Scientists used to lump protists into a single kingdom, and they still use this classification for some purposes. However, science has largely recognized that the taxonomic grouping known as Kingdom Protista actually includes a wide range of organisms that are not particularly related. Biologists are currently in the process of revising their classification to reflect the evolutionary relationships among this massive set of organisms.
TL;DR (Too Long; Didn't Read)
Biologists used to classify all protists as part of Kingdom Protista, but almost every rule used to describe them has some major exception. They are now attempting to revise the classification of the organisms in this kingdom. Some scientists have tried to sort protists in comparison to three of the other kingdoms: plant, animal and fungus. This has not worked because many protists exhibit characteristics from two or more of these kingdoms. The current attempts of classification focus on grouping protists into between three and ten kingdoms, based on species’ common evolutionary ancestors.
Mind-Boggling Diversity
The protists exhibit considerable diversity. Almost every rule used to describe them has some major exception. Some protists live as parasites, others as predators and still others as producers. Some have rigid cell walls, while others have more flexible cell membranes. Their methods of movement include passive drifting, swimming with flagella, swimming with cilia and creeping along with pseudopods. Even certain very basic criteria that have been used to define the group, such as the presence of nuclei and mitochondria, either don't exist or take on bizarre forms in some protists.
Cross-Kingdom Characteristics
Scientists have tried to classify the organisms within the protists as either plant-like, fungus-like, or animal-like. However, genetic testing and close examination have revealed that these categories often don't hold up, either. For example, Euglena have characteristics of both plant-like and animal-like protists. Euglena have chloroplasts like plants, in the sense that the chloroplasts allow them to gain energy from the sun through photosynthesis. At the same time, they have a tail or flagella which they use to swim, making them mobile, a very animal-like characteristic. Many other protists also have characteristics that make it difficult to justify keeping them all in a single group or subgroup.
Attempts at Sorting
Scientists have begun to use new criteria to sort protists. In fact, the protists can be sorted into between three and ten proposed kingdoms, depending on which researchers are doing the sorting. Scientists are trying to create these groups based on evolutionary relationships. The goal in forming kingdoms is to group all the descendants from a common ancestor into one group.
No Perfect Rule
Treating the protists as a single group can work in certain areas of biology. For example, it might not matter in medicine which specific protist kingdom is causing an infection in a patient if the treatment for all protist infections is the same. The only real constant rule is that all protists are eukaryotes, meaning that they are organisms with more complex cells than bacteria. Most protists have a single independent cell, though some seaweeds break this rule. Most protists have a single defined nucleus holding the bulk of their DNA, though some ciliates have multiple nuclei and flagellates do not have a defined nucleus. As a group, it is difficult to find a definition that describes all protists perfectly.