The road not taken: Part I

On 1 January, 2020

by Gayathri Sambamoorthy (and Karthik Raman)

Yamini, a class V student, is a very playful child who keenly observes everything around her and comes up with multiple questions that intrigued her. One morning she was angry with her mother that she did not permit her to play in the sand. Seeing the energetic kid unhappy, the teacher took the time to ask her about it.

After Yamini narrated, the teacher decided to explain, “Your mother is right! You may fall sick because of the microbes or microorganisms present.” Yamini, confused and curious, said, “I don’t see anything in the sand. How will I fall sick?” The teacher explained, “Microorganisms are small creatures that exist everywhere, and we cannot see them. We can see them only through a microscope!”. She explains and adds further, “There are both harmful and harmless microorganisms.” “What do they eat? They are so tiny!” exclaimed Yamini!

To quench her curiosity, the teacher decided to take her to Class IX, where I was on a visit to explain my research. And I started my research story from there:

“Microbes live everywhere. Microbes feed on the nutrients available to them, like how humans eat to obtain energy for survival. Now, how are these nutrients converted into energy? I am sure all of you have heard about metabolism, which is the breakdown of complex nutrients into simpler ones for the generation of energy. Similarly, every microorganism in the universe exhibits metabolism for converting the available nutrients into energy. Do we all eat the same food at our houses? No, right? Then how do we all get the energy from different food sources? Although there are differences in the food we take in, all of them are converted to energy. In a similar fashion, there are several nutrients that the microorganisms can take in, depending on the environment that they are present in.”

I asked the students, “How do you go from school to house?” They all replied differently: one answered “cycle”, another, “walk”, and yet another, “bus”, etc. Yamini remained silent to understand my question and quickly replied, “I go by auto.” I smiled, “Good. Just like there is a route to reach your house from the school, there is a path from the nutrients to reach the energy molecules. This path is called a metabolic pathway”. Then I asked them, “If you start from different places such as market, railway station, school, etc., to reach your house, will you go through the same routes?” The students nodded to my rhetoric.

“Similarly, metabolic pathways differ when different nutrients are converted to energy. Similar to different stops from school to your house, there are analogous stops in these pathways, called reactions. And the set of all these reactions in an organism is called a metabolic network, basically, a network of reactions that are involved in metabolism. These stops are different for different routes, and so are reactions in different pathways. So, to put it together now, microbes ingest various nutrients depending upon the environment they live in and through different metabolic pathways based on the nutrient, they gain energy. These pathways are composed of many reactions that are different for different pathways. Now the tricky question. Is there only one route to your house from school”. Pat came the reply, “No”, in chorus.

I continued, “There are different routes by which energy can be obtained from the same nutrient, similar to the different routes you have to reach your house from school. These are called alternating pathways since you go through one of the paths at a time. A reaction pair, one from each alternating pathway, is termed synthetic lethal, analogous to two bus stops from different routes. Say, for example, what happens if one of the routes to your home is blocked? You have another route, right? So, if one reaction of the synthetic lethal pair is absent, energy can still be produced for growth via the other reaction. Further, when an organism is present in an environment for a long time, they tend to evolve. Just like any other living being!”

(to be continued)

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