Plastids

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Upon seeing a myriad of colorful tents, I thought that I was at a carnival or an animal circus! But I was mistaken once again. It turns out that I am only at a camping site next to a small canal. Woah! What are these colors? No, I’m not talking about the tents. I think I saw a flash of green, or maybe something orange. Was it a leaf? It must be the next organelle that I must find, a plastid! See if you can find me or the plastid (a chloroplast) first!

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Plastids are unique in that they are enclosed in their own membrane layers and contain their own DNA sequence. It is hypothesized that plastids, similar to mitochondria, were once prokaryotes that were engulfed by the would-be eukaryote cell and merged in a form of endosymbiosis. Gradually, plastids became dependent organelles incapable of living outside the environment of the cell. Plants and algae both contain plastids, but animal cells do not contain plastids.

Structure

The specific structure may differ from plastid to plastid, but a plastid generally has multiple layers of membranes–usually an inner membrane and an outer membrane. Chloroplasts additionally feature a thylakoid membrane. Plastids do not contain organelles or nuclei, however, but instead have circular, single-stranded DNA like a prokaryote.

Function

Some plastids are responsible for photosynthesis, a process that builds organic molecules from H2O and CO2, providing energy for primary producers. Other plastids contain pigments, giving plants and fruits their distinctive color. Still others are used for storage and the synthesis of fatty acids, amino acids, and other compounds.

The chloroplast is perhaps the most important and well-known plastid of all. It exists abundantly in the cells of plant leaves and is crucial to photosynthesis. Moreover, its green pigment gives plants their characteristic color. On the other hand, some of these other plastids may sound less familiar: amyloplasts, chromoplasts, elioplasts, and proteinoplasts. The groupings below may help explain…

  • Leukoplast: usually specializes in storage. An amyloplast, for example, stores amylose (a type of starch) in plant roots and tubers. Amyloplasts, elaioplasts, and proteinoplasts are subcategories of leukoplasts.
  • Chromoplast: contains pigments, especially carotenoids, that give fruits and plants their yellowish hue
  • Chloroplast: link to chloroplast page

Amyloplast; Psilotum nudum is a fern-like vascular plant

Chromoplasts from red bell pepper

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