When diffusion constants and geometry are constant and Cp is negligible, the absorption rate is proportional to which term?

When diffusion constants and geometry are constant and Cp is negligible, the absorption rate is proportional to which term?

• A. Cgi
• B. Cp
• C. D
• D. h

Answer: (A)

The key idea is that, under steady diffusion across a membrane, the rate of absorption scales with the driving concentration difference across the barrier. For a planar membrane with constant diffusion coefficient and fixed geometry, the flux is J ≈ (D/h) × (Cgi − Cp). If Cp is negligible and the geometry (area) and D are constant, this reduces to J ∝ Cgi. The total absorption rate, which is the flux times the membrane area, becomes proportional to Cgi as well. Since Cp, D, and geometry are fixed or negligible in this scenario, the variable that sets the rate is the concentration at the entry side, Cgi.

The key idea is that, under steady diffusion across a membrane, the rate of absorption scales with the driving concentration difference across the barrier. For a planar membrane with constant diffusion coefficient and fixed geometry, the flux is J ≈ (D/h) × (Cgi − Cp). If Cp is negligible and the geometry (area) and D are constant, this reduces to J ∝ Cgi. The total absorption rate, which is the flux times the membrane area, becomes proportional to Cgi as well. Since Cp, D, and geometry are fixed or negligible in this scenario, the variable that sets the rate is the concentration at the entry side, Cgi.