Rebecca Levenson
external image the_fish.jpg Copyright 2002, Microbus, All rights reserved

Diagnostic characteristics:

(15 BMB) An example of protists with cilia
(15 BMB) An example of protists with cilia

Although there are organisms that do not fit into the category of Protista, there really is no obvious set of diagnostic characteristics that defines this group. The protist group consists of organisms that do not fit into any of the other groups. Protists can be single-celled organisms, colonial organisms, meaning each cell has to get its own food, or multicellular, which differs from colonial organisms because some of the cells perform different tasks for the overall benefit of the organism. Althought there are many exceptions, the most common protist is single-celled. Protists do not have specialized cells to form particular tissues with separate functions (SES 13). They can be either photosynthetic, meaning they make their own food, heterotrophic, meaning they eat other organisms to get nourishment, or mixotrophic, where the organism is both photosynthetic and heterotrophic. They can be motile, having the capability to move around, or stationary, because they cannot move. This may be because of the presence or lack of flagella or cilia, which are pieces of the membrane which keeps the cell together that help it to move. The following video that is taken from is just a sampling of some of the many kinds of organisms that are considered protists. Although the types of organisms considered protists are diverse, all protists are eukaryotic, which means that their cells and the organelles, which are pieces that make up the cell, are enclosed in a membrane, uniting all of the protists in at least one sense. This may seem like an insignificant characteristic, but in fact it would mean that prokaryotic cells, or those that are not surrounded by a membrane, are not protists, helping Protista to define its boundaries (1).

Because Protista is the most diverse kingdom, most scientists have split it into smaller units called subkingdoms. Others have gone so far as to say that Protista is not a kingdom and that these subkingdoms should replace Protista. Systematists, which are scientists who work on classifying animals, have split the Protista kingdom into as many as twenty different kingdoms. Because the organisms of Protista share a common ancestor, however, we can call Protista paraphyletic, to use the word that systematists would use (1). (Although a paraphyletic group of organisms are descended from a common ancestor, the group does not include all of the organisms that are descended from that ancestor.)

Major types:

The three majors types of organisms in the Protista Kingdom, to use the inclusive term, are ingestive protists, or protozoa, absorptive, or fungus-like, protists, and plantlike protists, or alg ae. The following list, however, is based on how scientists might group some protists together into subkingdoms, like the ones described in the diagnostic characteristics of Protista (1). Particularly, protozoa are believed by many scientists to be one of the most key players in a microbial community. They are known to be the principal hunters and grazers of bacteria, algae, and other microbes, and they are also an important food source for larger creatures and are the basis of many food chains. Some protozoa are naturally found in parts of the guts of some insects and mammals, helping to break down complex food particles into simpler molecules during digestion. A very small number of species cause disease in people; for example, Plasmodium vivax can cause malaria. (7) (SS)

Diplomonadida and Parabasala:
These two groups of protists lack mitochondria, which are organelles in a cell that control cell respiration and release energy to power the cell, or the ability of the cell to change oxygen into carbon dioxide. Both diplomonads and parabasalids have flagella, allowing them to move. Diplomonads have two separate nuclie, a simple cytoskeleton (the tubes that control the inner workings of a cell) and lack plastids (plant organells.) Giardia lambia, an example of a diplomonad is a parasite that is often found where human feces contaminate water, and causes bad cases of diarrhea and cramps because it infects the intestines of the human. Parabasalids include trichomonads which inlcude species like Trichomonas vaginalis which can infect the vagina or urethra of humans.(SV)
These protists are characterized by the pockets, or anterior pockets where their flagella come from. Also, euglenoids have a storage molecule called a glucose polymer, which is called paramylon. Most euglenoids are photosynthetic, but some may be mixotrophic or heterotrophic. Included in this subkingdom are kinetoplastids, which are recognized by their kineoplast, which holds some extra DNA.
Although this group contains flagellates (organisms with flagella), parasites, and ciliates (organisms which use cilia to move), they are united by the characteristic of having alveoli, which are small cavities under the surface of their membranes (1). Biologists are unsure of the function of the alveoli, but there is speculation that they form a part of a complete inner membrane system and also that they aid in ion transport and structural stabilization of the cell membrane (2) (DP).

A formal name for the group containing flagellates is the dinoflagellates. They make up a large population of organisms called phytoplankton. Phytoplankton includes algae and photosynthetic bacteria (an organism is photosynthetic if it makes food for itself using light energy from the sun). They are the main producers (organisms that make their own food, for example, through photosynthesis) for many marine and freshwater food webs. Most dinoflagellates are unicellular (having only one cell), but some are colonial. Dinoflagellates have plates of cellulose that give them a specific shape (cellulose, often found in plants, is a hard material made of sugar molecules). Some dinoflagellates are bioluninescent, meaning that they give off light because of biological processes within them. Red tides are caused by large population growths of dinoflagellates, which may release toxic substances.The red color is caused by red pigments in the plastids of dinoflagellates (1). [HZ]
T014322A.jpg(This is an image of a dinoflagellate that have a whiplike structure. YS)
This group, although it does have a fancy name, could also be called the brown and golden algae group. This group got its name because of the fine hairs on the flagella which help them to move called stramenopila ("stramen" means flagellum in Latin, and "pilos" means hair). This group also contains planktonic diatoms and water molds, such as the pathogen that infected potatoes and was at the root of the infamous Potato Famine. (16 Nangia)
This group name could also be simplified to be red algae. Red algae have no flagella, and despite their name, they are not always red in color. Most red algae are multicellular, although none are as big as the giant brown kelps.
This is the green algae group, and much like plants, green algae has chloroplasts which give them their green color and help the cells do photosynthesis. A good example of a chlorophyte is the one at the top of the page.

Mycetozoa, or slime molds, may seem to have similar characteristics to those of the fungi kingdom, but they belong to Protista. Even though they have similar ways of doing things, their actions are similar, or analagous, but not the same, or homologous. Basically, slime molds form a multicellular organism for part of the time, and when they need to feed, they change into a "puddle" of cells that all eat by themselves (1).

The white sporangia of a slime mold, which, although it resembles the Kingdom fungi, is actually a protist. (ER)(13)
The white sporangia of a slime mold, which, although it resembles the Kingdom fungi, is actually a protist. (ER)(13)

external image fuligo_septica.jpg
This is a picture of slime molds, which are like the fungi in that they have a similar life cycle. Under unfavorable conditions, they form sporangia like fungi, which form spores (these can grow into new organisms) (11) [HZ].

Three different types of Protista
(HS 5)


The habitats that protists live in vary greatly depending on the type of protist in question. However, most protists are aquatic, meaning they live in the water or near to where there is water. This includes any bodies of water, like ponds, or oceans, moist places, like damp soil or under a rock, and some live in the body fluids of another organism, including its cells (1). Protists living within the intestinal tracts of other organisms perform vital digestive processes that help the organism through the process of digestion (SES 13).
external image
external image
This is an image of the Great Salt Lake, which is the home to many floating species of algae in the prostista kingdom. (GR, 12).

Basic anatomy:

The basic anatomy of an organism that is considered a protist differs depending on the type of protist that you are talking about. Most of these cells are unicellular, so their anatomy may seem to be basic in comparison to multi-cellular organisms. But this is not completely true. Each cell has to be able do all of the functions within its minute boundaries that specialized cells perform as a unit (1).
Most protists contain several mitochondria and plastids (3). Protists, like eukaryotic cells, have membrane bound organelles. Since different kinds of protists have different kinds of environmental adaptations and live in a variety of habitats, the anatomical structures used to make energy vary dramatically. Some protists perform photosynthesis and therefore have plastids. Plastids are membrane bound organelles that help convert sunlight into chemical energy. The color the plastids will determine the color of the protists. For example, a large colony of protists with green plastids will appear green (4, NK).

Transport of materials:

Because the majority of protists are single-celled organisms, each cell has to be able to acquire its own food from the surrounding environment, so transport is not necessary for most protists. This is also true in colonial organisms, where each cell has to be able to get its own food, whether it be through photosynthesis or phagocytosis, which is the enveloping of single-celled around another single-celled organism to eat it. After a protist acquires its food through phagocytosis a food vacuole (a food-sac with a membrane) is created. Then the food vacuole pairs with a lysosome (an organelle that is responsible for digestion) and digests the food for the protist. (Jesse Carmen) [6] This happens when the membrane surrounding the cell stretches out, to form a sort of mouth, and drags the cell inside of the original one so that it can break it down into a usable form of energy (1. The firgure below is an example of this.(12)(SJB)


Because the majority of protists are uni-cellular, or single-celled, protists tend to produce asexually, meaning a cell will just split to produce another organism like itself. However, if the organism just kept making copies of itself and it was not immune to a certain disease, an entire population of that type of organism could be wiped out. That is why, even in asexual reproduction, the organism has to have some kind of genetic shuffling, or what scientists like to call conjugation.
Even though most protists are unicellular, they can still reproduce sexually as well. The processes that they use are called meiosis and syngamy. Meiosis is the splitting up of a set of paired chromosomes in a cell's nucleus into two sets of unpaired chromosomes. When the cell splits into two cells with unpaired chromosomes, it is called a haploid cell, meaning it has half the number of chromosomes. The chromosomes will be paired up again once one of the new haploid cells finds another haploid cell and joins it through the process of syngamy (1).
As seen above, is a mating reaction between two protists. There are several ways in which reproduction can occur. In isogamy, mobile gametes (haploid chromosomes used in reproduction) of the same size fuse. In anisogamy, mobile gametes fuse, but the female's is larger than the males. In oogamy, there are larger, non-mobile female gametes that meet with smaller mobile male gametes (8) AR.

Environmental adaptations:

If most protists have to live in or around water, how can there be any protists that live outside of the water, like when the tides go out? That is an interesting question, because how can seaweed, without real roots or any cellulose in its membrane to help keep it upright, even stand up? Although a seaweed does not have true roots, it does have something called holdfast, which holds the thallus, or the plantlike part of the seaweed, in place.The stipe, which is a stem-like structure, holds up the blades, or the leaves of a seaweed. Even though the seaweed is not a plant, it has adapted some similar characteristics in order to survive in its environment (1).
Because algae are autotrophic organisms, at least one point in their lifetime, there are several environmental adaptions they have in order to make sure they can do photosynthesis (9, DJ). Algae can only use photosynthesis with enough light, and algae can only get sunlight if they remain in the photic zone of the water (this is not possible if algae sink) (9, DJ). If algae sink, they will die if current or another force brings them up back into the sunlit photic zone (9, DJ). To ensure that the algae stay afloat, there is an environmental adaptation of high surface to volume ratio (9, DJ). The algae are smaller, store lipids (light storage compounds), and have surface projections. In addition, algae release mucilage, a low density excretion, which slows down the sinking rate (9, DJ)
external image Chlorococcum_Key108.jpg
The image displays the thin mucilage around Chlorococcum (a type of algae). (14 VM)

Review Questions:
1. What causes red tides? [OZ]
2. Explain Protista clssification. Do taxonamists all agree when it comes to classifying protista? In your opinion, does the 5-Kingdom or 3-domain system better fit the classification of protista? (RK)
3) What are the adaptations that some protists posess that allow them to survive out of water? (KN)
4. What do protists have structurally that differentiates them from eubacteria and archaea? (JAC)
5. What is the main habitat of protist organisms? (RJS)

1. Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.
2. "Alveolata." Palæos. 1 Dec. 2008 <>.
3. "II. Anatomy and Physiology." Protista. 3 Dec. 2008. <>
4. Hinkle, Gregory. "Protista." 14 Apr. 2008. 3 Dec. 2008 <>.
(5) "Kingdom: Protista." 2004. 3 Dec. 2008 <>.
[6] Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.
7. "Protozoa." Microbe World. American Society for Microbiology. 7 Dec. 2008 <>.
8. "Kingdom Protista." 2008. Botany 1050. 7 Dec, 2008. <>.
9. Simmons, Kent. "PROTISTS AND THE ORIGIN OF EUKARYOTES." Evolution, Ecology and Biodiversity. 2008. University of Winnipeg. 7 Dec. 2008 <>.
11. "Introduction to Slime Molds." University of California Museum. 14 Dec 2008 <>.12.
12. "The Great Salt Lake : Algae." 1998. 18 Dec. 2008 <>.13.
13. "Introduction." Protista. 1997-2008. Microsoft Encarta Online Encylopedia. 18 Dec. 2008. <>
14."Green Algae." 18 Dec 2008. <>
15. "Kingdom Protista." 18 Dec. 2008 <>.
16. "Protists II - Kingdoms Stramenopila, Rhodophyta, and Chlorophyta." Penn Sate. 19 Dec. 2008 <>.

Page Edited by: DP, Sarah Vlach, NK; Hilary Stepansky, Jesse Carmen, Hanna Z; SS, AR, Daisy Joo, Rachel Kornetsky, Kevin Nayer, Josh Czik,Sam Blatchford, GR, Ethan Richman, Vonai Moyo, Sarah Schwarzschild, Brittany Marcus-Blank, Meru Nangia