THE ENDOSYMBIOTIC ORIGIN OF CHLOROPLASTS

This chapter describes endosymbiotic origin of chloroplasts. The chloroplasts that cyanobacteria most closely resemble, and whose ancestry is in consequence, are those of the red algae. The chloroplasts are only semiautonomous. They lack a cell wall and have a reduced prokaryotic type of genome, which is present in multiple copies. The chloroplast stroma with its thylakoids is the homolog of the cyanobacterial cytoplasm but, although photosynthesis is maintained as the principal metabolic function, many other functions have been lost. Cyanobacteria and red algal chloroplasts both have chlorophyll a as the primary photosynthetic pigment and phycobilins as the main accessory pigments. The chloroplasts are enclosed within an envelope of two membranes. The chloroplast stroma contains strands of prokaryotic-type DNA and ribosomes that are of similar size (70S) to those in cyanobacteria but smaller than those found in the cytoplasm of eukaryotes (80S). Ultrastructural studies carried out on the chloroplasts of cryptomonads are largely responsible for the introduction of the hypothesis that some chloroplasts also evolved from eukaryotic algal endosymbionts. © 1993, Elsevier Science Publishers, B.V.