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275 - 297
- Zusammenfsg.
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Acetabularia appears, morphologically, as a highly polarized cell at almost all developmental stages. A review of the spatial distribution of organelles and important molecules shows that the genetic information is apically accumulated together with the protein synthetizing machinery. The transcellular bioelectric current and the phosphorylating activity display opposite gradients. Calcium and calmodulin are both accumulated at the apex of a growing alga; their gradients disappear at cap formation. Peroxidase and ascorbic acid display parallel negative apico-basal gradients; both dissappear at cap formation. The transcellular bioelectric current and the cytoplasmic movement have been reported to be modulated by circadian rhythms. So are cyclic AMP and oleic acid contents. Numerous chloroplast activities and several chloroplast molecules contents also display circadian rhythms. Moreover, circadian rhythm parameters have been found to vary according to their localization along the alga. Growth and differentiation are regulated by external signals (mainly light), whose transduction involves the calcium-calmodulin complex as secondary messenger. Auxin also has a regulatory role to play. There is evidence that the different steps involved in light reception and transduction display a circadian rhythm in susceptibility which peaks during the first part of the dark phase. Spatio-temporal organization at the cellular level is critical for morphogenesis.
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