Which property of a membrane determines which molecules may enter or leave?

The selective permeability of a membrane determines which molecules may enter or leave. This property arises from the lipid bilayer's composition and the presence of specific proteins embedded within it, which can facilitate or restrict the passage of substances. Small, nonpolar molecules typically diffuse freely, while larger or polar molecules often require transport proteins. Additionally, factors such as concentration gradients and charge play significant roles in regulating molecular movement across the membrane.