Facilitated Diffusion Practice Test

Cells are exposed to a fluorescent dye that is polar and cannot cross lipid bilayers unaided. Fluorescence inside cells increases only when a specific membrane channel is present and open. The increase occurs without detectable ATP consumption and slows as internal dye concentration rises. Which explanation best accounts for dye entry into the cells?

A researcher studies transport of polar solute R across a membrane. With a functional transporter present, R enters cells rapidly when outside concentration exceeds inside concentration. When the transporter gene is deleted, R entry becomes negligible despite the same gradient, and ATP inhibitors have no effect in either strain. Which explanation best accounts for these results?