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I know plants are green due to chlorophyll.

Surely it would be more beneficial for plants to be red than green as by being green they reflect green light and do not absorb it even though green light has more energy than red light.

Is there no alternative to chlorophyll? Or is it something else?

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I find it even more puzzling why plants don't absorb all of the visible spectrum altogether (resulting in the leaves being black). –  Adam Zalcman Jan 3 '12 at 19:48
@CrazyJugglerDrummer - I thought about it but didn't want to get the chlorophyll response without people reading =P –  Joe Clarke Jan 3 '12 at 20:38
It is even more strange becouse green algae evolved in water. And red light is absorbed by water. Rhodophyta (red algae) are red due to phycoerythrin but it seems that red color is advantageous for them only in sea depths. –  Marta Cz-C Jan 3 '12 at 20:39
@AdamZalcman: There's probably no organic substance with this absorption spectrum, and adding further pigments to cover the entire VIS spectrum might not pay off. –  Tobias Kienzler Jan 4 '12 at 11:27
Surely it would be more beneficial: not if the red pigment is less efficient in converting light to energy... –  nico Jan 4 '12 at 13:04

6 Answers 6

up vote 56 down vote accepted

Surely it would be even more beneficial for plants to be black instead of red or green, from an energy absorption point of view. And Solar cells are indeed pretty dark.

But, as Rory indicated, higher energy photons will only produce heat. This is because the chemical reactions powered by photosynthesis require only a certain amount of energy, and any excessive amount delivered by higher-energy photons cannot be simply used for another reaction1 but will yield heat. I don't know how much trouble that actually causes, but there is another point:

As explained, what determines the efficiency of solar energy conversion is not the energy per photon, but the amount of photons available. So you should take a look at the sunlight spectrum:

Solar Radiation Spectrum

The Irradiance is an energy density, however we are interested in photon density, so you have to divide this curve by the energy per photon, which means multiply it by λ/(hc) (that is higher wavelengths need more photons to achieve the same Irradiance). If you compare that curve integrated over the high energy photons (say, λ < 580 nm) to the integration over the the low energy ones, you'll notice that despite the atmospheric losses (the red curve is what is left of the sunlight at sea level) there are a lot more "red" photons than "green" ones, so making leaves red would waste a lot of potentially converted energy2.

Of course, this is still no explanation why leaves are not simply black — absorbing all light is surely even more effective, no? I don't know enough about organic chemistry, but my guess would be that there are no organic substances with such a broad absorption spectrum and adding another kind of pigment might not pay off.3

1) Theoretically that is possible, but it's a highly non-linear process and thus too unlikely to be of real use (in plant medium at least)
2) Since water absorbs red light stronger than green and blue light deep sea plants are indeed better off being red, as Marta Cz-C mentioned.
3 And other alternatives, like the semiconductors used in Solar cells, are rather unlikely to be encountered in plants...

Additional reading, proposed by Dave Jarvis:

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I would also add the fact that blue light reaches earth surface greately scattered due to Rayleigh scattering, so the absolute amount of energy carried blue light is comparable (or even less) than that of other parts of the visible spectrum. –  Alexander Galkin Jan 5 '12 at 14:10
@AlexanderGalkin: True, but that should already be included in the red curve (together with atmospheric absorption): for blue you can see a stronger deviation from sunlight (yellow curve) than for red/IR (Unfortunately I don't have the dataset but it would be better visible for the photon density) –  Tobias Kienzler Jan 5 '12 at 15:06
If leaves were black, they would get too hot. –  David LeBauer Apr 8 '12 at 2:44
Also don't forget - evolution only produces something which is good enough - not optimal. If green turned out to be good enough, then there would be impetus to develop black leaves. –  William Mioch Oct 11 '12 at 23:29
I'd note as well that there are other photosynthesizers that utilise the energies that plants do not - so-called purple bacteria - and these may predate green photosynthesizers. It's plausible that the "green gap" originally evolved as a means to exploit light not exploited by competing organisms. –  Jack Aidley Jan 29 '13 at 17:05

I believe it is because of a trade off between absorbing a wide range of photons and not absorbing too much heat. Certainly this is a reason why leaves are not black - the enzymes in photosynthesis as it stands would be denatured by the excess heat that would be gained.

This may go some of the way towards explaining why green is reflected rather than red as you suggested - reflecting away a higher energy colour reduces the amount of thermal energy gained by the leaves.

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I think the solar spectrum is also quite relevant. –  Tobias Kienzler Jan 4 '12 at 11:29
It is more to do with the specific photon energy level (see Tobias) than the thermal heat supplied per se. Heat is probably not a concern for temperate/arctic plants which are also green. –  Poshpaws Jan 4 '12 at 14:25
Heat is the least of a plant's problems. That is why we can breed black tulips without self-cooking petals. ;) –  S. Albano Oct 11 '12 at 6:02

There is quite a fun article here which discusses the colours of hypothetical plants on planets around other stars.

Stars are classified by their spectral type which is dictated by their surface temperatures. The Sun's is relatively hot, and it's spectral energy distribution peaks in the green region of the spectrum. However the majority of stars in the Galaxy are K and M type stars which emit mainly in the red and infrared.

This is relevant to this discussion since any photosynthesis on these worlds would have to adapt to these wavelengths of light in order to proceed. On planets around cool stars plant life (or its equivalent) might well be black!

OK, this is not entirely pie in the sky astrobiologist rubbish. It is actually quite relevant to the search for biosignatures and life on other planets. In order to model the reflectance spectrum of planets we observe (i.e. the light reflected from the primary star) we need to try and take into account any potential vegetation.

For example, if we take a reflectance spectrum of the Earth, we see a characteristic peak in the red "the red edge" which is due to surface plant life.

NASA also has a short page on this here.

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nice links, though I doubt black plants are likely to happen anywhere since that would most likely need too many different pigments –  Tobias Kienzler Jan 11 '12 at 8:03
OK, maybe not completely black but very dark. There are various terrestrial plants which have very dark leaves (e.g. Oxalis triangularis). Admittedly, many are cultivars but this does demonstrate that there are some dark pigments available (I suppose anthocyanins?) –  Poshpaws Jan 12 '12 at 11:08
good point. Maybe in most cases absorbing green was just the first thing that happened (by evolution) and it was sufficient, but I'm just speculating... –  Tobias Kienzler Jan 16 '12 at 9:19

There are two factors at play here. First is the balance between how much energy a plant can collect and how much it can use. It is not a problem of too much heat, but too many electrons. If it were a question of heat, a number of flowers selected for their black pigmentation would have their petals cooked off. ;)

If a plant does not have enough water, is too cold, is too hot, collects too much light, or has some other condition that prevents the electron transport chain from functioning properly, the electrons pile up in a process called photoinhibition.

These electrons are then transferred to molecules that they should not be transferred to, creating free radicals, wreaking havok within the plant's cells. Fortunately, plants produce other compounds that prevent some of the damage by absorbing and passing around the electrons like hot potatos. These antioxidants are also beneficial to us when we eat them.

This explains why plants collect the amount of light energy they do, but does not explain why they are green, and not grey or dark red. Surely there are other pigments that would be able to generate electrons for the electron transport chain.

The answer to that is the same as why ATP is used as the main energy transport molecule in organisms rather than GTP or something else.

Chlorophyll a and b were just the first things that came about that fulfilled the requirement. Certainly some other pigment could have collected the energy, but that region of parameter space never needed to be explored.

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I think you are right, that the bottleneck is not in collecting the photons, but in processing them without creating too much singlet oxygen. One of important factors might be that chlorophyll plays also a big structural role: It is not only a collector of light, but also allows separation of charge in the special pair of the reaction center and constitutes many pigment-protein complexes structurally. –  Irigi Oct 14 '14 at 20:10

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 thought to have evolved from a common ancestor that is a sort of green alga – with the idea being that chlorophyll has evolved only once. (reference)

Plants and other photosynthetic organisms are largely filled with pigment protein complexes that they produce to absorb sunlight. The part of the photosynthesis yield that they invest in this therefore has to be in proportion. The pigment in the lowest layer has to receive enough light to recoup its energy costs, which cannot happen if a black upper layer absorbs all the light. A black system can therefore only be optimal if it does not cost anything (reference).

Red and yellow light is longer wavelength, lower energy light, while the blue light is higher energy. It seems strange that plants would harvest the lower energy red light instead of the higher energy green light, unless you consider that, like all life, plants first evolved in the ocean. Sea water quickly absorbs the high-energy blue and green light, so that only the lower energy, longer wavelength red light can penetrate into the ocean. Since early plants and still most plant-life today, lived in the ocean, optimizing their pigments to absorb the reds and yellows that were present in ocean water was most effective. While the ability to capture the highest energy blue light was retained, the inability to harvest green light appears to be a consequence of the need to be able to absorb the lower energy of red light (reference).

Some more speculations on the subject. (reference)

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thank you for the useful infor.. –  Jose Javier Garcia Jun 2 '14 at 11:44

There are several parts to my answer.

First, evolution has selected the current system(s) over countless generations through natural selection. Natural selection depends on differences (major or minor) in the efficiency of various solutions (fitness) in the light (ho ho!) of the current environment. Here's where the solar energy spectrum is important as well as local environmental variables such as light absorption by water etc. as pointed out by another responder. After all that, what you have is what you have and that turns out to be (in the case of typical green plants), chlorophylls A and B and the "light" and "dark" reactions.

Second, how does this lead to green plants that appear green? Absorption of light is something that occurs at the atomic and molecular level and usually involves the energy state of particular electrons. The electrons in certain molecules are capable of moving from one energy level to another without leaving the atom or molecule. When energy of a certain level strikes the molecule, that energy is absorbed and one or more electrons move to a higher energy level in the molecule (conservation of energy). Those electrons with higher energy usually return to the "ground state" by emitting or transferring that energy. One way the energy can be emitted is as light in a process called fluorescence. The second law of thermodynamics (which makes it impossible to have perpetual motion machines) leads to the emission of light of lower energy and longer wave length. (n.b. wavelength (lambda) is inversely proportional to energy; long wavelength red light has less energy per photon than does short wavelength violet (ROYGBIV as seen in your ordinary rain bow)).

Anyway, chlorophylls A and B are complex organic molecules (C, H, O, N with a splash of Mg++) with a ring structure. You will find that a lot of organic molecules that absorb light (and fluoresce as well) have a ring structure in which electrons "resonate" by moving around the ring with ease. It is the resonance of the electrons that determine the absorption spectrum of a given molecule (among other things). Consult wikipedia article on chlorophyll for the absorption spectrum of the two chlorphylls. You will note that they absorb best at short wavelengths (blue,indigo,violet) as well as at the long wavelengths (red,orange,yellow) but not in the green. Since they don't absorb the green wavelengths, this is what is left over and this is what your eye perceives as the color of the leaf.

Finally, what happens to the energy from the solar spectrum that has been temporarily absorbed by the electrons of chlorophyll? Since its not part of the original question, I'll keep this short (apologies to plant physiologists out there). In the "light dependent reaction", the energetic electrons get transferred through a number of intermediate molecules to eventually "split" water into Oxygen and Hydrogen and generate energy-rich molecules of ATP and NADPH. The ATP and NADPH then are used to power the "light independent reaction" which takes CO2 and combines it with other molecules to create glucose. Note that this is how you get glucose (at least eventually in some form, vegan or not) to eat and oxygen to breath.

Take a look at what happens when you artificially uncouple the chlorophylls from the transfer system that leads to glucose synthesis. Notice the color of the fluorescence under UV light!

Alternatives? Look at photosynthetic bacteria.

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