h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This question has tickled my fancy somewhat.

It's not really me after a definitive answer, I'm just wondering what hootoos SExperts think.

It's from http://www.moleclues.org/index.php?option=com_fireboard&Itemid=28

A series of prize winning questions put to real scientists by school children.

Quite an interesting question I hope you'll agree


Black absorbs light more effectively from the Sun than green. Why haven't plants evolved to become black in order to take advantage of this extra energy?

Perhaps because the evolution of chlorophyll only occurred once in the history of the world, and it works?

I don't really understand why plants are green. Sunlight is actually slightly green, rather than being yellow like most people think. The greatest amount of energy is in the green part of the spectrum. So it does seem a waste that that is the one colour that the plants don't use but reflect back at us.

Well the blue/violet end is actually highest in energy in the visible spectrum but I agree with your general point.

I think there is no real answer to 'why...?' Because really the answer is... because that's how it is.

Really I suspect it is simply because there was no need to - and natural selection works from a selective driving force.

Blue/violet light is so high in energy it can actually be damaging to cells though (e.g. sunburn) so absorbing too much high energy light might not be the advantage one might imagine.

Of course that latter point doesn't preclude the simultaneous evolution of defence mechanisms against such damage...

They will usalise light at some other wavelengths too, err if only I could remember what the other* pigment is other than chlorophyll, I know its not as important to most plants; but I seem to recall that some plants, particularly those that end up growing underneath (E.G., forrest floor type plants), will often tend to have more of the other pigment proportionally to chlorophyll as plants higher up in the canopy have alreayd 'filtered' out much of the wavelengths chlorophyll absorbes....
Unfortuantley I avoided the vegetative bits of biology and concentrated on the animal side of things
I wonder is there any particular advantage to Chlorophyll, and the colours/wavelengths it utalises as compaired to having it use differnt wavelengths/and hence be a differnt colour... maybe its linked into the DNA dammage/repair systems, which I guess also depends a bit on chronologically when each evolved and developed, I'd hazzard a guess that the DNA repair systems were about before Chlorophyll to an extent, but I guess photosynthesis itself must have been pretty early, I wonder if the origional photosynthesis used chlorophyll or chlorophyll like substances as they now do, of if it was somewhat differnt wavelengths to start with and it eventually then settled on the chlorophyll for wahtever selective advantage that gave over its predisesor

why green not black?
Well, clearly that is because Mother Nature is a hippy and not a goth.

Could be worse... Mother nature might not have only been goth, but could have been EMO too

It occurs to me also that in order to be black, plants would have to absorb in *all* regions of the visible spectrum.

Now that means absorption in the red region also and this, I think, would be more or less chemically useless.

The reactions chlorophyll dependent enyzmes catalyse need photons of sufficient energy to do chemical reactions - that is, transfer electrons from one species to another (that is what chemical reactions are essentially).
Now red light is not of high enough energy to cause electron charge transfer in the vast majority of compounds.

So absorption in the red is very unlikely to advantageous to a plant.

In addition, in metabolic pathways, much energy is released (as in burning of glucose...) and not all that released energy can be utilised and is wasted as heat. In order to avoid burning up, plant cells, like any other cells, must radiate this heat somehow. A very useful way of doing this is by radiation of infra-red light - if you absorb too much into the red you risk heating up in the sun and not being able to cool down...

But the thging is that plants are green because they DO absorb radiation aqt the red end of the spectrum.

Yeah I guess that's true, I was thinking of red leaved plants when I wrote that bit.

Having just read up on it a bit, it seems green plants absorb strongly in the red and the blue/violet region and it seems that the photosystems that perform the electron transport use the red light mostly. Other coloured plants use photosystems that absorb in the blue/green region.

Well, I learned something there and that's what it's all about in the end. I was talking on my feet there and the idea here was to start a debate. I'm sure noone really *knows* the answer to this question for sure.

I guess I'll stand my original statement that it's just not necessary to absorb throughout the entire visible spectrum - which is nothing special anyway it just happens to be the region of the em spectrum we utilise to sense the outside world.

Icky got Quote of the Day/Weekend!!


>>Well, clearly that is because Mother Nature is a hippy and not a goth.<<

Doesn't it come down to whether pollinators can distinguish them from their background?

Not really because that's to do with flower colour, and flowers are not the main photosynthetic organ of the plant (although they can still do it).

I did? Never noticed though

Black grass.......an EMO lawn....WOW!!!....it cuts itself

David Niven as Bond, in the original Casino Royale;

"A b-black rose. Not dark red, but black. As a raven's wing at midnight.

Gentlemen, I would not exchange one petal of that flower for anything your world has to offer, including an Aston M-M-Martin complete with lethal accessories."

I've seen it once or twice.