Water and Solute Permeability of Plant Cuticles: Measurement and ...

2.3 Steady State Diffusion Across a Stagnant Water Film Obstructed by Cellulose and Pectin 39
Fig. 2.5 Schematic drawing (not to scale) of a water film having the thickness ℓ and separating
plasmalemma and cuticle
smaller than permeance of cell walls, and accumulation of solutes in the cell wall
is a highly unlikely event. This can be shown to be true in a more formal way by
considering the cuticle and the cell wall as two resistances in series.
2.3 Steady State Diffusion Across a Stagnant Water Film
Obstructed by Cellulose and Pectin
Cellulose and pectin occupy some of the volume of the cell wall, and this can be
expected to slow diffusion of solutes. The question is how much, and this can again
be estimated without having to conduct the experiments. The volume fraction of
water is
volume of water
volume fraction of water = (2.10)
volume of cell wall
and in the present example we work with a volume fraction of water of 0.9, which
means that 90% of the cell wall is made up of water. Volume is A ×ℓ, and with the
thickness of the cell walls being constant the area fraction of water is equal to the
volume fraction of water. In other words, 90% of the cell wall area is occupied by
water, and we can modify (2.9) by including the area fraction: