Membrane Permeability in Blood Cells

Steps:

• Blood cells, like erythrocytes, have a phospholipid bilayer membrane impermeable to ions but permeable to water.
• Evaluate statement 1: Sodium ions, being charged, cannot diffuse through the hydrophobic bilayer without channels.
• Evaluate statement 2: Water moves across the membrane down its concentration gradient via osmosis, a true process for these cells.
• Evaluate statement 3: Water molecules diffuse directly through the lipid bilayer due to their small size and partial permeability.

Why D is correct:

• Osmosis (2) is water diffusion driven by concentration gradients across semipermeable membranes, occurring via direct bilayer passage (3), per Fick's law of diffusion.

Why the others are wrong:

• A includes 1, but ions require protein channels, not bilayer diffusion.
• B includes 1 (false) and excludes 3 (true water diffusion).
• C includes 1 (false) and excludes 2 (true osmosis).

Final answer: D