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

40 2 Quantitative Description of Mass Transfer
J = Awater
×
Acellwall
D
ℓ (Cdonor −Creceiver). (2.11)
The flux of urea will be reduced by a factor of 0.9 if the solids of the cell wall add
up to 10% of the total volume. We have implicitly assumed that these small amounts
of pectins and cellulose do not affect D.
2.4 Steady State Diffusion of a Solute Across a Dense
Non-Porous Membrane
In the previous examples, the membrane was a stagnant water film. There was a
water continuum between donor, receiver and membrane because all were aqueous.
Cellulose and pectin obstructed the diffusion path and reduced the area available for
diffusion. They were considered impermeable to urea solute molecules, which is a
good assumption.
Now we consider diffusion across a dense membrane which does not contain
water. Donor and receiver are aqueous solutions as before. A solute can enter and
diffuse across the membrane only when it is soluble in the membrane. This type of
solid solution may appear strange to some, but it is a very common phenomenon.
Stained wax candles, fibres and plastics are examples. Histological sections are usually
stained to better visualise different organs and tissues. The cuticle depicted in
Fig. 2.6 stained orange using the lipophilic stain Sudan III. The stain dissolved in
cutin, and this is a good example of a solid solution.
Cuticles are lipid membranes, composed mainly of cutin and waxes. Solutes
can be classified as hydrophilic or lipophilic. Solutes having high water solubility
such as amino acids, sugars or inorganic salts are hydrophilic. Solutes which
better dissolve in lipid solvents (i.e., ether, hexane, chloroform, octanol, olive oil)
are lipophilic. When working with cuticles it is useful to classify solutes according
to their solubility in cuticles and in water. The ratio of the two solubilities is the
partition coefficient K, which can be determined easily. A piece of cuticle is equilibrated
in an aqueous solution of the solute. When equilibrium is established, the
concentrations in the cuticle and in water are determined. As it is not easy to determine
the volume of the piece of cuticle precisely, molal concentrations (molkg −1 )
Fig. 2.6 Photomicrograph of a thin cross-section of an adaxial Clivia leaf epidermis. The lipophilic
cuticle is stained orange with Sudan III, and the acidic cell wall is blue after staining with Toluidine
blue at pH 4.0