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Why are leaves green?

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Why are plants green?

Postby Hadrian » Thu Apr 14, 2005 1:34 pm

Hello!
I`m a new member, and because, my first language isn`t English I`ll probaply have some spelling errors so I apolgise for them in advance. To be honest I`m not a biologyst , but I`m interrested int it. To be precise I study electrotechnics. A few days ago we learned about light, and our professor said that in the ordinary white light-ray the green component, which has the wavelength of some 550nm carries the most energy. Now if it`s true, then I have a little logical problem. I know from my previous physics studies that if an object reflects a specific wavelength in much greater measure than the other wavelengths than we see this object in that color wich is represented by that most reflected wavelength. So my question is,that if the aim of the photosynthesis is to make organic material out of energy and unorganic materials, than it`s normal that plants " want" as much energy from the sun as possible. Now if all that`s right, than why are they reflecting the most valuable wavelengths of the suns light-rays? Other words WHY ARE THEY GREEN?

IP.s: I`ve asked my physics professor and she said that she simply doesn`t know why, so I would be very pleased if some of you would be so kind to explain me this thing

Thank you in advance
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Postby Chris4 » Thu Apr 14, 2005 1:56 pm

The topic was about digestion but as your new i guess its ok. :wink:

I found this from here: http://www.madsci.org/posts/archives/ap ... .Bt.r.html

Chlorophyll gives plants their green colour. There are other pigments in the leaves too, such as xanthophylls (yellows) and carotenoids (yellows, oranges and reds). These pigments are also used in photosynthesis but occur in lesser quantities than the green chlorophyll. The combinations of the different pigments make different shades of green.

Now the reason that plants look green is that they are trying to obtain energy from the sun using a particular part of the light spectrum, mainly the red and infra red wavelengths. If you remember from your physics classes the colour you see is the colour that is reflected from the object, the other colours are absorbed. So in the case of green plants, the green wavelength is reflected and all the other colours, especially reds and blues, are absorbed to drive the energy cycle in the plants.

Chlorophyll does best in the red (around 670 nm) and blue (around 500 nm) areas of the spectrum. That's why many plants have the additional pigments (xanthophylls and carotenoids) called accessory pigments that feed light energy to chlorophyll "a" from light. Chlorophyll is almost useless in the green part of the spectrum, and doesn't absorb that colour. That is why most plants are green.

About the 550nm wavelength having the most energy. I think this is wrong. I knew this before but checked it here: http://www.scienceofspectroscopy.info/t ... IGHT_3.HTM
The shorter the wavelength the more energy it has. So violet has the most with 380-420nm. White light is about 400-700 btw. There are simple formulas to prove all this on the website above.
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Postby lohita » Mon Apr 18, 2005 1:27 am

because of the present of chlorophyll pigment the leaves appear green. :D
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Postby mith » Mon Apr 18, 2005 3:43 am

That's not the question, he's asking why plants do not produce a pigment to absorb the green wavelength of light i.e. why leaves don't appear black/red.
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Postby biostudent84 » Mon Apr 18, 2005 11:00 am

Well, actually, they do. When leaves turn red in the fall, it is because they lost all their green pigmented chlorophyll, but retained their red, orange, and yellow pigmented chlorophyll for a little while longer.

During the summer, leaves have all color chlorophyll except blue and violet (if I recall correctly). The reason green shows up is because it is the dominant type of chlorophyll, and green light is mostly reflected.

Blue and purple are the only colors not reflected because it is visible light that is absorbed. As it has the shortest wavelength, it has the highest energy. This is the dominant color of visible light that is used in photosynthesis.
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Postby mith » Mon Apr 18, 2005 8:29 pm

Well, I meant red as why are they not usually red. As you said, the light wavelength with the most energy should be absorbed so blue/violet is absorbed but why isn't green, a higher energy wavelength than red? Why did plants evolve the pigments to absorb the lower energy reds/oranges but not the greens?
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Re: Why are plants green?

Postby rezkalla » Tue Apr 19, 2005 11:17 am

the green portion of light which have the wave length of 550nm doesn't carry the {biggest}energy content because the energy content of the wave length is inversely proportional with the wave length.
i mean if for example there is a light with 400nm ,this light will have greater energy content that that of the green light so the green light do not have the highest energy content.so when plant reflect the green light [not another light] they are already absorbing a higher energy content that that of green light.i wish i could satisfied you.
Hadrian wrote:Hello!
I`m a new member, and because, my first language isn`t English I`ll probaply have some spelling errors so I apolgise for them in advance. To be honest I`m not a biologyst , but I`m interrested int it. To be precise I study electrotechnics. A few days ago we learned about light, and our professor said that in the ordinary white light-ray the green component, which has the wavelength of some 550nm carries the most energy. Now if it`s true, then I have a little logical problem. I know from my previous physics studies that if an object reflects a specific wavelength in much greater measure than the other wavelengths than we see this object in that color wich is represented by that most reflected wavelength. So my question is,that if the aim of the photosynthesis is to make organic material out of energy and unorganic materials, than it`s normal that plants " want" as much energy from the sun as possible. Now if all that`s right, than why are they reflecting the most valuable wavelengths of the suns light-rays? Other words WHY ARE THEY GREEN?

IP.s: I`ve asked my physics professor and she said that she simply doesn`t know why, so I would be very pleased if some of you would be so kind to explain me this thing

Thank you in advance
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Postby b_d_41501 » Mon Apr 25, 2005 10:21 pm

Yeah, the leaves appear green because the only wavelength of light not absorbed by the pigment produced by chlorophyll is the wavelength of light for the color green. Thus if it is not absorbed, it is seen by us in the Visible Spectrum.
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It green because....

Postby victor » Fri May 20, 2005 2:04 pm

Yeah, I think the leaves are green because they don't absorb the sunlight but the sunlight is used to 'break' the chlorophyll and get in to the photosystem 1 and 2. (siklic and non-siklic). After that they form their chlorophyll again...
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Postby jhadler » Tue Jun 27, 2006 5:21 pm

I think the responses are not quite adressing the question. The question was this: Since the spectral peak of the sun is in the yellow-green portion of the visible spectrum, why doesn't chlorophill absorb at this wavelength? Why instead, does it reflect the most abundant wavelengths of light (not the most -energetic-).

I'm rather interested in finding out what the biological reason for this is.

Thanks,

-Josh
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Postby herb386 » Tue Jun 27, 2006 8:36 pm

I had a look on google and found a couple of arguments.

The first is that chlorophyll may have been easy to evolve due to it's similarity to cytochromes so a modification on an already existing pathway was all that was needed to capture some wavelengths of light.

Another argument is that very few plants actually have a shortage of energy input from light. It is only those that tend to grow in the shade of other plants that require further adaptations to get more light. Adaptations such as more chlorophyll and larger leaves may be easier to produce than another pigment.

The most convincing argument I found is that all plants have evolved from aquatic plants. Green light does not penetrate sea water very well so there was no advantage to being able to capture green light while plants were aquatic. Once they moved on to land the light levels were much higher than you get in water anyway so there was no need for further adaptation. There would have been no selection to develop another pigment.
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Postby mkwaje » Sat Jul 22, 2006 12:21 pm

I agree with that herb86, many photosynthetic plants /algae found some distance below the surface contain red pigments so that they can utilize the red wavelength, which is basically almost the only light wavelength that can reach deeper waters.

Another thing, many scientists believe that sometime in the past, chloroplasts are actually living cells that have been engulfed or had symbiotic relationship with other cells. This will explain why chloroplasts and mitochondria have their own DNA that is separate from the genomic DNA of photosynthetic cells.

So its possible that in the past, if these chloroplast ancestors were predominantly green pigmented, the leaf color that we see now are simply a carry over charateristic of those cells.
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