Question

Due to their unique combinations of characteristics, the protists cannot really be divided among the categories of plant or animal. Provide an example to illustrate this point and explain why such a division is misguided from a phylogenetic standpoint.

Answer 1

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Protists are eukaryotes, which means their cellshave a nucleus and other membrane-bound organelles. Most, but not all, protists are single-celled. Other than these features, they have very little in common. You can think about protists as all eukaryotic organisms that are neither animals, nor plants, nor fungi.

Although Ernst Haeckel set up the Kingdom Protistain 1866, this kingdom was not accepted by the scientific world until the 1960s. These unique organisms can be so different from each other that sometimes Protista is called the “junk drawer" kingdom. Just like a junk drawer, which contains items that don't fit into any other category, this kingdom contains the eukaryotes that cannot be put into any other kingdom. Therefore, protists can seem very different from one another.

Unicellular or Multicellular?

Most protists are so small that they can be seen only with a microscope. Protists are mostly unicellular (one-celled) eukaryotes. A few protists are multicellular (many-celled) and surprisingly large. For example, kelp is a multicellular protistthat can grow to be over 100-meters long (Figurebelow). Multicellular protists, however, do not show cellular specialization or differentiation into tissues. That means their cells all look the same and, for the most part, function the same. On the other hand, your cells often are much different from each other and have special jobs.

diverse group of organisms in this classification system is found among the Protists. This group contains plant-like, animal-like, and fungus-like organisms. This grouping is not based on evolutionary relationships but is more a grouping of convenience — the protists are the eukaryotes that aren’t plants, animals, or fungi. This may seem to violate the precision that seems important in a classification system, but this is what has been used historically and is commonly taught in K-6 classrooms and beyond. There will likely be changes in this grouping as the science of classification progresses.

Characteristics of Protists

A few characteristics are common between protists.

1. They are eukaryotic, which means they have a nucleus.
2. Most have mitochondria.
3. They can be parasites.
4. They all prefer aquatic or moist environments.

Classification of Protists

For classification, the protists are divided into three groups:

1. Animal-like protists, which are heterotrophs and have the ability to move.
2. Plant-like protists, which are autotrophs that photosynthesize.
3. Fungi-like protists, which are heterotrophs, and they have cells with cell walls and reproduce by forming spores.

But remember, protists are not animals, nor plants, nor fungi

The majority of organisms classified as protists are unicellular though there are a few multicellular organisms. For example, kelp (“seaweed”) is technically a protist even though it is multicellular. Kelp is not grouped with plants, however, because it lacks the cellular complexity present in plant cells. There are also “colonial protists,” which are aggregates of individual cells of the same species that function together as a group.

Plant-like protists make their own food, and many have cell walls made of cellulose. Animal-like protists cannot make their own food and ingest it. Some animal-like protests have “shells,” called tests, which are made of silica or calcium carbonate. Other animal-like protists lack a cell wall or protective test. The fungi-like protists are actually not evolutionarily related to fungi, although they do take in food by absorption. Like plants, however, their cell walls are made of cellulose.

Animal-like protist (amoeba)	Plant-like protist (Volvox)	Fungi-like protist (slime mold)

How are protists important to people?

Animal-like protists include some well-known species. Paramecia and amoeba are frequently used examples of single-celled organisms. Malaria, a world-wide disease occurring in tropical climates, is caused by an animal-like protist, the Plasmodium. In the ocean, many plant-like protists live at the surface where they perform photosynthesis. These organisms, such as diatoms, dinoflagellates, and foraminiferans, serve as a food source to many planktonic, or floating, animals. However, these plant-like protists are not always beneficial. Red tide is caused by a population explosion in poisonous dinoflagellates and contaminates the ecosystem, killing fish and making shellfish unsafe for human consumption. The fungus-like protists are commonly known as slime molds, and include the brightly colored organisms found growing on decomposing trees and the pesky mildew growing in a bathroom.