Cellular location of starch synthesis and evolutionary origin of starch genes

  • Raven J
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Polysaccharides are major energy- and organic-car- bon-storage materials in a very wide range of organ- isms. These compounds represent a compact means of storage: 1mole of organic C in starch occupies 0.02L, while 1mole organic C in a solution of the disaccharide sucrose, even when at such a high concentration as to be isosmotic with seawater, occupies 0.11L (computed from data in Weast 1969/1970 and Boyd and Grad- mann 2002). A polysaccharide storage compound is more eco-physiologically flexible than a soluble low- molecular-mass compound, because variations in the content of polysaccharide energy storage compounds have a much smaller effect on the osmotic and volume regulation of cells, or on the content of compatible sol- utes, than do variations in a soluble compound. At the other extreme, lipid provides a greater mass-based and, despite a lower density, volume-based content of energy and organic carbon storage than do polysaccha- rides (Schmidt-Nielsen 1983). However, mobilization of stored lipid is biochemically more complex than that of polysaccharides (Hochachka and Somero 1984), es- pecially when the lipid is used for growth with a re- quirement for sugars and hence glyoxylate cycle activity. Furthermore, stored lipid is less readily used to support anaerobic metabolism, although this may not be as great a problem for most algae as for certain other organisms. Thus, the widespread occurrence of polysaccharides as organic carbon and energy stores can be readily rationalized, although we must beware of ‘‘facile adaptationism’’ and so must not think that the natural selection significance of these storage com- pounds has been subjected to the required experi- mental falsification.

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  • John A. Raven

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