Plastid

Plastids are a class of organelles found in plant and algal cells. They may differentiate into several forms depending upon which function they need to play in the cell. In plants, undifferentiated plastids are called proplasts and may develop into any of the following plastids:

• Amyloplasts: for starch storage
• Chloroplasts: for photosynthesis
• Etioplasts: chloroplasts that have not been exposed to light
• Chromoplasts: for pigment synthesis and storage
• Leucoplasts: for monoterpenes synthesis

In algae, the term leucoplast or leukoplast is used for all unpigmented plastids. Its function may differ from that in plants. Etioplast, amyloplast and chromoplast are plant-specific terms.

Plastids are thought to have originated from endosymbiotic cyanobacteria. Due to a split-up into three evolutionary lineages, the photosynthically active (and ancestral) plastids are named differently: chloroplasts in green algae and plants, rhodoplasts in the red algae and cyanelles in the glaucophytes. The plastids differ by their pigmentation, but also in ultrastructure. The chloroplasts e.g. have lost all phycobilisomes, the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but - only in plants and closely related green algae - contain stroma and grana thylakoids not found in the other groups. The glaucocystophycean plastid - in contrast to the greens and the red algae - is still surrounded by a remains of the cyanobacterial cell wall. All these primary plastids are surrounded by two membranes in the cells.

Complex plastids originate from a secondary endosymbiosis (i.e. a eukaryote engulfs a red or green alga) and are surrounded by more than two membranes. Algae with complex plastids derived from a secondary endosymbiosis event with a red alga are the heterokonts, haptophytes, cryptomonads, and most dinoflagellates. Those with endosymbioses with green algae are the euglenids and the chlorarachniophytes. The Apicomplexa also have complex plastids, which stopped photosynthesis and turned into leucoplasts. These plastid is extremely reduced and it is not yet clear whether they are derived from red or green alga.

Kleptoplastids are found in some dinoflagellates. The dinoflagellates take up algae as food. They profit from the photosynthesis of their prey for a while, but then digest the alga.