Osmotic Selectivity as a Deterministic Function of the Fine Structure Constant

This paper derives the osmotic selectivity coefficient σ as a deterministic function of the fine structure constant α through a seven-step chain: α → Bohr radius a₀ → O–H bond length → water molecule diameter → aquaporin pore width → σ → Banach fixed point ω₀ = 0. Each link is either an exact definition, a quantum-mechanical eigenvalue, a geometric identity, or a constrained optimum confirmed by experiment. One conventional parameter (van der Waals radius of hydrogen) and one empirical input (measured pore width) enter the chain; neither affects the qualitative result: water passes, ions don't, q