Explain the passive transport mechanisms across a cell membrane, differentiating between simple diffusion and facilitated diffusion.

Substances move down a concentration gradient in passive transport, meaning that no energy input (ATP) is required. Hydrophilic molecules will pass down the membrane using facilitated diffusion, as they cannot pass through the hydrophobic phospholipid bilayer (the membrane). These are polar molecules such as mineral ions. Hydrophobic, fat-soluble, substances will pass through the cell membrane through simple diffusion at small, lipid-soluble molecules, pass through the hydrophilic region of the membrane easily.