Which type of diffusion is used by lipid insoluble molecules to enter a cell?

Lipid insoluble molecules, due to their hydrophilic nature, cannot easily pass through the lipid bilayer of cell membranes, which is primarily composed of phospholipids that provide a hydrophobic environment. Facilitated diffusion is the correct process that allows these molecules to enter cells with the help of specific carrier proteins or channels embedded in the cell membrane. These proteins create a pathway for the lipid insoluble substances to move across the membrane down their concentration gradient, without the need for energy input.

The other processes listed, such as active transport, involve the movement of molecules against their concentration gradient and require energy, making them unsuitable for lipid insoluble molecules that benefit from facilitated pathways. Simple diffusion applies to small, nonpolar molecules that can directly permeate the membrane without assistance. Endocytosis involves the engulfing of larger particles or volumes of material but is not typically utilized for the simple entry of lipid insoluble small molecules. Thus, facilitated diffusion is the most appropriate mechanism for these types of molecules to cross the cell membrane.