Why Are Plants Green?
This is the type of question that tends to puzzle many -- adults and 6 year olds alike. And surprisingly, its the kind of simple question that doesn't beckon a simple answer.
In this article I will attempt to explain the physics behind the greenness of leaves -- while also dabbling in a little bit of their evolutionary biology.
First, in order to understand why plants are green, we need to understand why they aren't black.
White light, such as the light from the sun, can be described as a spectrum -- with higher energy and lower energy wavelengths all existing together.
Upon initial glance, the lack of black leaves in nature seems to make no sense -- black things absorb all colours of light, and, therefore, black leaves would conduct the most photosynthesis. Right?
Well, yes; sort of. Black leaves would indeed absorb the largest amount of sunlight. However, they would likely absorb so much sunlight that their little leaf proteins would fry right up -- leaving the leaves burnt, bleached and shrivelled.
Alright, so that's why leaves aren't black. But why do they have to be green?
The white light emitted from the sun does not emit all colours of light equally. It tends to emit far higher amounts of red, yellow and orange light than purple or blue light -- and green, as far as visible light goes, is pretty up there in terms of emittance. So then, why do plants have to be green, if that means wasting so much sunlight? Why aren't they purple, a colour that is not nearly as abundant in sunlight?
This proves to be a far more interesting question.
In order to understand why leaves aren't purple, we need to have a stronger understanding of the interactions that occur between photons and atoms.
The concept of the photon is one that has already been covered in previous several articles on this very blog, but just to recap: photons are essentially little packets of light energy. The amount of energy that a photon has when it hits a molecule results in different interactions. A photon with little energy -- such as those from the infrared part of a light spectrum -- would simply cause the molecule to vibrate slightly. A photon with a lot of energy -- such as those from the UV part of the spectrum -- could cause electrons to be ripped from their host atoms entirely.
Neither of these reactions are particularly desirable for a leaf. So instead, leaves seek to absorb light in the visible spectrum only. But what is the interaction between visible light photons and atoms, you may ask? When an atom absorbs visible light, this typically causes an electron in an orbital to get excited and jump up to a higher energy state, farther away from the nucleus. The electron can't remain excited, however, and will soon jump back down to a lower energy state. When it does so, it will emit a photon of a certain amount of energy. This photon can then be absorbed by a neighbouring atom, and the whole process will repeat again.
The chain of photon emission and reabsorption will continue on and on till the photon reaches a reactant molecule (such as water or carbon dioxide).
Leaves, therefore, would like to absorb mostly visible light. Great! How exactly do they do that?
As it turns out, it is far easier to absorb light within a smaller range of wavelengths as opposed to attempting to capture the entire spectrum of visible light. What does this mean? Well, this means that in most leaves, there is one pigment that will absorb red-yellow light, and another, separate, pigment that will absorb blue-purple light. These two pigments work in tandem to absorb a pretty decent amount of sunlight -- leaving the only colour that isn't absorbed - green.
Alright fine, but even this answer doesn't seem satisfying enough. Why is red and blue light absorbed specifically? Couldn't plants choose to absorb another two arbitrary colours of light?
As of right now, the best answers to those questions are speculative.
For example, here is a hypothesis that many of the early adaptations of photosynthesising plants were first found in the bottom of the ocean. Much of the green light would have been absorbed by surface-level algae, leaving only blue and red light to those ocean floor plants. So, those plants had to learn how to best absorb and utilise red and blue light -- as green light wasn't available to them.
There is, also, a second answer -- although it is, perhaps, a bit infuriating: evolution doesn't breed what's best, it breeds what works.
Maybe green plants aren't the most efficient, but they work. Maybe dark purple plants would be better, but it was the green-leaf mutation that developed first, and it was the green-leaf mutation that was good enough to spread far and wide, across the whole planet.
So there is the full answer!
http://scienceline.ucsb.edu/getkey.php?key=1110
https://www.quora.com/Why-did-chlorophyll-evolve-to-be-green-as-opposed-to-black-which-would-absorb-more-energy
http://scienceline.ucsb.edu/getkey.php?key=4979
https://www.maxpixel.net/Ivy-Green-Leaf-Plant-Textures-Green-Leaves-Leaf-1665200