Photosynthesis, as we learned about in elementary school, is the process in which plants (as well as algae, phytoplankton and some bacteria) create glucose from sunlight (and water and carbon dioxide). Glucose, the main nourishment plants use as their food source is created when the molecule chlorophyll absorbs sunlight and transforms that energy through a complex chemical process which results in oxygen being the waste product. Chlorophyll absorbs light best at the red and blue wavelengths of the electromagnetic spectrum but not at the green wavelength and thus green light is reflected back which gives plants here on Earth their uniform color.
What is interesting is that if our sun “transmits most of its energy in the green part of the visible spectrum” then why haven’t plants evolved to be more efficient at absorbing this frequency of light? Even the human eye is more sensitive to green “which is why images from night-vision goggles are tinted green”.
For the answer we have to go back in time millions and millions of years.
Shil DasSarma, a microbial geneticist at the University of Maryland believes that chlorophyll actually evolved after another molecule called retinal. Retinal is a simpler molecule than chlorophyll and it can thrive in lower oxygen environments (such as the early earth). Retinal also absorbs green light much more efficiently than chlorophyll and it reflects back red and violet light (purple).
In the early days of the earth there very well may have been an abundance of simple microbes utilizing retinal to get their energy from the sun. Areas where their would have been high concentrations of these microbes would look purple so instead of the green algae seas we see today we would see purple blotches all over the place.
The trouble with retinal is that since it is a simpler molecule than chlorophyll it is not as efficient either. Early life may have been able to utilize retinal because of it’s relative simplicity to create but as these microbes evolved, chlorophyll became dominant.
Shil DasSarma goes on to say that “you can imagine a situation where photosynthesis is going on just beneath a layer of purple membrane-containing organisms”, thus competing with the retinal. Since chlorophyll is more efficient, it soon took over as the dominant technique of deriving energy from the sun.
Retinal didn’t go away though, in fact you would not be able to read this article if it weren’t for that little molecule. Retinal is found in the photorecptor cells of the retina in the eye and is responsible for how light is turned into a nerve impulse which in turn is interpreted by the brain as a visual signal.
Since early microbes had decided to use chlorophyll as their main tool, the less successful microbes evolved and over millions of years used retinal to find their stronger cousins with a primitive eye. They then used the plants as their main food source (and not the sun) and thus the first split in life on this planet occurred – plants AND the evolution of the animals which eat them.
So what about on another planet? What could the plants there look like? By analyzing what wavelengths of light are emitted by other stars scientists can infer what part of the spectrum plants on alien worlds are absorbing and then deduce what parts they are reflecting. If we were able to see these plants up close some of them may be blue, red, yellow or even green like here on earth.
I bet it would be interesting to “expand your mind” laying in a field of yellow grass under the shade of a tree with blue leaves.
Far out, man. Really, really far out.