Why are plants green?

Then don't make comments like this:

which suggest to most native anglophones that you're working on a post as we speak.

If you don't have time to respond at length, then simply state 'I don't have time to respond at length at the moment, but will do so later'. I can assure that no-one on this forum will begrudge you this. We all understand that the carbon world has to take priority over discussions here.

However, suggesting you don't have time to respond at length, and then being online for two hours (continuously or not) posting one-liners, tends to make something of a farce of you claim, in that if you had spent the time working intermittently on a post, that you spent mocking people who take the time to address you, then by now you could have had a veritable essay prepared for us on why we should discount the evolution of atmospheric chemistry when considering the predominance of chlorophyll based photosynthesis in the plant kingdom.

Personally, I'm at work at the moment, however what I'm doing involves periods of time where I'm waiting for things to load, or calculations to be done, and so I compose a post in note-pad while I wait for things to happen.

There was no bullying involved, only a request for evidence to support your dismissive oneliners, and the suggestion that failure to do so might result in the closure of the thread. You have claimed that you do not have the time for an extensive refutation at this moment, and I am willing to give you the benefit of the doubt in this regard.

However, there is something you need to keep in mind. I'm not psychic. I don't know these things unless you tell me first. There's no point in throwing a temper-tantrum because I didn't know something I had no way of knowing. It's infantile.

I did not say that I didn't know - I said there were certain things that we may never know, and that one of them was whether or not alternative forms of photosynthesis had evolved. They may have, there could have been a myriad of them, however, chlorophyll based photosynthesis may have simply been the most successful.

Just like I feel a bit like I'm wasting my time now, for example, because you haven't taken the time to fully realize the implications of my post. But that's fine, I paid an absurd amount of money to train in Chemistry, and cross train in Geology and physics, and given the tone of some of your posts, I'd wager you have yet to make that decision.

The point that you missed is I listed four 'possibilities'. Two of those possibilities were nothing more than a paraphrasing of what was in Spidergoats article.

The other two were simply two different aspects of the same environmental pressure - Atmospheric chemistry.

To wit:

The first point I made was this:

To elaborate further, the evidence we have available to us suggests that the early composition of the Earths atmosphere resembled that of Saturns's moon, Titan. Titan's atmosphere has a distinct ruddy tinge to it, due to some complex atmospheric chemistry, caused by what is in essence, a photochemical smog. For it to acquire a ruddy tinge to it, it must be preferentialy absorbing the Blue-green end of the spectrum. If the Earth's early atmosphere resembled that of Titan, then it stands to reason the optical properties of the early atmosphere would have resembled that of Titan. In otherwords, it is likely that a similar photochemical smog existed as a layer when the earth's atmosphere reached some critical density, that reddened the light reaching the surface, by preferentialy absorbing the blue-green end of the spectrum. The implication of this observation is that if early Autotrops were evolving towards the most efficient method of collecting available light for photosynthesis, then they're going to converge on Molecules that absorb efficiently at the red end of the spectrum, because that's where the highest flux of light is.

The second point I made was this:

There's two components to this statement
The first is in regard to atmospheric chemistry, and it is implicitly prefaced with the statement "On the otherhand, if the earth's early atmosphere was as transparent as it is now, and the light recieved at the surface was of a similar spectrum to what it is now". It also assumes the prior knowledge that the presence of elemental Oxygen in the atmosphere (in more than trace quantities) came about as a result of the evolution of autotrophs and photosynthesis. And it requires the prior knowledge that all life on earth today is protected from the sun's ultraviolet radiation by Ozone, which requires the presence of oxygen to form. In other words, if the Earth's atmosphere was transparent, and without oxygen, then the flux of ultraviolet radiation would have been much higher than we see to do (this is almost tautological).

Why is this important?

It is UV radiation that is responsible for the fading of pigments in fabrics and plastics. It is UV radiation that causes the loss of transperancy of transperant plastics over time. This is why Hilighter ink fades so quickly when left in the sun.

Pigments generally also absorb in the UV part of the spectrum. Most of the time, this isn't a problem. The photon is absorbed, and re-emitted at a lower frequency a short time later. Most of the time this isn't a problem, however, sometimes, or rather, inevitably, for any given molecule, an irreversable chemical reaction occurs, and the molecule no longer absorbs light in the visible part of the spectrum, and so the colour fades with time.

How does this explain the dominance of Chlorophyll?

First off, you need to note the observation that I made earlier, that plants don't just employ chlorophyll as a pigment, they also employ other pigments as well, for example, carotenoids. These pigments absorb in the UV, as does Chlorophyll, and some of these have antioxidant properties.

Then you need to consider what happens to an autotroph when it has no pigment left - it dies.

So there's the first three parts of the answer. Chlorophyll is a pigment. Pigments fade when exposed to UV light. An autotroph without pigmentation will be expected to die.

There's another piece to this puzzle - Chlorophyll does its thing using the lowest frequencies of light available. Why is this important? Because the lower the frequency of light used, the lower the energy of the photon, and the less likely damage to the pigment becomes.

And there, from the perspective of evolution at least is the reason for the dominance of chlorophyll today. Still can't see it?

Ultraviolet light destroys pigments. Autotrophs rely on their pigments for their food source. Chlorophyll absorbs in the UV as well as the red. Chlorophyll is used in the presence of other pigments. These other pigments also absorb in the UV protecting it from the UV, as well as sweeping up the free radicals that are likely to damage the Chlorophyll. This combination of traits, confered by the combination of pigments would seem then to enhance survival value in an environment with a high flux of UV radiation (compared to modern values). This in turn would indicate that chlorophyll autotrophs would have had a survival advantage over their competitors, because their life giving pigment was less likely to be damaged, because it was protected from UV by the presence of auxillary pigments, and utilized the longest wavelengths, with the lowest energy, that still had sufficient energy to maintain photosynthesis.

But then, I would come to that conclusion - that it's a result of atmospheric chemistry, I am, after all, a chemist.

Do I need to elaborate on this any further?

 

From wikipedia:

The biologist John Berman has offered the opinion that evolution is not an engineering process, and so it is often subject to various limitations that an engineer or other designer is not. Even if black leaves were better, evolution's limitations can prevent species from climbing to the absolute highest peak on the fitness landscape. Berman wrote that achieving pigments that work better than chlorophyll could be very difficult. In fact, all higher plants (embryophytes) are believed to have evolved from a common ancestor that is a sort of green algae - with the idea being that chlorophyll has evolved only once.

Shil DasSarma, a microbial geneticist at the University of Maryland, has pointed out that species of archae do use another light-absorbing molecule, retinal, to extract power from the green spectrum. He described the view of some scientists that such green-light-absorbing archae once dominated the earth environment. This could have left open a "niche" for green organisms which would absorb the other wavelengths of sunlight. This is just a possibility, and Berman wrote that scientists are still not convinced of any one explanation.​

 

You are asking questions that no one can answer in 2011. It's only been about a century and a half since evolution was discovered, about one century since we've had any idea of how it works, and about half a century since we've been able to thoroughly analyze DNA. Before that, the only evidence we had for the development of lifeforms was fossils, and the further back we go into prehistory, the fewer fossils there are because A) the forces of nature work against their preservation and B) there were fewer kinds of hard tissue like bones that could be fossilized, so we're stuck with the occasional print of a leaf or other soft tissue in a sedimentary rock.

As a result we have few good clues about how the earliest lifeforms evolved into more complicated species, and even fewer about how the very first objects that could be called "alive" arose out of inert matter.

Despite that handicap we've made some amazing progress in understanding possible means of abiogenesis; we can't yet reproduce the whole process in a laboratory but at least we now have confidence that one day we will be able to do that.

The same is true of figuring out how the the Plant Kingdom and the Algae Kingdom stumbled onto the clever technique of using chlorophyll to harness the sun's energy.

The 19th is called The Century Of Chemistry and the 20th The Century Of Physics. The 21st has already been named The Century Of Biology and it's quite possible that you younger people will live to see some utterly amazing scientific discoveries. Perhaps you'll see abiogenesis in a beaker; more likely we'll solve some lesser mysteries, and those solutions will build to a larger solution.

Still, some of your questions will probably never be answered. Why did plants standardize on chlorophyll instead of some other pigment? One might as well ask why terrestrial life is based on carbon instead of silicon, or for that matter why life arose on Earth instead of Venus, where the temperature is perhaps a little more hospitable to silicon-based "organic" molecules.

Much of what happens in nature is purely random. One lifeform mutates to have chlorophyll in its surface tissue, another mutates to have some other pigment. There may very well have literally only been one of each! Maybe the other guy got burned up in a volcano! We all know that the survival of any one organism on this planet is a matter of luck. The only reason it teems with life is that there are so many of them that enough survive to keep the ecosystem going. One super-special organism with The Mutation That Will Change History? Hey, lotsa luck guy! The odds are strongly against you surviving long enough to make it into the 21st century biology books.

We have no idea how many different cosmic experiments were taking place several billion years ago when the earth was warmer and the environment was a little more conducive to mutations. For all we know lifeforms may have arisen that used every conceivable pigment to harness the sun's energy, and all but one became extinct as an individual before being able to establish itself as a species.

But the "engineering" of evolution is not directed. Nobody is sitting there with a clipboard saying, "Based upon what I've seen so far, I think I'll try this little mutation next." They all occur at random. There are lots of really good ideas that do not occur in nature, and there are lots of really bad ideas that have survived.

I agree that "engineering" may have been a poor choice of words in this discussion since it's perpetually on the verge of turning into a flame war over the use of language. Every definition of the verb "to engineer" revolves around the words designing, planning, managing, contriving, etc. In other words, intelligent direction. Nonetheless, when the forces of nature, working at random over billions of years, result in something rather complicated that superficially resembles engineering, it's reasonable to use the word "engineering" as a metaphor, so long as no one will take it the wrong way.

Obviously it's been taken the wrong way, so let's just eliminate it from the rest of the discussion. Okay, everybody?

Your remarks give a very strong impression that you do not believe that what you see around you arose at random. You indicate through your posts that you believe that during the development of lifeforms on this planet, a systematic procedure was used. In particular, you assume that every possible pigment was tried as an energy receptor, and all were rejected except chlorophyll, and all processes of energy conversion were rejected except photosynthesis.

You don't appear to follow the line of reasoning that since chlorophyll and photosynthesis are the only ones that survive, therefore the process must have been a random one rather than a managed one, otherwise plants would have better "engineering." This is creationism in a nutshell: focusing on the few things in nature that resemble managed processes, rather than the many, many more that are clearly random, and understanding that those few resemblances are, themselves, coincidences, i.e., the results of random processes.

It's the center of the solar spectrum. An animal with virtually no night vision, who lives by his wits and therefore must be highly visually oriented, so much so that he has binocular vision rather than the wider field of view that other herbivores use to avoid predators, would be better served by a sense of vision that receives the maximum amount of light, giving him the best possible view of his surroundings.

Almost all primates have binocular vision; I'd be interested to know if they all have the same spectrum sensitivity. It varies wildly among clades of animals. Most birds, for instance, have four color receptors instead of three. Not only does this allow them to see finer gradations of color in order to find ripe fruit and to spot lurking predators, but it also gives them vision up into the ultraviolet. We have always assumed that many species are not dimorphic and wondered how they tell the males from the females. It turns out that their dimorphism is in the ultraviolet spectrum!

Cats, on the other hand, have very poor color vision but their eyes take in about six times as much light as ours so they can see well to hunt at night.