I learned this today. Plants are green because they absorb light in the wavelengths that are red and blue, while reflecting light in the wavelength that is green.
Plants create energy through the process of photosynthesis. They convert light energy into chemical energy. Living organisms that convert light into energy are called autotrophs (photoautotrophs to be exact). This is a process that humans and animals are unable to do and so we are dependent on plants. We can only turn chemical energy into chemical energy. These kind of living organisms are called heterotrophs. We rely on the plants to make energy that animals eat. They turn it into chemical energy and then we eat the animals. Without plants, all life would die.
How do plants photosynthesize? They take in water from the soil and carbon dioxide from the air. Inside the plant’s cells, the water is oxidized, which means it loses its electrons. The carbon dioxide is reduced, which means it gains electrons. This changes the carbon dioxide into carbohydrate, a simple sugar, specifically glucose, and releases oxygen. The glucose is then stored in the plant and the oxygen is released into the atmosphere.
Any chemical process requires energy to perform, and this is where the sunlight comes in. Photosynthesis cannot happen without photons of light. One photon of light contains between 10-19 Joules and the plants use this energy to turn the carbon dioxide and water into carbohydrate.
So, why are plants green? They are green because photosynthesis can only happen with chlorophyll and chlorophyll is green. The chlorophyll is contained in the chloroplasts of the plant, and it is the driving force behind photosynthesis. Chlorophyll absorbs light energy and transfers that energy to an electron. This charged electron is passed to the carbon dioxide, reducing it. The chlorophyll then takes an electron from the water, oxidizing it. This creates the glucose that the plant can store.
So, why is chlorophyll green? It is green because it absorbs all of the light in the wavelengths that we see as red and blue and reflects a lot of the wavelength that we see as green.
We see different colors because our brain has learned to associate different wavelengths with different colors. For example, red isn’t red. It is light with a wavelength of 740 to 625 nm. And blue isn’t blue. It is light with a wavelength of between 500 to 430 nm. When the wavelength gets closer to 500, we see a very light blue, and when it gets closer to 430, we see a very dark purply blue. The length of the wavelength is changed by the amount of energy that the wave has. The shorter the wavelength, the more energy. So, purple and blue colors have the most energy and orange and red have the least.
Knowing this, why would plants absorb the blue and the red wavelengths but not absorb the green? It would surely make more sense for the plant to absorb all of the wavelengths because more energy must equal more glucose. Or just to absorb the purple light, which has the highest amount of energy. However, it doesn’t work like this, and this is why plants have evolved to not absorb light in the green wavelength.
By absorbing specific frequencies of light, the plants are protecting themselves and increasing their efficiency. The red light has a wavelength of 700 nm and has the minimum amount of energy needed for photosynthesis. It is a safe amount. Too much energy can overwork and stress the cells in the plant. It can do more damage than good. Red light has the perfect balance of energy and safety. However, the plant cannot always guarantee that it will have a steady supply of red light. Anything can happen. The wind can blow the leaves around. Shade can fall over the plant. It can become cloudy. At times like these, the plant needs to be able to absorb light with more energy, and this is where the blue wavelength comes in. The plants have evolved to absorb light at the two ends of the visible spectrum so that they have a balance and will always have a supply of energy.
If plants absorbed green light as well, they would be black, and they would heat up like a bicycle seat in the summer. Too much energy is not a good thing for a plant.
So, plants are green because they reflect the wavelength that we see as green. They do this because they can get more use from the energy in the blue and the red wavelengths of light. And this is what I learned today.
Sources:
https://www.jic.ac.uk/blog/why-are-plants-green/
https://www.quantamagazine.org/why-are-plants-green-to-reduce-the-noise-in-photosynthesis-20200730/
https://www.ambius.com/learn/plant-doctor/why-are-plants-green/
https://www.nationalgeographic.org/encyclopedia/photosynthesis/
https://www.nationalgeographic.org/encyclopedia/chlorophyll/
https://en.wikipedia.org/wiki/Photosynthesis
https://www.britannica.com/science/photosynthesis/Energy-efficiency-of-photosynthesis
https://www.vernier.com/2018/09/04/what-are-the-best-light-sources-for-photosynthesis/