Final answer:
Distance is not included in the Fick equation for diffusion across a membrane because the membrane is considered to be a very thin structure. The thickness of the membrane is negligible compared to the distance over which diffusion occurs. The Fick equation focuses on factors such as concentration difference, diffusion constant, and time.
Step-by-step explanation:
Distance is not included in the Fick equation for diffusion across a membrane because the membrane is considered to be a very thin structure. Diffusion through membranes is a process that occurs over microscopic distances, and the thickness of the membrane is negligible compared to the distance over which diffusion occurs. The Fick equation focuses on factors such as concentration difference, diffusion constant, and time rather than distance.