You are right that the sun gives off the most amount of its energy as visible light in the green region of the spectrum nm. All plants on Earth, even the single-celled plants that grow in the ocean, contain chlorophyll-a as their main light-absorbing pigment. Plants have other pigments for absorbing light as well, including chlorophyll-b, chlorophyll-c and pigments known as carotenoids , but chlorophyll-a remains the main light-absorbing pigment.
Chlorophyll-a absorbs light throughout the visible spectrum, but mostly in the blue and red regions and very little in the green region. In fact, of all the many pigments that plants use to absorb light, none of them absorb much green light. To us it might seem inefficient that plants don't take advantage of the one part of the spectrum that the sun emits most of its energy in. In the case of plants, it is the pigment chlorophyll which absorbs the light, and it is picky about which wavelengths it absorbs — mostly opting for red light, and some blue light.
When electrons are excited, they are promoted from a level of low energy to a level of higher energy. The energy in the light makes the electrons excited and removes energy from the light — this is an example of the first law of thermodynamics — energy is neither created nor destroyed it can only be transferred or changed from one form to another. That process takes place in specific compartments within cells called chloroplasts and is split into two stages;.
During these reactions, CO 2 dissolves in the stroma and is used in the light-independent reactions. This gas is used in a series of reactions which results in the production of sugars. Sugar molecules are then used by the plant as food in a similar way to humans, with excess sugars stored as starch, ready to be used later, much like fat storage in mammals. Therefore, the red end of the light spectrum excites the electrons in the leaves of the plants, and the light reflected or unused is made up of more of wavelengths of the complementary or opposite colour, green.
The unused green light is reflected from the leaf and we see that light. The chemical reactions of photosynthesis turn carbon dioxide from the air into sugars to feed the plant, and as a by-product the plant produces oxygen. The technique first used by NASA to grow crops in space uses extended day-length, enhanced LED lighting and controlled temperatures to promote rapid growth of crops.
It speeds up the breeding cycle of plants: for example, six generations of wheat can be grown per year, compared to two generations using traditional breeding methods. By shortening breeding cycles, the method allows scientists and plant breeders to fast-track genetic improvements such as yield gain, disease resistance and climate resilience in a range of crops such as wheat, barley, oilseed rape and pea.
Why green, and not blue or magenta or gray? If they absorbed more, they would look black to our eyes. Plants are green because the small amount of light they reflect is that color. But that seems unsatisfyingly wasteful because most of the energy that the sun radiates is in the green part of the spectrum. Recently, however, in the pages of Science , scientists finally provided a more complete answer. They built a model to explain why the photosynthetic machinery of plants wastes green light.
What they did not expect was that their model would also explain the colors of other photosynthetic forms of life too. Their findings point to an evolutionary principle governing light-harvesting organisms that might apply throughout the universe. They also offer a lesson that — at least sometimes — evolution cares less about making biological systems efficient than about keeping them stable. The mystery of the color of plants is one that Nathaniel Gabor , a physicist at the University of California, Riverside, stumbled into years ago while completing his doctorate.
Extrapolating from his work on light absorption by carbon nanotubes, he started thinking of what the ideal solar collector would look like, one that absorbed the peak energy from the solar spectrum.
In , Gabor and his colleagues modeled the best conditions for a photoelectric cell that regulates energy flow. But to learn why plants reflect green light, Gabor and a team that included Richard Cogdell , a botanist at the University of Glasgow, looked more closely at what happens during photosynthesis as a problem in network theory.
The first step of photosynthesis happens in a light-harvesting complex, a mesh of proteins in which pigments are embedded, forming an antenna. The efficiency of this quantum mechanical first stage of photosynthesis is nearly perfect — almost all the absorbed light is converted into electrons the system can use.
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