Molecular crowding inside cells is a critical factor that is sometimes overlooked in the lab. Biochemical reactions are drastically different in tighter spaces, resulting in significant physical and chemical consequences.
The reductive approach to chemical reactions is attractive to scientists, as it’s convenient when interpreting results and easier to work with. However, if a more simplistic experimental framework is adopted, these tests may overlook the critical aspect of intracellular crowding.
If there is less space available for each molecule, the resulting effective concentration is boosted, increasing the chemical potential. When more molecules are crammed together, there is also the possibility that reactants will link up, causing the chemical potential to respond drastically.
There is also a slowdown of molecular diffusion, particularly for larger molecules. Lower rates of diffusion imply that diffusion-limited reactions happen slower, especially those involving larger constituents. These two consequences, higher chemical potentials and lower diffusion rates, counteract each other when it comes to intermolecular reactions, making the resulting reaction rate difficult to predict. However, there usually is an optimal spot with enough coziness to encourage reactivity, but not too much to gum up the works.
The effects on thermodynamics and kinetics are so far-ranging that estimates of reaction rates and equilibria made in uncrowded solutions differ by orders of magnitude from those of the same reactions operating under crowded conditions.