011 #4

    Yes the idea that there is only one chemical capable of extracting energy from light just does not cut the mustard. There are numerous
  6. Jul 20, 2011 #5
    You are right, thermoregulation is a significant possibility. Although I slightly disagree with the idea of having reached a peak in the Fitness landscape, since chlorophyll need not be replaced by another more efficient molecule. It could as well be an accesory pigment if it existed.
  7. Jul 20, 2011 #6


    User Avatar

    Staff: Mentor

    Good discussion.

    This thread at the naked science forum raises some of the issues.


    This is really good.

    Last edited: Jul 20, 2011
  8. Jul 21, 2011 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    Indeed, but the fact that more than one exists doesn't mean that it will evolve across the board. http://en.wikipedia.org/wiki/Chloroplast" [Broken] evolved to utilise light, they happened to be green. This would have been a massive advantage, it's perhaps possible that another pigment could have evolved and we'd all be sitting here asking "why are the plants blue?" but it is also possible that there is a biochemical/regulatory reason as to why green was initially favoured.

    Agreed though I think that to evolve extra pigments would involve evolving a lot of extra metabolic pathways to run it and make it worth while. It's a bit like installing a different type of solar panel on your house without placing the infrastructure.

    Good links Evo :smile:
    Last edited by a moderator: May 5, 2017
  9. Jul 21, 2011 #8
    Clorophyll is simply more cost effective for plants to use... It is a small molecule capable of ejecting electrons fairly rapidly, it is only replaced with other (similar) molecules when the environment calls for it (like as has been previously stated by many algae and plants which receive mainly red light... Most often evolution has found the most energy efficient alternative already... If there was a black molecule (easily manafactured) that could eject electrons of a wavelength that could be accepted by a transport chain... nature would have done it already.
  10. Jul 21, 2011 #9


    User Avatar
    Staff Emeritus
    Science Advisor

    Potentially true unless the evolution of a black molecule would require a more complex support biology. Once green chlorophyll spread everywhere it could be that the required changes to develop a black molecule would lead down a http://en.wikipedia.org/wiki/Fitness_landscape" [Broken], thus would be selected against before it got there. I'm not saying this did happen, but it is possible.
    Last edited by a moderator: May 5, 2017
  11. Jul 21, 2011 #10
    Yeah I agree with you, I suppose there are enough environments on earth to 'guide' a chlorophyll containing organism into becoming a Black chlorophyll containing organism... but still a maybe
    Last edited by a moderator: May 5, 2017
  12. Jul 21, 2011 #11
    A bioessay Into the deep: new discoveries at the base of the green plant phylogeny
    from July 11, 2011 should be helpful to this discussion Why are plants green:

    Last edited by a moderator: May 5, 2017
  13. Jul 21, 2011 #12
    I think that the Plant FAQ at the Cronodon site Evo linked to hits the nail on the head - it's something that makes sense in the light of evolution. Given that eukaryotes picked up the ability to do photosynthesis from cyanobacteria, it's rather logical that they predominantly use chlorophyll. Of course, it can be helpful to examine the details of this as well. Some assorted comments on the topic -

    - If you look at the solar spectrum as recorded at sea level - see for example http://rredc.nrel.gov/solar/spectra/am1.5/ - it's definitely been attenuated from the extraterrestrial spectrum, and a bit flatter in the visible range.

    - In terms of biochemical efficiency, the chlorophyll special pair in Photosystem II (the oxygen-evolving complex) has an absorption maxima at 680 nm. When oxidized, it is capable of stripping the electrons from water and forming dioxygen. At this point, all an organism pretty much needs is sunlight, carbon dioxide, and whatever trace minerals are necessary. Cyanobacteria, of course, are capable of differentiating cells that fix nitrogen, and plants frequently play host to nitrogen-fixing bacteria.

    - Another biochemical consideration - modification of the porphyrin group with a different metal center could have given rise to potentially dangerous reactive species. For example, various cytochromes have iron-containing porphyrins that are redox-active such as cytochrome P450 and carry out harsh oxidative chemistry that needs to be carefully modulated. It might change the absorption properties, but it could also very likely change the chemical reactivity for the worse. Covalent modification of porphyrins can also change the spectral properties, although I'd have to check and see if they'd be capable of shifting the absorption maxima far enough (I have a recollection that methylation and other small organic functional groups tend to do things on the order of 10 to 20 nm in the peak shift, not 100 to 200 nm).

    - There are, of course, other mechanisms to harvest solar energy in nature, although not necessarily for carbon fixation. Bacteriorhodopsin uses the photoinduced conformational switch of a bound retinal molecule to power proton translocation across a membrane.

    - There are other photosynthetic pigments that organisms use, some of which are tetrapyrroles of one sort or another, but there are also a variety of carotenoids floating around that vary from organism to organism. I think there is something to the idea that evolution co-opted already existing biochemistry (tetrapyrroles/porphyrins are fairly ubiquitous) for photosynthesis.
  14. Jul 21, 2011 #13
    I'm sorry but I don't support that website. One reason is obvious by my previous post and the other is it recommends 'Adam McLean's Alchemy Web site and courses'.
  15. Jul 21, 2011 #14
    So recent advances in molecular phylogenetics are suggesting that things are not nearly so simple as once thought. Color me surprised! (Actually, don't - I've noticed over the years that the phrase "recent advances in molecular phylogenetics have caused us to reassess our understanding of XYZ" - or ones like it - crops up EVERYWHERE in the biological sciences, so if anything, it's rather interesting. And kind of amusing, but that may just be me.)

    Still, the fundamental point remains - there was an endosymbiotic event in the past involving eukaryotes and cyanobacteria, and suddenly photosynthesis was no longer the domain of prokaryotes. How it went from there, that's not really the point I was focused on in my earlier post. I was mostly focusing on the fact that the fundamental machinery found in plants is the same found in cyanobacteria - light-harvesting complexes, Photosystem II & Photosystem I - and it does the same essential biochemistry.

    I haven't looked at the rest of that website, so I can't offer any commentary on that.
  16. Jul 21, 2011 #15
    Like I said earlier Mike, I don't support your comment, "I think that the Plant FAQ at the Cronodon site Evo linked to hits the nail on the head - it's something that makes sense in the light of evolution."

    I provided evidence to the contrary in the bioessay Into the deep: new discoveries at the base of the green plant phylogeny dated July 11, 2011. If you read the bioessay it disputes the "cronodon" website that states:
    Mike, as a "chemist" I'm surprised you didn't explore the website prior to supporting it. Like I said before, I don't support the Cronodon website since it distorts the truth as I've mentioned above and I surely don't support 'Adam McLean's Alchemy Web site and courses' which is listed on the cronodon website:http://cronodon.com/Links.html

    I'm sure Evo didn't realize it was there as well.
  17. Jul 21, 2011 #16


    User Avatar

    Staff: Mentor

    I linked only to the plant FAQ, it appears to be factual based on my other reading, and it's explained clearly and in layman's terms. I am not concerned with what ever links they provide to other sites for reading other subjects.

    I appreciate you bringing up that other links to other websites may not be of the same quality, but it's not pertinent to this discussion.

    Edit: Actually that link is a treasure trove of ancient historical texts on alchemy. http://www.alchemywebsite.com/texts.html

    Can you post specifically where they dispute the information provided in the plant FAQ? I don't see it. I could have missed it, I am suffering from chronic sleep deprivation.
    Last edited: Jul 21, 2011
  18. Jul 21, 2011 #17


    User Avatar
    Gold Member

    Still, it seems preposterous that - despite evolution's incredible penchant for finding multiple ways to crack its nuts - for some reason chlorophyll has been virtually the only cracking of this particular nut for 3 billion years. And it's not just any nut - it is the nut upon which virtually the entire rest of the Earth's ecosystem is founded.
  19. Jul 21, 2011 #18
    Evo, I think we will have to agree to disagree, hoping in the future we use reputable websites.

    Carnegie Institute for Science on June 15, 2011 has a quality article entitled "What makes a plant a plant?" Understanding about a plant is important.

    I stand behind what I have contributed on the previous page.

    p.s. Dear Evo, you are a very sweet person whom I admire because you are a dedicated member of PHYSICSFORUMS but foremost above all else a kind person. I'm off to take a vacation with a bunch of ladies. Wish you were with us. FUN TIME with R&R! Take care.
    Last edited: Jul 21, 2011
  20. Jul 21, 2011 #19
    DaveC426913 – Your post reminded me that evolution may have tinkered with chlorophyll along the way. In the photosynthetic anaerobes, we see bacteriochlorophylls with a slightly different structure of the porphyrin ring that have an absorption maxima in the infrared. In cyanobacteria and others, we see that chlorophylls are predominant whose absorption properties have already been mentioned. In the anaerobes, the organisms rely upon fairly well-known reduced chemical species (hydrogen sulfide for one) as an electron source – but in cyanobacteria, algae, and plants, we see water being used.

    One factor –which I’ve indirectly mentioned – is that chlorophyll (and bacteriochlorophyll, for that matter) is used as in the electron transfer chain of photosynthetic reaction centers, most notably as the “special pair” component of said chain, and not just in light-harvesting. That chlorophyll serves in a part of the mechanism past the light absorption phase might serve as an additional constraint on tinkering by evolutionary mechanisms. If one strays too far in terms of a critical physicochemical parameter, electron transfer – and subsequent biochemistry – might take an unfortunate hit.

    Related to my earlier comment about modification of the porphyrin ring substituents – it can get complicated. And hard to keep straight in my head, especially as it’s been more than a few years now since I was keeping up with this material to some extent.

    ViewsOfMars – I hope your vacation is delightful. If you have the time when you return, I would sincerely appreciate a response to the following.

    I read the paper by Leliaert, Verbruggen, and Zechman. Their recapitulation of the evolution of photosynthesis in eukaryotes meshes with mine – an endosymbiotic event involving a eukaryotic ancestor and with a photosynthetic cyanobacterium-like prokaryote. This is basically where I stopped, as the subsequent evolution of photosynthesis in eukaryotes and green plants in particular is rooted in this event and was not directly relevant to the points I was hoping to make. I did work through the remainder of the paper in a modicum of detail.

    The authors go on to mention that after this event, three lineages arose and they spend some time explaining how things went from there. The one aspect that is mentioned that I consider relevant to the discussion of photosynthetic pigments is the persistence and success of the red pigmented organisms, which they note allows for greater efficiency underwater and might reflect differential usage of trace elements by the algae as a whole. The remainder of the paper essentially sketches out the advances in molecular phylogenetics and explorations of little-known deep-water seaweeds, as well suggesting that further studies of the hard-to-treach ecosystems where photosynthetic life may flourish might clarify genetic and evolutionary relationships.

    Nowhere in the article do I see them object to the use of evolutionary theory in explaining biological phenomenona. If anything, the entire paper is predicated upon it! Their mentions of chlorophyll and pigments mostly revolve around the aforementioned bit about red pigmented organisms, some differences in chlorophyll and pigment abundances in a group of the deep-water seaweeds, and what seems to be a mention of their use in understanding phylogenetic relationships. It does not address the physical and chemical details that are the topic of this thread in any explicit manner. The thrust of their paper is to propose that application of modern research methods and exploration of previously understudied ecosystems is likely to be very fruitful in elaborating the details of the evolutionary relationships between the various photosynthetic eukaryotes. While the molecular details that are of interest here in this thread will almost certainly be touched upon peripherally as a result of such proposed research, they are not the primary interest.

    Insofar as the efforts you linked to in the press release from the Carnegie Institution, again, the article flat-out states that the current understanding of how eukaryotes gained the ability to carry out photosynthesis involved endosymbiosis between a eukaryotic ancestor and cyanobacteria. The underlying biochemistry that governs pigment biosynthesis has its origins in the early days of life on Earth. Yes, as seen in the paper by Leliaert, Verbruggen, and Zechman, organisms can adapt, but the adaptations are apparently on the order of preferring different chlorophyll derivatives and from differences in trace element abundances.

    Insofar as to my not exploring that website – I saw the FAQ that was linked to in the thread, saw what it had to say, and judged it on its own merits. If I had found it full of numerous egregious errors, I would surely have not mentioned it in my original response. I thought that, in the end, its statement that the “chlorophyll decision” is due to how photosynthesis evolved is likely to be correct to a reasonable degree.

    I did poke around the site afterwards, though, a bit. They have the structures for amino acids correct. Its depiction of the formation of a peptide bond is accurate. While very succinct, the site presents protein secondary structure and above without any major errors that immediately popped out at me. I also thought that its presentation of lipids was short and sweet. That it links to a site which – at the very least – serves as a resource to those who research the topic in a serious manner is, at best, a minor distraction. Some people find discovering hidden delights in obscure texts to be enjoyable, others get their endorphin rush elsewhere.
    Last edited: Jul 22, 2011
  21. Jul 25, 2011 #20
    Mike, I explained everything on the previous page and this one. As far as your comment, "Some people find discovering hidden delights in obscure texts to be enjoyable, others get their endorphin rush elsewhere." Your comment refers to "alchemy" which I have stood my ground because I don't endorse alchemy since it is psuedoscience! (1) Like I earlier said, "I don't support the Cronodon website! And I did say to you, "Mike, as a "chemist" I'm surprised you didn't explore the website prior to supporting it. Like I said before, I don't support the Cronodon website since it distorts the truth as I've mentioned above[refer to msg. #15] and I surely don't support 'Adam McLean's Alchemy Web site and courses' which is listed on the cronodon website:http://cronodon.com/Links.html'. You seem to still support the website and alchemy whereas I don't!

    From Skeptic's Dictionary explains alchemy:

    1. http://skepdic.com/pseudosc.html
    Last edited: Jul 25, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook