Plant cell organelles primary

If the mitochondrion is a principal site of the PDC, hence the formation of acetyl-CoA, then the plant cell must employ a transport system which can export this highly reactive substrate from the matrix of the mitochondrion to the appropriate site where it is utilized for fatty acid biosynthesis. However, this problem may not be serious, since in seeds that contain high levels of lipid at maturity the proplastid appears to be the principal site of the conversion of pyruvate to acetyl-CoA by the PDC and of its efficient utilization for the biosynthesis of palmitoyl-and/or stearoyl-ACP. In the chloroplast, evidence for the PDC is not clear, although indirect evidence does suggest PDC activity (Murphy and Leech, 1978) in these organelles. Since acetate moves freely across the chloroplast membrane and since acetyl-CoA synthetase occurs in the chloroplast stroma (Jacobson and Stumpf, 1972), it is possible that the primary source of acetyl-CoA derives from its synthesis at a site other than the chloroplast, its transport into the chloroplast as the undissociated acid and/or free anion, and the conversion of acetate back to acetyl-CoA by the stroma acetyl-CoA synthetase. Further work is necessary to clarify this important point. It is difficult to explain the function of acetyl-... 

Early botanists referred to the obvious structural role of walls as forming little boxes around the cells. This was most apparent in the substantial thickenings of secondary walls in mature tissues. However, over the last few deeades an appreciation has developed for the complexity and multi-functional nature of cell walls. The primary wall is now regarded as a complex, dynamic organelle rather than an inert extraeellular matrix. It is now apparent that cell walls influence, and are integral to, many aspects of plant growth and development. 

Most of the plant polysaccharides that have been described have not been obtained from isolated cell walls, but rather from other plant organelles and tissues. We beUeve that in order to describe the structure of primary cell walls, it will eventually be necessary to study polysaccharides that have been isolated directly from primary cell walls. This review will comprehensively consider all those polysaccharides which have been isolated from primary cell walls. This review will also consider a selection of those studies of plant polysaccharides which have been isolated from sources other than primary cell walls, but which are thought to resemble primary cell wall polysaccharides. 

Arabans have been isolated from the cell walls of many dicotyledonous plants. Until recently, no homo-araban has been isolated specifically from primary cell walls. However, methylation analysis of the walls of suspension-cultured sycamore 123) and pea (55) cells strongly suggested that these primary cell walls possess arabans which are structurally similar to arabans obtained from other tissues or organelles. Recently, an araban, essentially free of other polysaccharides, has been isolated from a methylated sycamore cell wall polysaccharide fraction 5). 

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