1

FIGURE 19.04 Primary versus Secondary Endosymbiosis

Primary endosymbiosis yields organelles with two membranes. In this example, the original independent cyanobacterium has a cytoplasmic membrane, which is retained, and an outer membrane, which is lost during symbiosis. When the two cells associate, the host cell cytoplasmic membrane surrounds the cyanobacterium, which is therefore left surrounded by two membranes. In contrast to primary endosymbiosis, secondary endosymbiosis occurs when an ancestral host cell engulfs a photosynthetic eukaryotic alga. The alga already has a chloroplast with two membranes as well as a nucleus and other organelles. Since the host cell only needs the energy from the chloroplast, the other captured organelles degenerate and eventually disappear. However, the membranes often remain and the chloroplast is left with four membranes, rather than two.

The malarial parasite contains degenerate non-photosynthetic chloroplasts that are involved in synthesizing fatty acids.

single-celled eukaryote, known as Plasmodium. The malaria parasite and other related single-celled eukaryotes are members of the phylum Apicomplexa. Although these single-celled eukaryotes live inside humans and mosquitoes, far from the sunlight, they possess plastids as well as mitochondria. These plastids are degenerate, non-photosynthetic chloroplasts that contain a circular genome. In Plasmodium this plastid DNA is 35 kb and encodes rRNA, tRNA and a few proteins, mostly involved in translation (Fig. 19.05).

Apicomplexa Phylum of parasitic single-celled eukaryotes that contain both mitochondria and degenerate non-photosynthetic chloroplasts Plasmodium The malaria parasite, a protozoan belonging to the Apicomplexa rpo C

rpo B

FIGURE 19.05 Plastid Genome of Plasmodium

The circular genome of Plasmodium has genes for rRNA, tRNA and protein synthesis. The tRNA genes are denoted by the single-letter amino acid code, for example, S for the tRNA for serine.

rpo C

rpo B

FIGURE 19.05 Plastid Genome of Plasmodium

The circular genome of Plasmodium has genes for rRNA, tRNA and protein synthesis. The tRNA genes are denoted by the single-letter amino acid code, for example, S for the tRNA for serine.

LSU rRNA

SSU rRNA

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