Final answer:
Polar substances such as ions and polar molecules cannot easily cross cellular membranes, while lipid soluble molecules can pass through the lipid bilayer. Different substances cross membranes through different mechanisms, including simple diffusion, facilitated diffusion, and active transport.
Step-by-step explanation:
Polar substances such as ions and polar molecules cannot easily cross cellular membranes. These substances are impermeable to the lipid bilayer of the membrane. However, lipid soluble molecules can readily pass through the membrane due to their ability to dissolve in the hydrophobic interior of the bilayer. Additionally, certain impermeable substances can be transported across the membrane with the help of membrane transport proteins.
Different substances cross membranes through different mechanisms. Small nonpolar molecules can pass through the lipid bilayer via simple diffusion, while larger polar molecules and ions require transport proteins for facilitated diffusion. Some substances can also be actively transported across the membrane against a concentration gradient, using energy from ATP. The specific method of transport depends on the properties of the substance and the characteristics of the membrane.