Diffusion is a process in which there is a net movement of a substance from an area of higher concentration of that substance to an area of lower concentration.
In the figure below the triangle represents a concentration gradient and the arrow indicates the direction of net movement of the substance that makes up the concentration gradient.
movement of a substance down its concentration gradient is an exergonic process
The diffusion of a substance down its concentration gradient is an exergonic process as indicated by the symbol for free energy release.. The free energy released during diffusion can be computed from the following equation where R is the gas constant, T is the temperature and ln is the natural logarithm and C1 and C2 are the two extreme concentrations.
When a substance is moving from C1 to C2 it is moving down a concentration gradient. C2 is smaller than C1 and the lnC2/C1 is a negative number and the change in free energy is a negative number. Thus diffusion is an exergonic process:
Movement of a substance against a concentration gradient is an endergonic process
When a substance moves from C2 to C1 in the above figure then it is moving up its concentration gradient (moving against its concentration gradient).
In this case lnC1/C2 is a positive number and the free energy of the reaction is a positive number. Thus movement of a substance against its concentration gradient is an endergonic process and in order for it to proceed energy must be provided from an outside source.