Final answer:
Lipid solubility is a key factor for a molecule's passage across the cell membrane, as lipid-soluble substances can easily diffuse through the hydrophobic core of the membrane, whereas polar or charged molecules generally require specialized transport mechanisms.
Step-by-step explanation:
Lipid solubility significantly affects a molecule's ability to cross a cell membrane. Because the plasma membrane is amphiphilic, with hydrophobic (water-repelling) and hydrophilic (water-attracting) regions, substances that are lipid-soluble can more easily penetrate the hydrophobic lipid core. This allows lipid-soluble molecules such as fat-soluble vitamins (A, D, E, and K), certain drugs, hormones, and nonpolar molecules like oxygen and carbon dioxide to diffuse through the membrane more readily than polar or charged molecules, which may require specific transport mechanisms like protein channels or carrier proteins.
Temperature and the presence of cholesterol also affect membrane fluidity, which in turn influences solute permeability. Higher temperatures and more unsaturated fatty acids in membrane lipids enhance fluidity and, hence, the diffusion rate of certain substances. Conversely, cholesterol acts to stabilize the membrane and can decrease its permeability to some ions and molecules.
The ability for a molecule to cross a cell membrane via simple diffusion is largely determined by lipid solubility. However, transport proteins facilitate the diffusion of polar or charged molecules that the lipid bilayer alone would impede, such as glucose entering a cell through a glucose transporter.