Question

facilitated diffusion will occur only if there is a ?

Answer 1

Facilitated (means helped) diffusion, can only occur if a specific transport-molecule binds to the substance (which is going to be transported), and then "pull" it to the other side of the cellmembrane. The facilitated diffusion can only go from higher concentrationen to lower concentrationen like regular diffusion.

Answer 2

It's called concentration gradient or gradient of concentration, which ever way. It is going to define the concentration of a specific ion or substance on either sides of a cellmembrane, an example would be the the intermembrane transport protein GLUT-1 that transports 1 glucose molecule and 2 Na+ ions by symport.

Answer 3

One way in which cells can control diffusion is through the control of movement of particular types of molecules. Some molecules can pass freely through a cellular membrane. Large polar molecules and ions cannot diffuse through the lipid bi layer. Carrier proteins can move molecules across the membrane. Cells will have carrier proteins for specific molecules. These molecules will pass freely across the membrane while other molecules will not. The types of carrier proteins found on a cell are related to cell function.