Why are leaves green?

[MrMistery]

I said generally guys...
Yes there are two basic cases when "sharing" (what is actually called altruism) is good:
Kin altruism - when the two organisms share part of their alleles the reproduction of one will actually increase the fitness of the other, because it passes down genes they both have. From this you can derive the "two brothers or eight cousin case" postulated by G.B.S. Haldane, in that the minimum people you should be altruistic is is either two brothers or 8 cousins, as that is equivalent with 100% of your genes.
Reciprocal altruism - what you described with the bats. I think my old textbook had the same example, though it's been a few years. From what I remember, some of the bats stick around the cave because they need to take care of the young. The adult bats take turns in gathering food, and also feed the "baby-sitters" when they return.
About humans, it is a bit complicated: if a man is risking his life to save a complete stranger, then he is apparently risking his life to save completely different genes for no good reason. Generally in the human population this kind of altruism is never reciprocated, so i find it hard to examine how human altruism can be evolutionary good, at least in some cases. Nor do I care really.
About plants: I do not believe reciprocal altruism to be the case here, with the advent of green-reflecting pigments. It is still possible, but I honestly doubt it. In most ecosystems photosynthetic organisms are in terrible competition with each other for light, and I see no reason why this was not true when photosynthesis was just emerging.
I have to say I am not a physicist. The explanation I gave I learned from a botany book. If you think that does not agree with physics, I would be more than happy to listen to your opinions as to why not.
BTW, here is a person who apparently did some research on the topic: http://theforcethat.blogspot.com/2007/0 ... green.html

[Sumi]

I learned from a photovoltaic (PV) manufacturer that PV has over-heating problems. Commonsense tells me that to increase the energy output of PV, you can just increase the amount of light. I did this using a mirror. Doubling the amount of light increased the electrical output of PV but also it made the PV panels very hot. Hot PV fails for its wiring and other electrical systems.

A tree’s interest is to gain most amount of energy from the sunlight. It does not want to get fried in the process. Although black may be the logical color, the cooling system does not keep the leaves cool enough. Leaves have limited amount of air and water cooling mechanism. The nature’s solution is to collect some energy at the first layer of leaves, let the rest of the energy go through and catch them at the second layer of the leaves and so on. This does not overheat the leaves.
But why green?

I saw an experiment at a students’ science fair that compared the energy going through various colored glass. It was a simple experiment of placing a PV at the bottom with a light bulb at top, placing different colored glass plates in between. Green allowed the least amount of energy to go though next to black. In another word, green is the best color to collect energy but not overheating the leaves. By having the secondary layers of leaves, trees achieve their goal of the most effective energy collection.

[wrong16]

The leaves contain chlorophyll which is green in colour.
Mostly leaves make up of MOST chlorophyll out of all other pigment make the leave green,
and they reflect the green light (thus they appears green) and abosorb other lights.

This chlorophyll carries out photosynthesis: food making process; take in carbon dioxide and give out oxygen. Changes light energy to food. Only carries out when there is light, water and carbon dioxide.

The higher rate of photosynthesis and absorbance by green leaves are
in the red (chlorophyll a: 650 nm) and violet light (chlorophyll b : 400 nm) range.
While the lowest is green which is reflected and minimal green light is also absorbed.
[ more information check out Action & Absorption Spectra]

My opinion is :
"It not only about the 'quality' of light ; how much energy it gives"
"it is also about the quantity that is absorbed and make use of."

If it has two higher reading for violet and red wavelenght (which they absorbed and make use of)
red is kinda the longest, lowest energy wavelengths and
violet is kinda the shortest and highest energy wavelengths.

and green (510 nm) is in between of them.
so what do you think now ?

[MrMistery]

so why is it better to be in the middle?

[sanjid123]

leaves are green for the present of chlorophyll pigment and i hope everyone knows it well. I understood that english is not you 1st language and also mine as well.

[Linn]

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.

This would seem to make sense since many plant leaves with red pigments are found as understory plants, however it's not always the case as many are found growing in full sun.

[MrMistery]

it also depends on what "red pigments" you are considering.
The red pigment mkwaje was referring to is phycoerythrin, which indeed has the role to allow the organism to use light that would otherwise be useless. However, most land plants that we see colored in red owe their color to anthocyan pigments. these pigments are stored in the central vacuole of a cell (and thus have absolutely nothing to do with photosynthesis) and serve the role of attracting insect and bird pollinators.

I would advise caution when using the word "pigment". While many classical biologists (especially botanists) still classify substances by their color, and refer to anything colored as "pigments", most of these pigments are very different chemically and some have entirely different roles. A chemical/biochemical classification would be much better in my opinion.

[Linn]

well I just read the original post again and realized you said that green light has the most energy.
But really it's the blue and violet light that has the most at 400 nm wavelenght, followed by green which I think is 500. Also I think, if I remember right, that some pigments do utilize the green wavelength.

[DavidMaine]

The sun is considered a "yellow star" because most of its radiation is in the yellow-green portion of the visible spectrum. The Solar Irradiance - that portion of solar radiation that reaches the surface of the Earth, is also predominantly in the yellow/green wavelengths. The other spectra of light, both lower energies toward and including red, and higher energies toward and including blue, are also present, but in lower quantities. Other writers have indicated that plants most desire light in the reddish and blueish wavelengths. In order to avoid overloading the plant system with too much energy, plants evolved the ability to shield themselves from the excessive quantities of yellow-green wavelengths by reflecting green light away. They may use some of the green, but not all of it. This is my theory - please tell me where I'm wrong. (I really want to know why plants are green!)

[JackBean]

IMHO plants are more often suffering from insufficient irradiation, than from excessive. And even if so, they have some mechanisms, how to stay fine.

[Physicist0017]

All,
I am a physicist working in the solar industry. I have always wondered why leaves of different plants have their particular color, and why by far the most plant species have green leaves. I have thought about possible explanations and have read your comments.
If I can summarize, the main postulation raised in this forum is that the color is a left-over remnant from evolution, and was originally due to plants living under water, which does not easily transmit green light. Therefore, it was never necessary for plants to develop the mechanisms to absorb green light.

The concern I have with this statement arises when I look a little deeper into evolution of plants itself. It is not exactly known, but postulated that the earliest organism utilizing photosynthesis arose around 3 Billion years ago, and the first ‘land plants’ appeared around 500 Million years ago. If we believe in evolution, to me it seems unlikely that plants would carry over such a simple characteristic that is left unchanged for 500 Million years. In light of evolution, it is not logical to state that there was no need to absorb green light, especially considering the fact once plants went ‘on land’ that wavelength was available in abundance. Furthermore, as I will show below, I have confirmed that leaves absorb, with very high efficiency, photons of wavelength that are both shorter and longer than green light, thus showing that all photons in the visible range can be used for photosynthesis, including green light.

Therefore, we need to consider the possibility that it is unlikely the evolutionary mechanisms of photosynthesis is the true explanation as to why leaves reflect green light. I want to show you some measurements I have performed on leaves. My original question was this: If I can make a solar cell BLACK because I want to use all photons available to me from sunlight, why then are leaves GREEN, since when I am looking at a leaf, I am literally looking at BILLIONS of years of evolution – and therefore (if I believe in evolution) I must be looking at the ULTIMATE OPTIMIZED form of absorbing the ‘fuel’ of sunlight to generate energy (even though the output ‘energy’ is in a different form between a solar cell and a plant – the incoming ‘fuel’ is the same: sunlight).

In that regard, I have performed transmission and reflectance measurements on leaves, as a function of wavelength. I have discovered several interesting findings, which I will show below. Before I go on though, I must mention that there is an important point to consider: we must distinguish between the front side and the back side of a leaf, which can be thought of as a bifacial solar cell. If you look closely you will see that the two sides look quite different. However, in reality there is a substantial amount of light that can be reflected from the surface underneath the leaf, and back on to the leaf from the bottom, and we need to take that into account.

Here are the findings, all values are approximate and to simplify the discussion, for a single green leaf of a single species of plant:
(1) The front of the leaf absorbs with ~95% efficiency all photons in the 300nm – 500nm range (energy 2.5eV and above)
(2) The absorbance decreases to ~75% at ~540nm (2.3eV), and climbs back up to above 90% at 670nm.
(3) This ‘dip’ in absorbance is not symmetric in wavelength, that is there is a sharp dip from 95% at 500nm, to 75% at 540nm, followed by a gradual climb from 75% at 540nm to 95% at 670nm. I think this is very important. Looking at the data even closer, it is apparent that there are three distinct wavelengths where absorbance is reduced: at 540nm by 20%, at 590nm by 10% and at 620nm by 8%.
(4) The absorbance drops very sharply (exactly like an optical filter) from 95% at 670nm to 10% at 750nm, and stays at 10% from 750nm to 1100nm, except for a slight trough to 15% around 960nm. This is also very important, showing that the leaf is optimized not to absorb light at wavelength >~700nm (i.e. the near infra red – HEAT). The leaf does not want the heat. This makes sense because absorbing light at those wavelengths would heat up the plant and cause it to dry. It is immediately obvious that the leaf’s interaction with light has been optimized to the mechanics of photosynthesis from this standpoint.

Here is the raw data of the measurements on the front side of the leaf, in comma separated format:
wavelength (nm),ENERGY (eV),Transmission (front),Reflectance (front),Absorption (front)
300,4.132805722,-0.1083,5.162,94.9463
310,3.999489409,0.06034,5.221,94.71866
320,3.874505365,0.06543,5.184,94.75057
330,3.757096111,0.02548,5.239,94.73552
340,3.646593284,0.01933,5.131,94.84967
350,3.542404905,-0.01474,5.179,94.83574
360,3.444004769,0.01071,5.167,94.82229
370,3.350923559,-0.1436,4.984,95.1596
380,3.26274136,0.04095,5.245,94.71405
390,3.179081325,0.1646,5.558,94.2774
400,3.099604292,0.1678,6.05,93.7822
410,3.024004187,0.2932,6.422,93.2848
420,2.952004087,0.3153,6.636,93.0487
430,2.883352829,0.3518,6.696,92.9522
440,2.817822083,0.3335,6.724,92.9425
450,2.755203815,0.4274,6.759,92.8136
460,2.69530808,0.5088,6.816,92.6752
470,2.637961099,0.5315,6.863,92.6055
480,2.583003576,0.584,6.846,92.57
490,2.530289218,0.6306,6.874,92.4954
500,2.479683433,0.9116,7.192,91.8964
510,2.43106219,1.8,8.484,89.716
520,2.384310994,3.889,11.63,84.481
530,2.339323994,6.232,15.47,78.298
540,2.296003179,7.436,17.39,75.174
550,2.254257667,7.924,18.08,73.996
560,2.214003066,7.681,17.47,74.849
570,2.175160906,6.491,15.24,78.269
580,2.137658132,5.302,13.08,81.618
590,2.101426638,4.626,11.93,83.444
600,2.066402861,4.377,11.48,84.143
610,2.032527404,3.802,10.53,85.668
620,1.999744704,3.19,9.58,87.23
630,1.968002725,3.034,9.312,87.654
640,1.937252682,2.686,8.883,88.431
650,1.907448795,1.939,8.003,90.058
660,1.878548056,1.427,7.386,91.187
670,1.850510025,0.9225,6.794,92.2835
680,1.823296642,0.8207,6.713,92.4663
690,1.796872053,2.264,8.139,89.597
700,1.771202452,8.318,16.87,74.812
710,1.746255939,15.2,29.38,55.42
720,1.722002384,21.07,41.14,37.79
730,1.698413311,25.64,50.49,23.87
740,1.675461779,28.27,55.35,16.38
750,1.653122289,29.54,57.73,12.73
760,1.63137068,30.06,58.79,11.15
770,1.610184048,30.38,58.99,10.63
780,1.589540662,30.52,59.17,10.31
790,1.569419895,30.6,59.02,10.38
800,1.549802146,30.73,59.13,10.14
810,1.530668786,30.86,59.28,9.86
820,1.512002094,30.82,59.2,9.98
830,1.493785201,31.03,59.21,9.76
840,1.476002044,31.02,59.06,9.92
850,1.458637314,31.01,58.87,10.12
860,1.441676415,31.06,59.08,9.86
870,1.425105421,31.12,58.81,10.07
880,1.408911042,31.03,58.97,10
890,1.393080581,31.26,58.85,9.89
900,1.377601907,31.26,58.53,10.21
910,1.362463425,31.23,58.47,10.3
920,1.34765404,31.33,58.54,10.13
930,1.333163136,31.12,58.28,10.6
940,1.31898055,31.06,58.02,10.92
950,1.305096544,30.74,57.5,11.76
960,1.291501788,30.42,56.83,12.75
970,1.278187337,30.19,56.43,13.38
980,1.265144609,30.19,56.58,13.23
990,1.25236537,30.27,56.46,13.27
1000,1.239841717,30.49,56.65,12.86
1010,1.227566056,30.67,56.98,12.35
1020,1.215531095,31.03,57.15,11.82
1030,1.203729822,31.18,57.46,11.36
1040,1.192155497,31.44,57.78,10.78
1050,1.180801635,31.65,57.94,10.41
1060,1.169661997,31.89,57.98,10.13
1070,1.158730576,32.13,58.15,9.72
1080,1.14800159,32.24,58.17,9.59
1090,1.137469465,32.31,58.09,9.6
1100,1.127128833,32.4,58.01,9.59


Please plot it yourself and take a look – it is quite interesting. At this stage I will not put forth any suggestions as to what may be the true explanation why the leaf has reduced absorbance at certain wavelengths (although I do have some ideas). I will stop here and wait for responses and further discussion, before I flood you with more data and findings.
Thanks.

[JackBean]

1) just because plants absorb light doesn't mean they use it for photosynthesis. We absorb some light too yet we're not photosynthesizing

2) if is something beneficial, you do not need to change it even after long time. Look into histones, they are pretty much conserved, much more then proteins of chlorophyll biosynthesis. Additionally, the proteins are evolving, but to create whole new pathway you need a little bit more More additionally, if you even created new pathway for biosynthesis of new pigment, you would need new pathway for electron transfer.