O.K., this is about photosynthesis---I'm (fairly) sure that I understand it, BUT...?

Question

why is chlorophyll green, not blue-or any other color for that matter? Does it have anything to do with that particular wavelength of light? Maybe it wouldn't work with any other
color(?)...

Maybe I didn't ask it correctly---at least that's what I get after reading the first answer. What I asked was, "why green"
and not any other color? I didn't ask how
light and color work. That was not the
question.


Any other answers out there?

You folks are coming up with some GREAT answers! O.K., you're getting
it...Maybe one (or more) of you could
help me rephrase my question, & make
it more exact...

P.S. I get why chlorophyll appears green-I
understood that since 2nd grade.

Answer 1

Chlorophyll is a green photosynthetic pigment found in plants, algae, and cyanobacteria. Its name is derived from ancient Greek: chloros = green and phyllon = leaf. Chlorophyll absorbs most strongly in the blue and red but poorly in the green portions of the electromagnetic spectrum, hence the green color of chlorophyll-containing tissues like plant leaves.

Chlorophyll and photosynthesis
Chlorophyll is an essential component of photosynthesis. Chlorophyll molecules are specifically arranged in and around pigment protein complexes called photosystems, which are embedded in the thylacoid membranes of chloroplasts. In these complexes, chlorophyll serves two primary functions. The function of the vast majority of chlorophyll (up to several hundered per photosystem) is to absorb light and transfer that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems. Photosystem II and Photosystem I have their own distinct reaction center chlorophylls, named P680 and P700, respectively. These pigments are named after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the chlorophylls in each photosystem are distinct and determined by each other and the protein structure surrounding them. Once extracted from the protein into a solvent (such as acetone or methanol), these chlorophylls loose those distinctions and become a homogenous mixture of identical molecules.

The function of the reaction center chlorophyll is to use the light energy absorbed by and transfered to it from the other chlorophylls in the photosystems to undergo a charge seperation, a specific redox reaction in which the chlorophyll donates an electron into a series of molecular intermediates called an electron transport chain. The charged reaction center chlorophyll (P680+) is then reduced back to its ground state by accepting an electron. In Photosystem II, the electron which reduces P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms like plants produce O2 gas, and is the source for practically all the O2 in Earth's atmosphere. Photosystem I typically works in series with Photosystem II, thus the P700+ of Photosystem I is usually reduced, via many intermediates in the thylacoid membrane, by electrons ultimately from Photosystem II. Electron transfer reactions in the thylacoid membranes are complex, however, and the source of electrons used to reduce P700+ can vary. The electron flow produced by the reaction center chlorohpylls is used to shuttle H+ ions across the thylacoid membrane, setting up a chemiosmotic potential mainly used to produce ATP chemical energy, and those electrons ultimately reduce NAD+ to NADPH a universal reductant used to reduce CO2 into sugars as well as for other biosynthetic reductions.


Absorbance spectra of free chlorophyll a (green) and b (red) in a solvent. The spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment-protein interactions.Reaction center chlorophylls are capable of directly absorbing light and performing charge separation events without other chlorophylls, but the absorption cross section (the likelihood of absorbing a photon under a given light intensity) is small. Thus, the remaining chlorophylls in the photosystem and antenna pigment protein complexes associated with the photosystems all cooperatively absorb and funnel light energy to the reaction center. Besides chlorophyll a, there are other pigments, called accessory pigments, which occur in these pigment-protein antenna complexes. They include other forms of chlorohphyll, such as chlorophyll b in green algal and higher plant antennae, while other algae may contain chlorophyll c or d. In addition, there are many non-chlorophyll accessory pigments, such as carotenoids or phycobilins which also absorb light and transfer that light energy to the photosystem chlorophylls. Some of these accessory pigments, particularly the carotenoids, also serve to absorb and dissipate excess light energy, or work as antioxidants. The large, physically associated group of chlorophylls and other accesory pigments is sometimes referred to as a pigment bed, though this term is losing prominence with the advent of detailed knowledge of the structural organization of the photosystem and antenna complexes.


The different chlorophyll and non-chlorophyll pigments associated with the photosystems all have different spectra, either because the spectra of the different chlorophylls are modified by their local protein environment, or because the accessory pigments have intrinsically different absorption spectra from chlorophyll. The net result is that, in vivo the total absorption spectrum is broadened and flattened such that a wider range of red, orange, yellow and blue light can be absorbed by plants and algae. Most photosynthetic organisms do not have pigments which absorb green light well, thus most remaining light under leaf canopies in forests or under water with abundant plankton is green, a spectral effect called the "green window". Some organisms, however, contain phycobilin pigments of cyanobacteria and red algae can absorb green light relatively well and they can exploit the little remaining green light in these habitats.

Answer 2

This is a very interesting question. Chlorophyll is a green pigment, and as such, it is best at absorbing red light. Why would a photosythetic organism evolve a pigment that is best at absorbing red light, which is among the weakest that the sun produces in any quantity? It almost seems stupid in a way. Why not absorb blue or even ultraviolet, which are much more energetic? The sun radiates visible light as a black body with a peak wavelength of ~5,500 Angstroms in the yellow-green part of the rainbow. Just over 90% of the total energy output of the sun is in the visible range. If you were doing an "Intelligent design" you would certainly choose a pigment that absorbes the peak wavelength the best, wouldn't you? I would. This would be purple, by the way. Your human eye contains just such a light absorbing pigment called rhodopsin, which is indeed purple. So why aren't plants purple also? It seems that in the early evolution of photosynthesis, the Earth's atmosphere was much denser, cloudier, and dustier (due to far more numerous volcanoes) than today's atmosphere. In those days, the red light got through to the greatest extent and the most reliable extent. So the so called "bluegreen algae" prokaryotes that chloroplasts are decended from found that a green pigment was the most advantageous at the time, and now we are stuck with that decision, as is so often the case. Once a convention gets started, it is difficult to break. Just try to get the US to adopt the metric system if you don't believe me. But if we someday find alien plant life on other planets, what color will they be? Maybe purple, maybe green, or maybe red, as in the recent Tom Cruise movie "The War of the Worlds", or something else. That will be weird, won't it?

Answer 3

You give the impression that you don’t; really understand how light, colour and vision work at the most basic level.

Chlorophyll is the colour it is for the same reason that anything is the colour it is: because it emits light at that frequency. In the case of chlorophyll it is the yellow and green wavelengths that are emitted, with the red and blue wavelengths in sunlight being absorbed.

So naturally chlorophyll won’t work with any other wavelengths of light. It needs to absorb light energy to work. If the light energy is in any wavelength outside the red and blue parts of the spectrum then the energy can’t be absorbed and so the pigment obviously can’t work.

I really get the impression that you lack the basic knowledge to even know what question you intend to ask.


>>>>I didn't ask how light and color work. That was not the
>>>>>question.

In fact that was precisely the quetsion you asked: why is chlorphyll the colour it is and how is that related to light wavelengths. It is totally imposible to answer the quetsion you asked in any way aside from how light and colour work.

>>>> What I asked was, "why green" and not any other color?

And as I said, it is green because it emits green wavelengths and absorbs red and blue. If a susbtance absorbs red and blue wavelengths and emits green and yellow it can't BE any other colour but green.

I still don't think you really know what question you are want to ask. There is no answer to why anything is any colour beyond "those are the emitted wavelengths".

Would you like me to expand on how emitted light produces observed colours in the eye?

Do you want to know why chloroprhyll evolved to absorb red and blue wavelngths?

Think about what information you actually want and ask that question. Asking the same question that has already been answered is unikely to get us anywhere.

Answer 4

well you almost answered your own question - the chlorophylls (not just one type!) catches only some wavelengths. so that the rest of the spectra of sunlight is not totally wasted, plants have other auxiliary pigments, the carotenes, xanthophylls that are different colors and catch different spectra and hand over the energy to chlorophylls. like if you make a separation experiment and separate the pigments of a perfectly green leaf, you will see that there are also yellow pigments, the xanthophylls, for sure.
in a simplified way: make an ethanol extract of leaf (crush it simply in as LITTLE amount as possible of ethanol or even vodka) and put a strip of filtering/blotting paper as a wick in it (put a lid over it so that it doesnt dry out too fast) - you will see different colors separated as they seep through the paper , but chlorophyll will dominatë.

why the most important pigment is not other color? that is because every wavelength carries different amount of energy - so the wavelengths somewhere form the visible part of spectra are jusk enough - not too much (that would cause damage), not too little (not enough to make the chemical reactions work). so the kind of red light that is captured by chlorophylls-the dominant pigments- is just the perfect one...

Answer 5

it seems everyone is answering the question in terms of the electromagnetic spectrum. there is a simple point forgotten here however.
the chlorophyll is a pigment just like the haemoglobin in our blood, the iron in the latter being replaced by magnesium in the former.
magnesium is an alkaline earth element and thus needs little energy for the excitation of its electrons to higher energy levels for which even the higher wavelengths of red light are sufficient.
so.Mg absorbs red and thus emits out the complimentary green light, appearing green to our eyes.
and yes green pigment is absolutely essential for photosynthesis because other colours would not cause electron excitation in chlorophyll.

Answer 6

The reason why it's green is because when white light hits it, it reflects only green light and absorbs all others.

..or is it the other way round?

Answer 7

its because of (renina) Spanish. its an aminoacid that pigments plants.