Final answer:
The transport maximum refers to the point at which the number of carrier proteins in the epithelial cell membrane are fully saturated, beyond which transport cannot increase.
Step-by-step explanation:
The transport maximum is dependent upon the number of carrier proteins in the epithelial cell membrane. Carrier proteins and channels are integral proteins that facilitate the movement of substances across the cell membrane. Carrier proteins bind to the molecule that needs to be transported and change their shape in a way to shuttle the molecule across the membrane. When all the transport proteins are occupied, or saturated, the rate of transport cannot increase even if the concentration gradient increases, since there are no more proteins available to carry more molecules. This saturation leads to a maximum rate of transport known as the transport maximum.