Saturday, September 8, 2012

Why are plants green?

Yes, I know, because chlorophyll is green. But why is that? This is a genuine question, so if any readers of this blog can answer it for me, I'd be grateful.

Let me explain a bit more. Yesterday I stumbled across an old post by Peter Coles asking why the Sun isn't green. If you haven't wondered this before, you should have a read. The reason for asking is that the intensity of light emitted by the Sun — which is very similar to a blackbody spectrum at a temperature of close to 6,000 Kelvin — peaks at the wavelengths associated with green.
Some example blackbody spectra. The Sun's surface temperature is about 5,800 K.
Naively therefore one might expect the Sun to appear green, rather than the colour it does, which is ... white (depending a little on where it is in the sky).

I won't explain here why the Sun doesn't appear green, or indeed why we don't see any green stars at all, because that's been done very well elsewhere on the internet. In particular you might want to check out this video explanation. As perhaps you might have suspected, the answer is less to do with physics than with biology of the human eye.

In fact, it's quite easy to come up with a simple ex post facto rationalisation of why we should have evolved so as to perceive sunlight as neutral white colour. 'White', after all, is a somewhat vague and flexible concept, as anyone who has operated a digital camera would know.

But in light of this, what I don't have such a simple explanation for is the fact that plants are green. Chlorophyll appears green because it uses light from the red and blue parts of the spectrum to power photosynthesis, and reflects green wavelengths. But why has it evolved in such a way as to reject precisely that part of the solar spectrum where the intensity of light is highest? Would it not have conferred an evolutionary advantage to make better use of this incident energy?

As I said, I don't know the answer to this little puzzle. Perhaps the explanation comes from chemistry — maybe there is simply some restriction on the possible chemical pathways by which CO2 and H2O can be converted into organic molecules, which all evolutionary variants would have to respect, and which renders light of certain frequencies unsuitable. Or perhaps it comes from some quirk of evolutionary history — one of those little inefficiencies that occur due to historical accident, and which are themselves evidence for the action of evolution, like the panda's thumb.

Either way, if you know the answer or can hazard a guess at it, please do tell me.


  1. On a recent (well it was on more than 3 months ago!) TV program about plants (I've forgotten the name but it was a BBC program) they claimed that the first photosynthesising organisms arrived later on the scene and there were already organisms using the green spectrum (they were purple coloured). Then used what was available: red and blue, and did so efficiently. I've found a reference to this theory in Wikipedia:

    With some creative interpretation they then killed off all their competitors using deadly oxygen and took over the planet: I think the BBC program did use this creative interpretation.

    The first two references on this Wikipedia page: look like they head to published work but I don't have access to the journals.

    I should finally add that I have no knowledge in this field, I came across your post from a tweet by Shaun from The Trenches of Discovery and then I got curious and tried to figure out what the BBC were basing their claims on. Sadly I don't have access to any (or I'm too lazy to find more recent open access articles) journals and gooling just turns up Wikipedia results which reference subscription based papers.

  2. Hi Gareth, and welcome to this blog! Thanks for the pointers. I don't have access to the journals at the moment either, but the Wikipedia entry on chlorophyll does have a link to an article on LiveScience with quotes from some of the scientists.

    I have to say I'm very sceptical of some aspects of this Purple Earth competition hypothesis. Specifically, they seem to be arguing that the purple organisms based on retinal were already "using up" all the green light, so chlorophyll based organisms evolved to make use of what was left. But this seems to treat the green component of sunlight as a resource similar to say the nitrogen content of soil - there's still sunlight around after the retinal-based organisms have used it, but with all the green removed. I can't see how that is true. Surely any organism with access to sunlight has access to the green wavelengths.

    On the other hand, the argument that perhaps absorbing green light as well leads to problems with heat dissipation sounds at least plausible.

  3. Another thing that a friend pointed out in a different forum is that of course the spectrum of light which arrives at sea level on the Earth is different to that actually emitted by the Sun. It is less strongly peaked at green wavelengths and there is a bit of a drop-off at the very highest (blue) frequencies.

  4. I assumed they meant something more like a change in spectrum caused by the retinal organisms so the peak energy density was at a different wavelength.

    I believe this was also happening in the sea and chlorophyll based organisms were deeper than retinal, as retinal already occupied the region close to the surface. Again changing the spectrum.

    I also imagined it was a simplification and probably just one of many factors ;)

    Thanks for the welcome, now I'm going to have to read your other posts :)

    1. Hmm, yes, I hadn't considered that this was all probably happening in the sea. But were the retinal organisms supposed to be translucent then? I suppose I should just wait until I can access the actual journal articles themselves.

  5. Hi Sesh,

    Not only can excess light be damaging, but the energy from light may not be the primary limiting factor in photosynthesis. Have a look at

    I think the limiting factors in photosynthesis are generally carbon & water, particularly since there's a tradeoff in the availability of the two: in order to take up more carbon, plants have to open their stomata which means they lose water more quickly.

    Having said that, I'm not sure if there's a consensus about a simple, straightforward answer to your question. These are just my musings on a pleasant Friday morning...