Final answer:
Facilitated diffusion is limited by substrate concentration, carrier protein in the membrane, and size of pores in the membrane. It is a process of moving substances down their concentration gradient with the help of selective transport proteins.
Step-by-step explanation:
Facilitated diffusion is limited by all of the above: substrate concentration, carrier protein in the membrane, and size of pores in the membrane.
Facilitated diffusion is the process of moving substances down their concentration gradient with the help of transport proteins in the cell membrane. These transport proteins are selective and can only allow certain substances to pass through. The rate of facilitated diffusion is directly proportional to the number of transport proteins available.
For example, glucose cannot cross the lipid bilayer of the cell membrane via simple diffusion because it is large and polar. To move glucose into the cell, a specialized carrier protein called the glucose transporter is used to facilitate its inward diffusion.