2023-05-16 by Shakunthala Natarajan

Why Are Plants Green? Science Blog

Listen to the episode, here.

It is a nice sunny day and you take a walk through your garden. The plants in your garden sway and dance to the tunes of the wind, catching your attention. While you look at them, their green colour is something that you first observe, right? Now you must be curious and thinking ‘why are most plants green?’ and so are Utsuka and Jigyasa! Let us travel with them into the world of plants to understand why plants are painted in green colour and get to learn a range of other interesting aspects about them.

Light that can be seen by human eyes is made of the seven rainbow colours - ‘VIBGYOR’. The colour of an object depends on the light it reflects, from among these colours in the visible spectrum. Green plants have a specialized pigment molecule called chlorophyll, that reflects green light and makes the plants appear green. Surprisingly, a huge portion of energy radiated by sunlight lies in the green spectrum of visible light. Chlorophylls reject green light and instead absorb a major portion of red light and some portion of blue light. The energy obtained by absorbing these parts of the sunlight powers the photosynthesis reactions in the plant kitchens, similar to fuel supply for stoves in the human kitchens!

Green light however, has a higher energy associated with it, while red and blue lights have lower amounts of energy. So why should plants waste the powerful green photons, and go for the less energized red and blue ones? To understand this, let us imagine the chlorophyll molecule to be a device that needs electricity to function, but lacks an in-built fuse system to protect it from short-circuits and high current load. The photons from the Sun are the source of electricity for the chlorophylls in our example. The photon supply process from the Sun to the plants is sometimes disturbed by obstacles like clouds that prevent sunlight from reaching the plants. Plants frequently experience such moments of shade where their photon supply is cut. Since plants cannot move, they cannot follow sunlight throughout. In such a situation, if they opt to take in green photons, it would resemble huge amounts of current supply reaching the chlorophylls after short periods of pause in the flow. This would lead to frequent short circuits in the chlorophyll devices as they lack fuses. These frequent disturbances can eventually destroy the photosynthetic machinery and threaten the survival of plants! Hence plants have beautiful risk-sensing molecules called ‘chromophores’ within chlorophylls, which help them avoid the green photons, make food and survive on the long term.

Now, Utsuka and Jigyasa are greatly fascinated by this intelligent selection mechanism shown by plants and are curious to know more on this from Dr. Gitanjali Yadav, a plant scientist. Let us continue travelling with them as they explore further. Although we have looked at why plants are green coloured, we are yet to see why some plants are not green in colour. Dr. Yadav answers this by explaining that, ‘greenness’ is a character generally associated with photosynthesis in plants. There are however some plants that are not green in colour, but can still photosynthesize. For example, ornamental plants with purple leaves have low amounts of chlorophyll in their leaves which carry out photosynthesis. The greenness of chlorophylls is masked by the presence of other coloured pigments that are present in higher amounts. But some non-green plants like the ‘ghost plant’ do not photosynthesize and get their food through parasitism. Thus, some non-green plants do photosynthesize, while others do not.

Having gotten so far, Utsuka and Jigyasa are wondering, how plants got to develop the photosynthesis function, that makes them magically prepare food out of sunlight and thin air! They are surprised to learn from Dr. Yadav that plants did not first evolve the oxygenic photosynthesis mechanism. The first organisms to perform such a kind of photosynthesis that resulted in the production of oxygen, were small blue-green algae (cyanobacteria), living in the oceans. These microbes split up water molecules to power the photosynthesis reactions and produced oxygen as a by-product. This breakthrough by the tiny bacteria was a turning point in the living world that eventually led to green plants evolving to possess a similar trait. Thus, photosynthesis in the plant world is a result of millions of years of fine tuning and adjustments by the tiny microbes and mighty plants in the path of evolution.

We have looked at the various wonders of the plant world with Utsuka and Jigyasa. But I bet, you might be interested in joining them live in their adventure and getting to hear their first-hand narratives. It is now time to gear up your headphones and listen to their voices in the podcast!

References:

1. Difference between oxygenic and anoxygenic photosynthesis (2022) BYJUS. BYJU'S. Available at: https://byjus.com/neet/difference-between-oxygenic-and-anoxygenic-photosynthesis/ (Accessed: April 13, 2023).
2. Monotropa uniflora (2023) Wikipedia. Wikimedia Foundation. Available at: https://en.wikipedia.org/wiki/Monotropa_uniflora (Accessed: April 14, 2023).
3. Ortega, R. P. Why are plants green? to reduce the noise in photosynthesis. Quanta Magazine (2020). Available at: https://www.quantamagazine.org/why-are-plants-green-to-reduce-the-noise-in-photosynthesis-20200730/. (Accessed: 25th March 2023).
4. Sánchez‐Baracaldo, P. and Cardona, T. (2019) “On the origin of oxygenic photosynthesis and cyanobacteria,” New Phytologist, 225(4), pp. 1440–1446. Available at: https://doi.org/10.1111/nph.16249
5. Sezen, U. (2019) How does the purple bacteria photosynthesize? Beckman Institute-UIUC (2015), Nature Documentaries. Nature Documentaries.org. Available at: http://naturedocumentaries.org/10031/purple-bacteria-photosynthesis-chromatophore-molecular-structure/ (Accessed: April 13, 2023).
6. Society, N.G. (2022) Photosynthesis, Education. Available at: https://education.nationalgeographic.org/resource/photosynthesis/ (Accessed: April 12, 2023).
7. Sonuhoney (2016) Fascinating facts about photosynthesis of non green plants, Blog.Nurserylive.com. Blog.Nurserylive.com. Available at: https://blog.nurserylive.com/2016/07/27/fascinating-facts-about-photosynthesis-of-non-green-plants-and-gardening-in-india (Accessed: April 14, 2023).
8. Why are plants green? John Innes Centre (2021). Available at: https://www.jic.ac.uk/blog/why-are-plants-green/. (Accessed: 25th March 2023).
9. IndiaAsksWhy's blog on Dr Gitanjali Yadav: https://www.indiaaskswhy.org/blog/2023-05-17-indiaaskswhy-meets-gitanjali-yadav/

Credits

• Cover Image (J Swanstrom, Flickr)