TL;DR : Over hundreds of millions of years the cholorophyll molecule that you find in plants has been shaped by time and evolution in order to harvest the “best” possible light. It absorbs light in the blue end of the spectrum, thus harvesting the most energy possible. Yet plants aren’t necessarily green are they? You may also find red or orange plants.
Why green and not another color?
We’ve already talked a bit about the resources needed by plants to grow and thrive when we talked about why plants needed water and about what makes plants green. One of these reasons is, of course, for photosynthesis. Plants get the energy they require to grow through a process called photosynthesis, in which a few basic elements are needed. Why are we talking about this again? Well, it’s because chlorophyll has an crucial part to play in this process. Large numbers of chlorophyll molecules act as the antenna that harvest light energy and convert it into chemical energy, a form that is useful for the plant’s growth. Here’s where the absorbent properties of the chlorophyll molecule come into play, the molecules absorb the sunlight (or certain parts of the sunlight, for most plants the red and blue part) and transform it into usable energy.
What is light?
Light is complicated, and the physics of it are quite hard to grasp.
Light is made up of electromagnetic waves (like microwaves, radio-waves, and X-rays). You know how when you pass electricity through a coil of wire it generates a magnetic field (which is how electric motors work)? And you know how when you move a magnet through a coil of wire it generates electric current in the wire? That is basically what light is: an oscillation of a magnetic field, that generates an electric current, which in turn generates a magnetic field, and so on and so on.
Only it’s moving at 300’000 km per second (or 186’000 miles per second). Each wave (or photon) has a frequency, which is a measure of how many times it oscillates (goes through its cycle) per second. Visible light has frequencies of 430–750 teraHertz (million-million oscillations per second!).
Put it another way, the wavelength (i.e. the distance covered by the wave during one oscillation) covers 400–700 nanometers (billionth of a meter). The faster it oscillates, the shorter the distance it travels. This image bellow might help you understand. It shows the relationship between wavelength and frequency on a pretty neat scale.
The really cool thing (I think) about light is that it carries energy. The higher the frequency, the higher the energy of the light. So electromagnetic waves with very high frequency (and so a very short wavelength), carry loads of energy (which is why gamma-rays, whose wavelengths are a million times shorter than visible light are dangerous!).
On the other end of the electromagnetic wave spectrum are radio-waves, which can have wavelengths in hundreds of kilometers! So visible light is just a short part of the electromagnetic spectrum, that goes from 400 nm (violet, blue light) to around 700 nm (red). Colour of the highest energy is therefore the bluish-violet end of the visible light spectrum. Which is somewhat confusing, since we say those are “cold” colours, as opposed to “warm” oranges and reds.
What happens when light hits a molecule?
When light hits a molecule, the energy from the electromagnetic wave is transmitted, usually by some form of excitation. Some compounds change their 3D structure (some part of the molecule rotates for example), which is what happens in receptors in our eyes! For other compounds (notably plant pigments!), the energy is used to split water molecules to obtain electrons – in a complex process called photophosphorylation. This reaction (the splitting of water) usually requires HUGE amounts of energy (strong electric current, or temperatures above 2000°C), but plants do it routinely in their leaves all around us – just one more reason that plants are next-level awesome! These electrons are then transferred through a complex cycle to be stored by the plants’ cells as stable chemical energy to be used later (to turn atmospheric carbon into sugar for example)!
Anyway, that’s another story. Plants (and other photosynthetic organisms) have evolved a whole range of pigments, each specialized in a particular wavelength. So some photons get absorbed by (green) chlorophyll “a” (there are several different chlorophylls), others by (redish) carotenoïds, etc… By covering a wide range of wavelengths, plants are able to harvest the sunlight with little waste.
Light that is NOT harvested by the plants’ little light centers is mostly reflected, and those photons are then absorbed by your eyes, blinding you forever! HAHAHAaaaaa! Day of the Triffids all over again. Or not.
Chlorophyll, a molecule shaped by hundreds of millions of years of evolution to harvest the “best” possible light, absorbs light in the blue end of the spectrum, thus harvesting the most energy.
Here is a graph showing the spectrum absorbed by different pigments found in most plants:
Notice that there is an absorption peak on the right (620-690 nm), which means that chlorophyll also absorbs red light – with relatively low energy, which scientists don’t fully understand as of yet. It could also just be a side-effect with little to no evolutionary cost.
And why don’t they absorb greenish light? Well that could be because of competition with microbes called Halobacteria in the early days of life on Earth that already used up all the green spectrum (incidently, this gave them a purple tinge – imagine if the whole world was purple instead of green!). It could also be an adaptation to reduce the amount of energy absorbed from the sun, since leaves can easily get saturated with light on a sunny day, which can end up causing damage by destroying the photoreceptors.
Plants have developed auxiliary pigments (carotenoids) to fill in even more of the light spectrum. Carotenoids are richly colored molecules that are the source of the yellow, orange or red colors of many plants or algae for example. They are also the reason we get to decorate our aquarium with red plants, yippee!
Are all plants green?
It is believed that plants would have a hard time achieving a pigment as effective as chlorophyll for their light absorbing needs. This is why plants have remained green over thousands of years. In fact, all higher plants (embryophytes: basically all plants except algae) are thought to have evolved from a common ancestor that is a sort of green algae. But not all plants adopt the lush green hues you are used to seeing as their color.
You may also have heard about carotenoids when reading up about health or tanning (https://en.wikipedia.org/wiki/Sunless_tanning). Fruit and vegetables provide most of the carotenoids in the human diet. An example of carotenoids?
Xanthophylls are the typical yellow pigments of leaves. Xanthophylls are present in all young leaves as well as in etiolated leaves (leaves that grow without access to sunlight, for example if you lock up a plant in a cupboard). Because of the combination of chlorophyll and carotenoids, plants can absorb up to 85% of the solar spectrum.
As leaves change colour in the autumn, it’s because the pigment composition changes in the leaves, and the chlorophyll is the first pigment to be degraded, which lets the orange and reds of the other pigments to shine through! When fruits ripen (like apples that turn red when ripe), rising sugar concentration make the fruit start building up anthocyanin, a red pigment. The same thing happens as avocados ripen, with the darkest, sometimes nearly black or purple fruits having the highest concentration of anthocyanins.
Some algae have phycobilins (or phycobilliproteins – e.g. phycocyanin and phycoerythrin), the light-capturing color pigments found in cyanobacteria and in the chloroplasts of red algae that are not found in green algae or in plants. These phycobilins can absorb green light to use in photosynthesis. This allows this type of algae to grow in ecological niches where the red and blue light has been used up by the more classic photosynthetic organisms.
For example, the red algae belong to Phylum Rhodophyta, a large group of aquatic algae that is made up of about 6000 species. These pigments mask the color of the chlorophylls that these organisms also have!
A small group of microorganisms called Dinoflagellates have their very own pigments, of the xanthophyll family, typically dinoxanthin and diadinoxanthin. These pigments give many dinoflagellates their typical goldenbrown color. These small phytoplankton are responsible for the so-called “red-tides”.
Finally, some animals can absorb light. Our favourite is the green sea-slug Elysia chlorotica, that eats so much green algae (called Vaucheria litorea) that it absorbs their chloroplasts, giving it a lovely green color. It’s also stolen some genes from the algae, thus having the genetic machinery necessary to maintain the chloroplasts functional, enabling it to live through long periods without food, as long as the sun is shining!
Finally, the story wouldn’t be complete without a shout-out to arguably the most important group of living things ever. Cyanobacteria have been around for over 2.5 billion (yup, Billion) years, and are responsible for pumping all the oxygen into the Earth’s atmosphere. These very simple organisms are still extremely diverse today, and are found more or less all over the world (and that includes frozen rocks in Antarctica). It is thought that the chloroplasts in all photosynthetic plants are descended from Cyanobacteria that were incorporated (enslaved) by some other Bacteria, a long long time ago.
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