  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 simple theory, known for decades and based on the simple thermodynamics
of mixtures of ideal gases, which 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.
