What is the process called when fatty substances dissolve into the lipid part of the membrane to cross it?

• A. Dissolution
• B. Diffusion
• C. Active Transport
• D. Filtration

Answer: (A)

Nonpolar, lipid-soluble molecules cross the cell membrane by simple diffusion, moving directly through the lipid bilayer down their concentration gradient without using energy. If fatty substances dissolve into the lipid part of the membrane to cross it, they are utilizing this passive diffusion process. Dissolution is just solubility in a solvent and doesn’t describe a mechanism for crossing the membrane. The other options involve energy or different movement methods: active transport uses energy to move substances against their gradient, and filtration relies on pressure to push water and small solutes through membranes. In practice, oxygen, carbon dioxide, and steroid hormones move across membranes this way.