Which of the following substances is most likely to cross a cell membrane by simple diffusion rather than facilitated diffusion?

• A. Amino acids
• B. O\(_2\) (oxygen gas)
• C. Na\(^+\) (sodium ion)
• D. Glucose

Answer: (B)

Verified step by step guidance

1. Understand the concept of simple diffusion: Simple diffusion is the movement of molecules across a cell membrane without the need for transport proteins. It occurs when molecules move from an area of high concentration to an area of low concentration, driven by their kinetic energy.
2. Identify the characteristics of molecules that can cross the membrane by simple diffusion: Molecules that are small, nonpolar, and uncharged are most likely to diffuse directly through the lipid bilayer of the cell membrane. Examples include gases like O\(_2\) and CO\(_2\), and small hydrophobic molecules.
3. Analyze the properties of each substance: Amino acids are polar and often charged, making them unlikely to cross the membrane by simple diffusion. Na\(^+\) is a charged ion, which requires facilitated diffusion through ion channels. Glucose is a large polar molecule, which also requires transport proteins for facilitated diffusion.
4. Compare the properties of O\(_2\): Oxygen gas (O\(_2\)) is small, nonpolar, and uncharged, making it ideal for crossing the cell membrane by simple diffusion. It can easily pass through the hydrophobic core of the lipid bilayer.
5. Conclude that O\(_2\) is the substance most likely to cross the cell membrane by simple diffusion, based on its physical and chemical properties.