Classification and Characteristics of Protoctists

The protoctista contains eukaryotes that are generally regarded as identical or similar to the ancestors of modern plants, animals and fungi. It includes organisms which resemble early plants, early animals and early fungi. It also includes a group known as the slime moulds which produce spores like fungi but can creep slowly over surfaces and are therefore motile like animals. The earliest eukaryotes were probably unicellular organisms which moved by beating flagella.

The group is fascinating to those interested in evolution because these organisms are the link between prokaryotes and the more modern eukaryotes like plants and animals. For example, during the 1960s it was discovered that mitochondria, the powerhouses of cells that provide energy in aerobic respiration, contained their own DNA and now good evidence, based on examination of the base sequences in the mitochondrial DNA, that mitochondria were formerly aerobic bacteria that invaded an ancestral eukaryotic cell and learned to live symbiotically within it. Now all eukaryotic cells contain mitochondria, and the mitochondria can no longer live independently.

Like mitochondria, chloroplasts, the chlorophyll-containing organelles responsible for photosynthesis, also contains their own prokaryotic DNA and ribosomes. These seem to have evolved from photosynthetic bacteria which invaded heterotrophic animal-like cells, turning them into algae which are autotrophic. It is also likely that red algae may have evolved in this way from blue-green bacteria and that green algae evolved from green bacteria known as prochlorophytes.

The theory that mitochondria and chloroplasts are the descendants of symbiotic bacteria is known as the endosymbiont theory. An endosymbiont is an organism that lives symbiotically inside another organism.