Which transport pathway involves substances passing through the cell membrane itself rather than between cells?

• A. Paracellular transport
• B. Facilitated diffusion
• C. Transcellular transport
• D. Active transport

Answer: (C)

Movement through an epithelial barrier can occur either by crossing between cells (paracellular) or by going through the cell membranes themselves (transcellular). The phrase “through the cell membrane itself” points to transcellular transport, where the substance enters the cell across the apical membrane, traverses the cytosol, and then exits across the basolateral membrane. This route relies on transport proteins, channels, or endocytosis to move substances across the membranes, and it can be passive (facilitated diffusion) or energy-dependent (active transport). Paracellular transport, by contrast, bypasses the cell membranes altogether and passes between cells via tight junctions.