Oxygen diffusion across phospholipid bilayers to mitochondrial matrix

Steps:

• Assume X is extracellular space and Y is mitochondrial matrix, the site of oxidative phosphorylation.
• Oxygen first crosses the plasma membrane, a phospholipid bilayer (2 layers).
• Next, it crosses the outer mitochondrial membrane, another bilayer (adds 2 layers, total 4).
• Finally, it crosses the inner mitochondrial membrane, a third bilayer (adds 2 layers, total 6).
• No additional barriers exist for direct diffusion.

Why D is correct:

• Biological membranes are phospholipid bilayers; crossing three membranes (plasma, outer mitochondrial, inner mitochondrial) requires passing 6 phospholipid layers.

Why the others are wrong:

• A: Counts only the three membranes, ignoring each as a bilayer.
• B: Accounts for two bilayers (4 layers) but misses the third membrane.
• C: Accounts for all three bilayers (6 layers) but subtracts one layer erroneously.

Final answer: D