Electrophysiology and Phototropism - Communication in Plants: Neuronal Aspects of Plant Life - page 355

cryptochromes and phototropin share many similarities they have differ-
ent transduction pathways. Cry1 plays a significant role in the synthesis of
anthocyanin and the entrainment of circadian rhythms. Cry2 plays a part
in the photoperiodic flowering and cotyledon expansion. Cryptochromes
were found to be predominantly in the nucleus.
Blue light and UV irradiation influence the opening of the stomata (Folta
et al. 2001). It is suggested that zeaxanthin is a blue/green light photore-
ceptor (Eisinger et al. 2000).
Phototropism is one of the best-known plant tropic responses. A positive
phototropic response is characterized by a bending or turning toward the
source of light. When plants bend or turn away from the source of light,
the phototropic response is considered negative. A phototropic response is
a sequence of the four following processes: reception of the directional light
signal,signaltransduction,transformationofthesignaltoaphysiological
response, and the production of directional growth response.

24.3
Electrochemical Circuits

The ends of a correctly constructed electrochemical circuit measuring
the electrical potential difference must always have metals or conductors
with identical chemical composition. This is usually achieved by simple
connection of two metals by copper wires. The inclusion between two
metal conductors of a third metal conductor according to Volta's law does
not change the potential difference at the output of a circuit (Volkov et al.
1998). The potential difference in an electrochemical circuit at equilibrium
iscausedbythechangeofGibbsfreeenergy

∆ G during the appropriate

electrochemical reaction:

= ∆ / E − G nF , (24.1)

= where F 96,500 C is the Faraday constant and n is the number of electrons.

The electrical potential difference of an electrochemical circuit at equilib-
rium ( E ) is the electromotive force. The value of nFE characterizes the
maximum electrical work that can be received through an electrochemical
circuit. Equation 24.1 is the basis for the calculation of the Gibbs free energy
for different electrochemical reactions. The electrode potential at a given
temperature and pressure is determined by the magnitude of the standard
electrode potential and the activities of the substances taking part in the
electrode reaction. The standard potential is a constant, which is specific for
each electrode, while the activities of the reacting species may be different,
depending on the concentration of the reaction medium.