Which molecule most easily diffuses across a biological membrane's lipid bilayer, without facilitation?

• A. H₂O.
• B. O₂.
• C. H₂PO₄^–.
• D. Glucose.
• E. Na⁺.

Answer: (B)

Verified step by step guidance

1. Understand the structure of a biological membrane: Biological membranes are primarily composed of a lipid bilayer, which is made up of phospholipids. These phospholipids have hydrophobic tails and hydrophilic heads, creating a barrier that is selectively permeable.
2. Identify the properties of molecules that can easily diffuse across the lipid bilayer: Small, nonpolar molecules can diffuse across the lipid bilayer more easily than polar or charged molecules. This is because the hydrophobic interior of the lipid bilayer is more compatible with nonpolar substances.
3. Evaluate the given molecules based on their properties: Consider the polarity, charge, and size of each molecule. O2 is a small, nonpolar molecule, H2O is polar, H2PO4– is charged, glucose is large and polar, and Na+ is charged.
4. Apply the concept of passive diffusion: Passive diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration without the need for energy or facilitation. Nonpolar molecules like O2 can passively diffuse across the lipid bilayer.
5. Conclude which molecule most easily diffuses: Based on the properties discussed, O2, being small and nonpolar, can diffuse across the lipid bilayer without facilitation, unlike the other molecules listed.