  Biological units such as macromolecules, organelles, and cells are directed
to a proper location under gradients of relevant chemicals. By considering a
macroscopic element that has binding sites for a chemical adsorption reaction
to occur on its surface, we show the existence of a thermodynamic force that is
generated by the gradient and exerted on the element. By assuming local
equilibrium and adopting the grand potential from thermodynamics, we derive a
formula for such a thermodynamic force, which depends on the chemical potential
gradient and Langmuir isotherm. The conditions under which the formula can be
applied are demonstrated to hold in intracellular reactions. The role of the
force in the partitioning of bacterial chromosome/plasmid during cell division
is discussed.
