Final answer:
The rate of entry of test compounds into cells is best correlated with lipid solubility, confirming that the plasma membrane's permeability is lipid-dependent and selectively permeable.
Step-by-step explanation:
The rate of entry of test compounds through the plasma membrane is best correlated with lipid solubility. This is because plasma membranes consist of a bilayer of phospholipids that are amphipathic, meaning they have both hydrophilic and hydrophobic regions. Substances that are nonpolar or lipid-soluble can easily slip through the hydrophobic lipid core of the membrane, thus allowing a faster diffusion rate. Molecular size does affect diffusion, but it's not as significant a factor as lipid solubility for substances crossing the membrane. This confirms that the nature of the plasma membrane is lipid-dependent.
The plasma membrane is highly selective, exhibiting a selectively permeable characteristic that allows some substances to pass through while hindering others. This selective permeability is largely dependent on the lipid solubility of the substance trying to cross the membrane. The modern understanding of the plasma membrane as described by the fluid mosaic model confirms that it is the fluidity and mosaic nature of the lipids and proteins in the membrane that enable this selectivity.