Fick's law of diffusion of a gas through a lipid membrane: rate of diffusion is directly proportional to surface area and diffusion coefficient and inversely proportional to membrane thickness. In the equation V = D × A × (P1–P2) / T, which variable represents thickness?

Fick's law of diffusion of a gas through a lipid membrane: rate of diffusion is directly proportional to surface area and diffusion coefficient and inversely proportional to membrane thickness. In the equation V = D × A × (P1–P2) / T, which variable represents thickness?

• A. Diffusion coefficient
• B. Surface area
• C. Thickness
• D. Pressure gradient

Answer: (C)

Diffusion rate through a lipid membrane rises with how easily the gas moves (diffusion coefficient), how much membrane area is available (surface area), and the driving force pushing the gas (the partial pressure difference), but it falls as the membrane thickness increases. In the equation V = D × A × (P1–P2) / T, the thickness is represented by the symbol in the denominator. Because it sits in the denominator, increasing thickness reduces the rate of diffusion, while decreasing thickness increases it. So the thickness is the variable T. The other factors—diffusion coefficient, surface area, and the pressure gradient—play the other roles described: D for ease of diffusion, A for the available area, and (P1–P2) as the driving force.

Diffusion rate through a lipid membrane rises with how easily the gas moves (diffusion coefficient), how much membrane area is available (surface area), and the driving force pushing the gas (the partial pressure difference), but it falls as the membrane thickness increases. In the equation V = D × A × (P1–P2) / T, the thickness is represented by the symbol in the denominator. Because it sits in the denominator, increasing thickness reduces the rate of diffusion, while decreasing thickness increases it. So the thickness is the variable T. The other factors—diffusion coefficient, surface area, and the pressure gradient—play the other roles described: D for ease of diffusion, A for the available area, and (P1–P2) as the driving force.