Agriculture Reference
In-Depth Information
When concentration gradients of solutes exist across a membrane the solutes
will diffuse according to their individual concentration gradients. Because of
differing mobilities, an electric potential exists between diffusing ions—the dif-
fusion potential (Section 2.2)—and as a result the faster ions speed up the slower
ones and vice versa, so that electrical neutrality is maintained everywhere in
the solution. Thus the rates of transfer of negative and positive charges are
equal. However an electric potential difference across the membrane persists and
is measurable.
Plant cells have several internal compartments separated by plasma membranes.
The main compartments controlling the ionic relations of the cell are the cytosol
and vacuole. The vacuole occupies more than 90% of the cell volume and so
contains the bulk of the ions. Many different ions permeate the membranes, but
in general K
+
,Na
+
and Cl
−
have the greatest concentrations and permeabilities.
Studies of ion relations in plant cells have led to the following conclusions:
K
+
is accumulated in both the cytosol and vacuole by passive diffusion, except
when the external concentration is small in which case its is actively taken up;
•
Na
+
is actively pumped out of the cytosol into the apoplasm and vacuole;
•
H
+
generated in metabolic processes in the cell is also actively pumped out of
the cytosol to maintain a neutral pH; as a result the pH in the apoplasm and
vacuole may be two units lower;
•
Cl
−
and all other anions are actively taken up into the cytosol.
•
All the ions also diffuse passively according to their electrochemical gradients.
It is possible to calculate the resulting diffusion potential across the membranes.
Typically the diffusion potential expected from the movements of K
+
,Na
+
and
Cl
−
is in the range
−
80 to
−
50mV. But measured membrane potentials are gener-
ally much more negative, often
−
200 to
−
100mV, indicating that the membrane
potential has a second component. The excess potential is generated by elec-
trogenic H
+
-ATPases in the plasma membrane which pump H
+
ions out. The
energy provided by hydrolysis of ATP is used to pump out H
+
against its elec-
trochemical gradient. This drives the movements of other ions and molecules
across the membrane via various transporters which both enhance and regulate
the transfers.
Three types of membrane transporter are found:
channels, carriers
and
pumps
(Figure 6.10). Channels are transmembrane proteins that function as selective
pores through which ions or uncharged molecules can diffuse passively. Their
selectivity for solutes depends on the size of the pore and the density of surface
charges lining it. These are altered in response to external and internal stimuli in
the plant, so regulating the transport.
Carriers consist of proteins that extend completely across the membrane. A
solute being transported is initially bound to external sites on the carrier protein;
subsequent changes in the conformation of the protein result in the transfer of
the bound solute across to the other side of the membrane where it dissociates.
