Which factor would result in a faster net flux of molecules across a membrane?

A large concentration difference is indeed the factor that results in a faster net flux of molecules across a membrane. This phenomenon can be explained through the principles of diffusion, where molecules move from an area of high concentration to an area of low concentration. The greater the disparity in concentrations on either side of the membrane, the stronger the driving force for diffusion. This increased concentration gradient enhances the rate at which molecules diffuse across the membrane, leading to a more rapid net flux.

In contrast, a low permeability coefficient indicates that the membrane is less permeable to certain molecules, which would slow down their movement across the membrane. Large molecular size typically restricts the ability of molecules to pass through the lipid bilayer or membrane channels, making it more difficult for those molecules to cross quickly. Additionally, lower temperatures can decrease molecular energy and movement, resulting in slower diffusion rates. Therefore, while these other factors might influence the movement of molecules, they do not promote a faster net flux compared to a large concentration difference.