The Oxford Science Lecture Series

Professor Julia Yeomans FRS

University of Oxford

"Nature's Engines:  Powering Life"

 Dorothy Hodgkin Memorial Lecture 2023

Somerville College,  Oxford,  9th March 2023

Professor Julia Yeomans is the Head of Department of Theoretical Physics at the University of Oxford and a Fellow of the Royal Society. She did her MA   and   DPhil   in   Physics   at   Oxford   and   was   a   member   of   Somerville College. She spent two years as a post-doc at Cornell University before returning to the UK initially as a lecturer at the University of Southampton and then back in Oxford. In her research, she applies techniques from theoretical   and   computational   physics   to   problems   in   soft   condensed matter and biophysics.

Professor Yeomans began her lecture by describing how Dorothy Hodgkin saw   patterns   in   nature   through   her   study   of   crystal   structure.   She explained that she also looks at patterns but in ‘active matter’, which is a system that takes energy from its surroundings in order to do work. This covers systems as small as proteins in cells through to animals. These self-propelled   systems   have   ‘nematic   symmetry’   but   this   symmetry   is unstable in the presence of flow. Professor Yeomans applies mathematical principles to all these systems to analyse the patterns of their motion. 

Proteins   move   around   within   cells   using   ‘feet’   with   which   they   follow tracks formed by micro-tubules in the cells. Professor Yeomans illustrated this   with   a   beautiful   animation   from   Erik   Schaffer’s   lab  (https://www.youtube.com/watch?v=plvQCOE9s_k).   Bacteria,   which   are slightly   larger   at   about  10  µm  in  size,  have  a   motor   across   their   cell membrane which powers the movement of the bacterial tail. The motor is built from a stack of different proteins which can be contracted to give the movement required. Professor Yeomans explained that if you look at a large   number   of   bacteria   swimming   together   you   see   turbulence   in patterns that look very similar to those seen with the proteins moving on the microtubules. You also see these  patterns   with   epithelial   cells in a dish, with fish swimming in shoals and with  murmurations  of starlings. She suggested that if we could learn from the way these patterns evolve we could use that information to design better engines.

Using a video to show the way dye spreads when injected into a pot of glycerol  she   demonstrated   a   fundamental  difference   depending   on   the speed of flow of the glycerol. If the glycerol is stirred very slowly, then the spread of the dye can be reversed by reversing the direction of stir. At higher stirring speeds the turbulence formed cannot be undone. Similarly if you swim with a slow movement forward and backward then you stay in one   place   and   ‘tread   water’.   However,   active   particles   tend   to   have   a preferred direction of movement. A large number of particles moving in the same direction will form regular patterns, but this can be distorted by   small   defects   in   the   surface   of   the   particles.   Professor   Yeomans investigates the physics behind these patterns and those that form from movement around different defects.

Professor Yeomans went on to compare the behaviour of pseudomonas bacteria to the fable of the hare and the tortoise. In two dimensions, the bacteria   pull   themselves   around   the   surface   at   different   speeds, depending   on   how   many   ‘legs’   they   have.   If   you   seed   fast   and   slow bacteria in parallel tracks, the slow ones with fewer legs start off more slowly   but   after   time   they   win   the   race.   The   faster   bacteria   start   to change   direction   and   form   rosettes   due   to   a   combination   of   defects coming together. This pushes the bacteria out of the plane, which mean they point upwards and stop moving forwards. These patterns of motion and defects can be seen  in many other areas of biology, including  the movement of cells in the formation of embryos and in the protective cells around organs where the defects form holes in the membrane which allow inward migration of cancer cells. Professor Yeomans concluded by saying that understanding the mathematics behind this movement will help our understanding of multiple areas of biology.

The evening concluded with formal questions in the hall and over drinks.

Prof. Carolyn Carr

Associate Professor of Biomedical Science

University of Oxford