Key biology you should have learned in physics class: Using ideal-gas mixtures to understand biomolecular machines

The biological cell exhibits a fantastic range of behaviors, but ultimately, these are governed by a handful of physical and chemical principles. Here, we explore a simple theory, known for decades and based on the simple thermodynamics of mixtures of ideal gases, that illuminates several key functions performed within the cell. Our focus is the free-energy-driven import and export of molecules, such as nutrients and other vital compounds, via transporter proteins. Complementary to a thermodynamic picture is a description of transporters via "mass-action" chemical kinetics, which lends further insights into biological machinery and free energy use. Both thermodynamic and kinetic descriptions can shed light on the fundamental non-equilibrium aspects of transport. On the whole, our biochemical-physics discussion will remain agnostic to chemical details, but we will see how such details ultimately enter a physical description through the example of the cellular fuel ATP.