Biomedical Engineering Reference
In-Depth Information
1992). To prevent an explosive rupture of the membranes, the huge pressures have to be counter-
balanced by strong but flexible cell walls made of cellulose and other polymers. However, when
considering mobile, nastic structures, the walls must have also the ability to yield to the pressure
and so to allow plastic and elastic expansion. Not all cell expansions are driven by the pressure of
vacuolar osmotic motors though. The 15-fold volume increase in root cap cells occurs without the
formation and expansion of a large vacuole and — at least in part — is likely to be driven by a
colloid motor (Juniper and Clowes, 1965).
The osmotic motor is the most common of the three types of hydration motors used by plants.
Some plant species can also use osmosis to pump water and solutes into the upper shoot, an action
that becomes necessary when a highly humid air prevents normal ion transport by transpiration.
Pumping ions into the xylem vessels of the lower end of the root cylinder, roots generate a local
pressure increase of up to 0.1 MPa sufficient to push a water column 9 to 10 m above the ground.
This so-called root pressure is generated by only some plant species and becomes apparent in such
phenomena like guttation (the appearance of droplets at the leaf periphery of grasses and broad-
leafed plants) and the so-called bleeding of decapitated stumps (e.g., Stahlberg and Cosgrove,
1997).
Osmotic motors have the disadvantages that they depend on the intactness of a very thin, fragile
membrane that also must be permeable to the small water molecules alone. Freezing and subse-
quent thawing destroy these membranes and with it all osmosis-based mechanisms. The same
failure occurs when the ionic solutions are so severely dehydrated that they crystallize. The
shrinking of osmotically operating vacuoles that often occupy more than 90% of the cell volume
leads to harmful structural deformations of tissues (exceptions are discussed in Section 19.2.2.4).
Some plants, for example, in the genus
Selaginella,
can repeatedly dry and rehydrate without
structural damage. They avoid critical cell deformations during severe dehydration by using
vacuoles of smaller size that are filled with tannin colloids instead of ions. Upon dehydration
these colloids undergo minimal volume changes (Walter, 1956). Nature itself points here to the
interesting alternative of replacing crystallizing small molecules with larger-sized colloids.
19.2.2
Colloid-Based Motors
Colloids are hydrating particles with a size ranging from 5 nm to 0.5 mm. Most colloids do not form
true solutions but suspensions that are not completely transparent and show light diffraction
(Tyndall effects) and other optical effects not found in true solutions. Many natural macromol-
ecules, such as starch, pectins, latex, nucleic acids, and proteins, fit this definition. Due to their
larger particle size, colloidal solutions cannot be as concentrated as osmotic solutions and their
osmotic effect is therefore considerably smaller. This is demonstrated in the following comparison.
A 10% (w/v) solution of glucose has an osmotic pressure of 1.35 MPa whereas a 10% solution of
the colloidal bovine serum albumin (BSA, a soluble protein) has only 3.2 kPa, that is, it is
osmotically almost three orders of magnitude less effective (Levitt, 1969).
For the purpose of constructing a colloid-based motor, there are primarily three desirable
characteristics of colloids: (i) the potential expandability, that is, the volume change they undergo
per volume absorbed water; (ii) the reversibility of the volume change; (iii) a low hydraulic
capacitance, defined as the volume change per unit applied pressure (Meidner and Sheriff, 1976).
A high force development per volume change is desirable for any motor and leads to the practical
question for both nature and human engineers of whether colloid-based motors can equal the
generated pressures of osmotically operating systems like vacuoles filled with ionic solutes. Natural
macromolecules bind water to different degrees, for example, 1 g of starch binds 0.8 g water. Due to
high particle size and molecular weight, colloid hydration looks more impressive if we express
it as the binding of 30,000 to 100,000 water molecules per molecule gelatin (Walter, 1957). This
high degree of hydration is not osmotic but due to the presence of adsorptive forces (called adhesion
or imbibition) that can equal and exceed the pressure of osmotic systems by reaching values of up to
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