I mean, I know plants are green because chlorophyll is green. But why is that? Is that particular hue any better or more efficient for plants, light absobtion or energy transfer? Was it just random that these molecules happen to be green? Or is green really better than other colors for this job?
In: Biology
So we know that Photosynthesis is how plants “turn sunlight into usable energy/food” right? Well photosynthesis can be broken down into multiple different steps in a very complex chain reaction of chemical reactions.
The very first step is: how do you convert sunlight into usable energy? And the answer for that is in the “electron transport chain”.
Basically, sunlight hits a special molecule in the chloroplast and “excites” an electron in the molecule. The electron now jumps up to a Higher energy state. Where in then can be “released” through another chemical reaction to harvest that energy.
But electrons are finicky things, to “jump to a higher energy state” they need to absorb certain specific amounts of energy, specific wavelengths of light. And it just so happens that these wavelengths for this molecule are reds and blues.
Meaning when white sunlight hits the plant, a lot of the red and blue wavelengths of light are absorbed by the plants, leaving most of the green light to be reflected away.
Chlorophyll absorbs energy from the light to carry out photosynthesis. It just so happens that it absorbs all the colour wavelengths except green, which gets reflected back. You do get red leaves as well
So the colour of something is what light doesn’t get absorbed by the plant and is reflected back to our eyes. So green plants absorb more of the reds and blues and less of the green so they appear green. But now this seems illogical because green is in the middle of the spectrum of visible light (ROYGBV) so shouldn’t it try to absorb more green? There’s a simple explanation, plants evolved from planktons way back when that filled the ocean. The plankton that was closer to the surface absorbed most of the green light and so the plankton deeper down adapted to focus more on absorbing red and blue light as green was more scarce. That deeper plankton was what eventually evolved and took over to be the main plant matter on land. There is some purple plants.
So the green colour is actually not the most efficient for light absorbtion, quite the opposite. But the green plankton in the ocean had more pressure to evolve and come to land to try to get more light than the purple plankton who was content staying in the ocean.
Because they absorb light that is red and blue leaving, leaving just green light to reflect back.
If you graph the frequencies of light that gets absorbed by plant in photosynthesis you will notice a few things:
First they absorb light mostly in the same part of the spectrum that we human use to see. That part makes sense because that is the part of the spectrum where the light emitted from the sun is the strongest so everything that makes use of it gets pushed by evolution to use that part of the spectrum.
A weird thing however is that plants don’t just absorb all the visible light. They absorb in two separate humps.
There are different types of chlorophyll which absorb slightly different wavelengths but hey all follow the same broad pattern:
Broadly one hump is in the 400nm to 500nm range and another hump in the 600nm to 700nm range leaving the part in the middle alone.
400-500 nanometer is blue from violet to blue with a slight green tine and 600-700 nanometer is orange to red.
Chlorophyll a has a tiny hump in the yellow part of the spectrum and Chlorophyll b absorbs a small amount of light throughout the entire visible spectrum and extends into the ultra violet part beyond.
Chlorophyll doesn’t use green light and thus reflects it back and appears green.
This is a bit weird because that means plants basically leave money on the table as far a solar energy is concerned.
It doesn’t seem to be entirely clear why that is.
One theory I have heard is that the ancestor to today’s chlorophyll evolved back when it was its own organism in an environment where green light was already absorbed by something else that no longer is around and evolution never filled the gap.
It may seem like a perfect plant would absorb all the light available and therefore be black but there is no point for a plant to absorb more light than it has the chemicals, primarily water and CO2, to make food with and for most plants if they **were** black strong sunlight would cook them. Chlorophyll a, the primary pigment which directly photosynthesises does absorb green light but it absorbs red and blue light far more readily so leaves look green because they reflect more green light or allow it to pass through. Plants in lower light conditions tend to use other pigments to capture green light then pass the energy to the chlorophyll to turn into plant food. So red algae can live at lower depths because it has accessory photosynthetic pigments which absorb blue-green light which penetrates the water better and purple understory plants can survive without much direct sunlight because their accessory pigments absorb the green light reflected or passed through by the green leaves above them. Seaweeds can be brown or black because they absorb more light but are water cooled which prevents the heat denaturing their tissues.
As with everything in evolution chlorophyll a came about by chance. You could argue that plants on Earth **should** be purple, primarily absorbing the yellow-green light most abundant in the sun’s light and sheding the dregs at either end of the spectrum. Scientists have even theorised that the first photosynthesisers could have been purple, and green plants evolved to take advantage of the scraps of light they left behind, then out competed them.
the graph [here ](https://www.simply.science/images/content/biology/cell_biology/photosynthesis/conceptmap/Photosynthetic_pigments.html) shows the absorption spectra for some common photosynthesisers and accessory pigments, compare to the [spectrum of the sun’s electromagnetic irradiation](https://www.sciencedirect.com/topics/physics-and-astronomy/solar-spectra)
Basically yeah it’s just random, what we need to understand about evolution is that is has no “intention” it happens and if it’s beneficial it tends to stay and otherwise it tends to dissapear so while it’s not inherently the best or the most efficient it could be it’s “good enough”. Think of it this way, porcupines didn’t develop spikes to fend of predators, they are able to fend off predator BECAUSE they “randomly” developed spikes.
Are they green? We only perceive them as being green.
Chlorophyll? More like borophyll!
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