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

• 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 consists of phospholipids with hydrophobic tails and hydrophilic heads.
2. Identify the properties of molecules that can easily diffuse across the lipid bilayer: Small, nonpolar molecules can pass through the hydrophobic core of the lipid bilayer without assistance.
3. Analyze the given molecules: Consider the size, polarity, and charge of each molecule. Nonpolar molecules like O2 can diffuse easily, while polar or charged molecules like H2O, H2PO4-, glucose, and Na+ require assistance.
4. Consider the role of membrane proteins: Charged or large polar molecules typically require transport proteins to cross the membrane, as they cannot pass through the hydrophobic core easily.
5. Conclude which molecule can diffuse most easily: Based on the properties discussed, O2, being small and nonpolar, can diffuse across the lipid bilayer without the need for a protein.