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The plastids. In the young plant cells, pro-plastids whose structure is comparable to that of mitochondria develop to give differentiated plastids. the chloroplasts show a grainy structure (figure 19.8). The wall consists of two membranes: one external and one internal. The internal membrane gives rise to crests and lamellae. In the numerous points of a chloroplast, sacculi or green discs are observed piled one on another. Each pile forms a granum. The numerous grana form chains at the interior of the plastid. In the disc membranes there are structural units called the quantasomes. A quantasome, as defined by Arnold in 1963, is the smallest photosynthetic unit, which embodies the minimum number of chlorophyll molecules necessary for the production of photosynthetic reactions.
Figure 19.8a Chloroplast:
1, 2 - external and internal membranes, 3 - thylakoids, 4 - granum, 5 - DNA, 6 - ribosomes, 7 - starch, 8 - stroma, 9 - lamella, 10 - lipidic granule.
Figure 19.8b Chloroplast
Figure 19.8c Chloroplast (electronic micrograph)
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| Figure 19.6 The peroxisomes (electronic micrograph) | | | Figure 19.8d Chloroplasts in the plant cell |