The third in a series of biophysical questions (one, two).
Finding food is an essential activity for any organism. We and many other animals interpret sights, sounds, and smells to make decisions about where to go or what to do to get things to eat. But what about a bacterium, a single-celled organism just a few tens of thousands of atoms long? Does it just passively take whatever comes its way, or can it process information about its surroundings and navigate to where food is more abundant?
My intent with this series is to illustrate some of the questions that characterize biophysics, not to give long answers, so I’ll just succinctly point out that the answer is “yes” — bacteria can navigate. The best studied, E. coli, decides whether to move forward or turn in a random direction based on how its perceptions of nutrients are changing over time, decisions which lead it, on average, to swim towards places with more abundant food. What’s more, it makes these calculations in ways that are close to optimal given constraints imposed by the microscopic motions of the food it wants and its own body. In other words, physics sets the possibilities for how well a microscopic object of any sort can sense, move, and navigate, and nature has constructed a machinery of proteins and membranes that realizes these possibilities.
I describe this further in Chapter 6 of my pop-science biophysics book, So Simple A Beginning, coming out in less than a month. (My description; publisher’s page; Amazon page.) Not all of these “What is Biophysics?” posts will connect to my book, by the way — there’s lots of other stuff out there!
For a more technical look at bacterial navigation, I highly recommend the deservedly famous Random Walks in Biology by Howard Berg, a giant in this field who passed away just a few days ago. It’s a wonderful, short book, and should be accessible to any sophomore-level physics undergraduate or any biology undergraduate who isn’t afraid of basic math. At a higher level, the 1977 article Physics of Chemoreception by Berg and Edward Purcell is a classic, and William Bialek’s advanced textbook Biophysics: Searching for Principles explores in greater depth how bacteria and other living things process information.
— Raghuveer Parthasarathy; January 9, 2022