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 is made up of phospholipids. These phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, creating a barrier that is selectively permeable.
2. Identify the properties of molecules that can easily diffuse through the lipid bilayer: Small, nonpolar molecules can easily pass through the lipid bilayer without the need for transport proteins. This is because the hydrophobic core of the bilayer allows nonpolar molecules to pass through more readily than polar or charged molecules.
3. Analyze the given molecules: Consider the properties of each molecule listed in the problem. O2 (oxygen) is a small, nonpolar molecule. H2O (water) is small but polar. H2PO4- (dihydrogen phosphate) is charged. Glucose is a large polar molecule. Na+ (sodium ion) is charged.
4. Determine which molecule fits the criteria for easy diffusion: Based on the analysis, O2 is the only molecule that is both small and nonpolar, making it the most likely to diffuse across the lipid bilayer without assistance.
5. Conclude with the reasoning: Since O2 is small and nonpolar, it can easily diffuse through the hydrophobic core of the lipid bilayer, unlike the other molecules which are either polar, charged, or too large to pass through without help.