Bridging the gap between metabolism and the physics of the cytoplasm

This thesis starts by drawing a bold hypothesis: that metabolic activity causes proteins to move inside cells, such that the intracellular environment is effectively "stirred up". While some degree of fluidity is required, we posit that too much movement is detrimental for the cells, and that they have to have mechanisms in place to control excessive release of (Gibbs) energy. Then, we set out to test some of the assumptions underlying our initial hypothesis. Specifically, we ask whether (i) there is a limit on the rate at which cells release Gibbs energy, and (ii) movement inside the cell depends on metabolic activity. After quantifying various parameters of cell growth and metabolic activity under different conditions, we conclude that cells can release Gibbs energy at a higher rate than previously assumed. Then, by using microscopy to track fluorescent particles inside cells, we were able to quantify intracellular "movement". Here, we observed that this movement varied significantly across growth conditions. Yet, neither our initial hypothesis, nor other parameters that have been previously described in the literature could satisfactorily explain the results. Instead, we saw that this "movement" correlated with the abundance of some cellular proteins – noteworthy for their tendency to associate with one another. Putting these two pieces of evidence together, we postulate that the varying degree with which large protein structures are formed inside the cell are the reason for the changes in "movement" that we observed.