The Mighty Chlorophyll Molecule
Introduction – Growers can be Scientists.
What is Chlorophyll? It is an essential element in plants that allows light to be converted into usable energy for plant growth. But, some of us are intimidated by all the science behind the scenes. So, in this article, we demystify this fascinating topic to help growers and budding scientists understand the details alongside the big picture.
Fig 1 – Photo by Pixabay
What’s so mighty about Chlorophyll?
When you think about it, everything that we eat, either directly or indirectly, comes from plants. Even the meat that we consume comes from herbivores or plant-eating animals. And it is Chlorophyll that triggers photosynthesis, leading to plant growth that becomes the building blocks of all food. So, Chlorophyll might be the most essential molecule, besides water, for life on this planet.
Fig 2 – All food is ultimately derived from plants – Photo by Lina Kivaka (Pexels)
Where are Chlorophyll Molecules?
The Chlorophyll molecule is part of the Photosynthesis process in plants, which was covered in a previous article (here). There, we mentioned that Photosynthesis occurs in the Plant > Plant Cell > Chloroplast > Thylakoid > Thylakoid membrane.
Fig 3 – Chlorophyll is part of Photosynthesis that happens in the Thylakoid Membrane (click image to expand).
The Thylakoid membrane contains a kind of factory that receives light and turns it into usable plant energy (ADP and NADPH). In this factory are PSII and PSI components; they contain light-absorbing Chlorophyll Molecules.
Fig 4 – The light-to-energy factory in the Thylakoid membrane
What is a Molecule? A little science.
It’s easy to understand what a molecule is by thinking about water. An “Atom” is a fundamental, universal element. For instance, there are oxygen atoms and hydrogen atoms. If you connect one oxygen atom with two hydrogen atoms, you will get an H2O molecule or simply water. Carbon Dioxide (CO2) is one Carbon and two oxygen atoms, all connected. So, a molecule is simply a bunch of atoms connected to make a unique substance.
But H2O is a liquid and CO2 is a gas. Their properties differ, but why? Because the characteristics of their molecular structures are inherently different: the mass of the atoms, charge, and sub-particles within each atom.
All that said, a Chlorophyll Molecule is simply a bunch of atoms connected. Its molecular structure determines its unique properties, which are to absorb light and trigger photosynthesis.
Fig 6 – The Chlorophyll Molecule
Fig 5 – Atoms and Molecules
Chlorophyll-a and Chlorophyll-b
As we mentioned, the molecular structure of Chlorophyll determines its ability to absorb light. But not just any color of light. Plants have evolved into two types of Chlorophyll, Chlorophyll-a and Chlorophyll-b.
Chlorophyll-a and Chlorophyll-b both reside in PSII and PSI of our factory. However, chlorophyll-b is better at absorbing blue light, while Chlorophyll-a absorbs better with red light. They are fundamentally similar except for a tiny piece in their molecular structure (Figure 7). But this small difference alters the light-absorbing capabilities of both.
Figure 7 – Chlorophyll-a and Chlorophyll-b, differentiated by a small formation on the head of the molecule.
Chlorophyll is a pigment molecule
As we mentioned, both Chlorophyll molecules can absorb light. Chlorophylls also reflect light, mostly in the green region of the spectrum, which is why plants appear green.
The ability of a molecule to interact with light makes it a “pigment molecule‘. Most objects that can exhibit color have pigment molecules, like paints and dyes, but there are also natural pigment molecules, for example in the color of our hair.
However, some objects, such as transparent glass or water, do not have pigment molecules.
The absorption chart or sensitivity curves (Figure 8) shows the Chlorophyll pigment molecule in action.
- Chlorophyll a: Main peaks around 430nm (blue) and 662nm (red).
- Chlorophyll b: Main peaks around 453nm (blue) and 642nm (orange)
Also notice that there is a large dip in the green-yellow area and that’s because instead of absorbing green, it is reflecting it or transmitting it through the leaf.
Fig 8 (Cholorophyll sensitivity curves) – image by Daniele Pugliesi CCA Share Alike 3.0
Fig 9 – Chlorophylls, use Blue and Red Light for photosynthesis. Below the canopy, Far-Red Light is more abundant and this is where Phytochromes come into play.
Why Blue Light and Red Light?
The answer to this question is a fascinating consequence of evolution.
Blue Light
Blue light has a shorter wavelength than other colors in the visual spectrum, and a shorter wavelength means that it contains more energy – in other words, blue light has more bang for the buck. Plants have evolved to take advantage of this high-energy boost.
Leaves that face the sun directly will get the full spectrum of colors (figure 10) falling on its surface. However, leaves not facing the sun but still facing the vast blue sky will take advantage of that blue light (figure 11). This is called indirect sunlight; many houseplants do just fine with that.
Red Light
What about red light? From Figure 8, you can see that Chlorophyll-a widens the range of available spectra available for photosynthesis.
Far-Red Light
The strategies mentioned above are evolutionary ways plants use to optimize light energy harvesting. Even under a dense canopy, some photosynthesis can still take place, as some Blue and Red light penetrates. However, the most abundant spectra of light under leaves are Far-Red, above 700nm (See Figure 12). This is where Phytochromes step in, absorbing Far-Red energy to increase Chlorophyll production and encourage stem growth. (See Phytochromes and Photomorphogenesis).
Fig 10 – Full spectrum of colors under direct sunlight – Plants employ both Chlorophyll-a and Chlorophyll-b.
Fig 11 – Spectrum of the blue sky for leaves facing away from the sun (indirect sunlight) – Chlorphyll-b can still employ blue light.
Fig 12 – Spectrum underneath a canopy where photosynthesis is challenged, but above 700nm is far-red where phytochromes come to the rescue.
Chlorophyll and the Cycle of Life
Chlorophyll kickstarts photosynthesis, which helps plants grow and become the food or derived food that sustains all life. From this life comes nitrogen-rich fertilizer (excrement) that begins the process all over again on farmlands. Photosynthesis also recycles the suffocating CO2 and produces the precious Oxygen we breathe.
It’s the quintessential model of a sustainable ecosystem – and it all starts with Chlorophyll.
In this article, we covered the what, where, and how about Chlorophyll for growers who want to be budding scientists. We hope we enlightened you or filled in some spaces about this mighty molecule and convinced you that science is not so bad after all.
Fig 13 – Photo by Joshua J. Cotten on Unsplash
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