Neco 2025 Biology Question 34

The thickness of a leaf mesophyll is \(2\times10^{-4}\) m. The diffusion coefficients for O\(_2\) and CO\(_2\) in the leaf tissue are \(D_{O_2}=2.1\times10^{-9}\) m\(^2\)·s\(^{-1}\) and \(D_{CO_2}=1.6\times10^{-9}\) m\(^2\)·s\(^{-1}\) respectively. Assuming one‑dimensional diffusion, the approximate time for a gas to traverse the leaf can be estimated by \(t \approx L^{2}/(2D)\).\n\n(i) Calculate the diffusion time for O\(_2\) and for CO\(_2\) across the leaf at the given thickness.\n(ii) If the stomata close, the effective diffusion path is halved. What is the new diffusion time for CO\(_2\) and by what percentage does the diffusion time decrease compared with the open‑stomata condition? Give the percentage to the nearest whole number.