Whether a cell uses facilitated diffusion or active transport depends on the specific needs of the cell. For example, the sugar glucose is transported by active transport from the gut into intestinal epithelial cells, but by facilitated diffusion across the membrane of red blood cells.

Epithelia form linings throughout the body. In the small intestine, for instance, the simple columnar epithelium forms a barrier that separates the lumen from the internal environment of the body (note that the internal environment in which body cells exist is the extracellular fluid or ECF). The epithelium forms a barrier because cells are linked by tight junctions, which prevent many substances from diffusing between adjacent cells. For a substance to cross the epithelium, it must be transported across the cell's plasma membranes by membrane transporters.
 

Not only do tight junctions limit the flow of substances between cells, they also define compartments in the