Biomedical Engineering Reference
In-Depth Information
8.5 Water Exchange between Aquatic Plants
and the Environment
Every living organism, whether multicellular or a single cell, needs to continu-
ously exchange with the environment water and various chemicals. It uptakes
water and nutrients and expels water and metabolic waste products. In case of
multicellular land plants, water uptake takes place mostly osmotically through
the root system. Water is then expelled in leaves. It transpires to the atmo-
sphere, mainly from the walls of mesophyll cells and from leaf cuticles. Water
can diffuse across cell membranes and cuticles; however, in many cases we
may observe a significantly faster permeation through water pores formed in
membranes by specialized proteins or water pores that spontaneously form
in cuticles. The majority of studies of such transport have only qualitative
character, and the details of transport mechanisms are not well understood
(Schonherr 2006).
In case of aquatic plants that live entirely submerged, the mechanisms
of water exchange with the surroundings are simpler but still not entirely
understood. Aquatic plants do not uptake water through roots, since they
have smaller roots that serve mostly to attach the plant. Water also cannot
transpire from leaf surfaces. Therefore we assume water is exchanged not by
specialized organs within a plant, but by each plant cell individually. This
exchange takes place across the cell membrane and is induced by thermody-
namic forces, such as osmotic pressure, ∆Π , or hydraulic pressure, ∆ P . One of
the main microscopic mechanisms of water transport is permeation through
a variety of pores, such as aquaporins or ion channels. Therefore this trans-
port can be described using practical KK equations (Chapter 2) or the ME
equations (Chapter 4). In this section, we describe biophysical foundations of
water exchange with the surroundings on the example of two aquatic plants:
Nitella translucens and Chara Corallina (Kedem and Katchalsky 1965; Hertel
and Steudle 1997; Dabska 1964).
8.5.1 KK Equations Applied to Water Exchange by
Aquatic Plants
We consider a model of an aquatic plant cell in an aqueous environment as
shown in Figure 8.4. The cell surrounded by a selective membrane contains an
osmotically active nonelectrolytic solute (s). The concentration of the same
solute in the environment, c so , is smaller than intracellular concentration, c si .
According to the KK formalism, the transport properties of the membrane
are characterized by transport parameters L p , σ , and ω . The reflection coef-
ficient has a value 0 <σ< 1 for a selective membrane. Therefore, the perme-
ation coecient is ω> 0. The concentration difference across the membrane
c = c si
c so induces a solute eux, given by the volume solute flux J vs .
 
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