Plant systems, electronic properties

B. Mammalian Systems. Investigations of the electronic properties of plant systems served as an excellent entree into the examination of mammalian membranes using the techniques already developed. Because of the writer s long-time interest in muscle physiology, this tissue was chosen for exploration with the objective of obtaining more intimate information regarding muscle behavior. 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

The herbicidal activity of the bipyridyliums depends on their redox properties. Their abilities as one-electron acceptors of the right redox potential (-350 mV for diquat and -450 mV for paraquat) allow them to siphon electrons out of the photosynthetic electron-transport system, competing with the natural acceptors. The radical anion produced is then reoxidized by oxygen, generating the real toxicant, hydrogen peroxide, which damages plant cells. Structure-activity relationships in this series have been reviewed (60MI10701). 

Hasson, E.P., West, C.A. "Properties of the system for the mixed function oxidation of kaurene and kaurene derivatives in microsomes of the immature seed of Marah macrocarpus. Electron transfer components." Plant Physiol., 1976, 58, U79-U8U. 

The intermediate electron transfer between the pool of quinones accepting electrons from the RC, and the water soluble proteins donating electrons to the RC (bacterial RC and the PSI-RC) is always promoted, at least in the systems studied so far in detail, by a multiprotein complex containing cytochromes and Fe-S proteins, the so called h/ci complex. The universal presence of this type of complex in many redox chains of respiration and photosynthesis has been recognized only very recently [109]. As far as photosynthesis is concerned, complexes of this kind have been characterized in facultative photosynthetic bacteria [110] in cyanobacteria [111], and in higher plant chloroplasts [112]. All these preparations share common characteristics and composition these properties are also very similar to those of analogous complexes isolated from mitochondria of mammals and fungi [109]. 

Photosystem II (PSII) is part of the photosynthetic apparatus in cyanobacteria, algae and higher plants and catalyzes the light-induced transfer of electrons from water to plastoquinone via a set of delicately arranged cofiictors. It has a well known bindii site for diverse chemical compounds in its so-called D1 subunit and the ability to convert such a binding event into signals which can be easily detected by optical, potentiometric or amperometric systems. Due to these inherent properties PSII can be considered as a natural biosensor and has consequentially been used for the detection of herbicides and other pollutants in pilot studies. ... 

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