Final answer:
Facilitated diffusion does not follow a linear graph like simple diffusion because it involves specific transmembrane proteins with limited transport capacity. As substrate concentration increases, the rate of facilitated diffusion initially increases but eventually levels off and does not increase linearly.
Step-by-step explanation:
The graph of the rate of facilitated diffusion versus substrate concentration is not linear like the graph of simple diffusion versus substrate concentration because facilitated diffusion involves the use of specific transmembrane proteins to transport molecules or ions across membranes. These proteins have a maximum capacity for transporting substances, known as Vmax. As the substrate concentration increases, the rate of facilitated diffusion initially increases rapidly, but eventually reaches a saturation point where all the transport proteins are occupied. At this point, the rate of facilitated diffusion plateaus and does not increase linearly with substrate concentration.