Which of the following substances can diffuse across the phospholipid bilayer due to being non-polar and fat soluble?

The correct choice identifies substances that are non-polar and fat soluble, allowing them to diffuse directly through the phospholipid bilayer of cell membranes. The phospholipid bilayer primarily consists of hydrophobic (fat-loving) tails that create a barrier to most polar or charged molecules. Non-polar substances, such as oxygen (O2), carbon dioxide (CO2), nitrogen (N), and ethanol (ETOH), can readily pass through this lipid membrane without the need for transport proteins or channels, which are necessary for polar and charged molecules.

Oxygen and carbon dioxide are particularly relevant as they are gases that play crucial roles in cellular respiration and gas exchange in living organisms. Ethanol, being a small non-polar molecule, can also traverse the membrane freely. These characteristics contrast sharply with the other sets of substances, which include polar molecules or ions that require specific channels or transport mechanisms to cross the membrane due to their hydrophilic nature.