Protists are not a closely related group. Rather they are grouped together because of common characteristics and because of what they are not. One way to define protists is as all the organisms that have a nucleus in their cells but are not animals, plants or fungi.
The name protist comes from a Greek word that means primary. It was given to this group because the protists were once thought to have been the first primitive organisms with a nucleus in their cells. Most protists are single-celled or simply structured. It turns out, though, that some protists are descended from complex organisms and are not primitive at all.
Types of cells
Cells are the foundation of life. They come in two varieties-eukaryote and prokaryote. Eukaryote cells are much larger and more complex. They developed from the prokaryotes. Prokaryote cells have no nucleus. Eukaryote cells have a true nucleus surrounded by a double membrane. Bacteria and archaea are prokaryotes. Plants, animals, fungi and protists are eukaryotes.
Organisms are grouped into clades according to their genetic relationships. A clade is a group of organisms that share a common ancestor. Classification is made through statistical and genetic analysis. Each clade is a distinct branch on the tree of life.
Taxonomy-the defining and naming of groups of organisms-was originally based primarily on appearance. Carl Linnaeus is considered the father of modern taxonomy, because he clarified the classifications of Aristotle and others in 1735.
In his time, and for long after, it was impossible to trace genetic relationships. Although it is easy to see that house cats and leopards are related, the science of genetics is needed to discover the relationships between some organisms, and also to discover when apparent relationships do not really exist.
The protists were grouped together because they seemed alike, in the way that all plants, animals, and fungi resemble one another. Each group was considered to be a Kingdom. However, the genetic makeup of protists shows that many are not nearly as similar as one plant or animal is to another.
The protists are still considered a Kingdom, but one that was lumped together as a catch-all and contains members that can be as different from one another as plants are from animals.
Some of the protists
Deadly malaria is caused by a parasitic protist, and so is sleeping sickness. On the other hand, the sea palm and kombu are two edible species of kelp, a protist.
The slime molds are a group of protists that were once considered fungi. They live as single cells or in aggregations according to circumstance, and specialize in decomposition.
Some amoebas can cause disease, but many are innocent and interesting water creatures, famous for their floppy, flowing shape under the microscope.
The blight that caused the potato famine in Ireland in 1845 was caused by a protist, Phytophthora infestans. It drove many Irish people to the United States to start new lives.
Most protists are single-celled microscopic organisms, but not all. Off the coast of California, and in other places, protist forests of giant kelp stretch from the ocean floor to the surface, sheltering one of the most rich and beautiful ecosystems on earth.