Plastids Are Organelles Unique to Plant Cells and Algae

Most of the biosynthetic activities in plants (including CO2 assimilation) occur in plastids, a family of self-reproducing organelles bounded by a double membrane and containing a small genome that encodes some of their proteins. Most proteins destined for plastids are encoded in nuclear genes, which are transcribed and translated like other nuclear genes; then the proteins are imported into plastids. Plastids reproduce by binary fission, replicating their genome (a single circular DNA molecule) and using their own enzymes and ribosomes to synthesize the proteins encoded by that genome. Chloroplasts (see Fig. 19-38) are the sites of CO2 assimilation. The enzymes for this process are contained in the stroma, the soluble phase bounded by the inner chloroplast membrane. Amyloplasts are colorless plastids (that is, they lack chlorophyll and other pig-

Iodin Starch Granules Leaf
FIGURE 20-2 Amyloplasts filled with starch (dark granules) are stained with iodine in this section of Ranunculus root cells. Starch granules in various tissues range from 1 to 100 ^m in diameter.

ments found in chloroplasts). They have no internal membranes analogous to the photosynthetic membranes (thylakoids) of chloroplasts, and in plant tissues rich in starch these plastids are packed with starch granules (Fig. 20-2). Chloroplasts can be converted to pro-plastids by the loss of their internal membranes and chlorophyll, and proplastids are interconvertible with amyloplasts (Fig. 20-3). In turn, both amyloplasts and proplastids can develop into chloroplasts. The relative proportions of the plastid types depend on the type of plant tissue and on the intensity of illumination. Cells of green leaves are rich in chloroplasts, whereas amylo-plasts dominate in nonphotosynthetic tissues that store starch in large quantities, such as potato tubers.

The inner membranes of all types of plastids are impermeable to polar and charged molecules. Traffic across these membranes is mediated by sets of specific transporters.

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  • steven
    What organelles are unique to plant cells?
    2 years ago
  • eleleta
    Can plastids reproduce by binary fission?
    1 year ago

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