Final answer:
Small, neutral, nonpolar molecules transport across cell membranes through simple diffusion, driven by concentration gradients and without the need for transport proteins, unlike polar molecules.
Step-by-step explanation:
The type of transfer used for small, neutral, nonpolar molecules is simple diffusion. These molecules can easily move across the cell membrane because the hydrophobic lipid bilayer provides a permeable barrier for such molecules. Movement is driven by the concentration gradient, without the need for energy or transport proteins. Unlike polar molecules, which require facilitated diffusion or active transport to pass through the membrane due to their charge or polarity, nonpolar molecules can pass through by slipping between the fatty acid tails of the membrane lipids.
It's important to understand the role of electronegativity in determining molecule polarity. Nonpolar molecules have an equal sharing of electrons due to a very small or zero difference in electronegativity between bonded atoms. Molecules like H2 and O2 have nonpolar bonds because of similar or identical electronegativity values of the atoms involved.
In contrast, polar molecules exhibit a significant difference in electronegativity leading to unequally shared electrons and a separation of charge within the molecule. This characteristic prevents them from easily crossing the hydrophobic core of the cell membrane without assistance.