Some members of Kingdom Protista are unicellular, others are colonial, and yet others are multicellular. Note that in the colonial forms, all the cells are similar with similar, generalized functions, whereas in the truly multicellular species, the “body” of the organism consists of a variety of types of cells, each type with its own specialized function. These organisms are all eukaryotes (they have a true nucleus). They all need some kind of a water-based environment — which can be fresh or marine water, snow, damp soil, polar bear hairs — in which to live. All are aerobic and have mitochondria to do cellular respiration, and some have chloroplasts and can do photosynthesis. Most of them reproduce or grow by mitosis, and some reproduce by meiosis and fertilization. Many can form cysts in adverse conditions. Protists are a major component of plankton.

Protists are grouped into three major, unofficial categories based on means by which they obtain nutrition. These are the Protozoa, the Algae, and the Fungus-like Protists. For some reason, botanists use the word “Division” to mean the same taxonomic level as “Phylum”, and since, way back everything was lumped in as either a plant or an animal, taxonomists who study Kingdom Protista (and those who study Kingdom Fungi) also still use the word “Division” to mean “Phylum”, so for example, when “Division Rhizopoda” is listed below, that means the same thing as saying “Phylum Rhizopoda”.


These protists are animal-like, especially in their nutrition. They ingest their food by phagocytosis. Some have mouth-like structures into which the prey are put while others use pseudopodia to move and to engulf prey. Typical prey include bacteria and other smaller one-celled organisms.

Division Rhizopoda:
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Amoeba Movie
An example of a member of this Division is genus Amoeba, a fresh-water dweller. Protists in this group are unicellular and have pseudopodia. Some secrete shells around themselves, while others do not. None of them have flagella, cilia, or meiosis. Entamoeba histolytica is a parasitic form that causes amoebic dysentery. These colonize the colon and feed on bacteria, causing symptoms that range from mild diarrhea to dysentery. Typically periods of watery diarrhea, often containing blood, may alternate with constipation, and often there is flatulence and abdominal cramping. Entamoeba can be directly spread (anal sex), or indirectly spread (by drinking contaminated water). Fresh fruits and vegetables may be unsafe if fertilized with human feces, watered with contaminated water, or prepared by a person with it on his/her hands.
Division Apicomplexa:
Asian Tiger Mosquito
Asian Tiger Mosquito (© DBF)
These are all parasites and form tiny, infectious spores. All have complex life cycles. An example is Plasmodium vivax, which causes malaria, for which certain species of mosquitoes are the secondary host. It is also possible to become infected with Plasmodium parasites from a transfusion from an infected person or if a drug addict shares a syringe with an infected person. One stage in this complicated life cycle grows in the mosquito, the next stage in the newly-infected person’s liver, and the next stage invades the person’s red blood cells, rupturing the RBCs as the parasites leave to invade other cells. Symptoms include cyclical alternating chills, fever, and sweating which at first, can be mistaken for flu. While usually less than 1% of the RBCs are infected, often malaria causes anemia due to the smaller number of RBCs. Often the spleen and liver become enlarged as they try to deal with the dying RBCs. Malaria has traditionally been treated with extract from the quinine tree. Frequently, school-aged children in areas where there is malaria have to be treated numerous times (if medical care is available in their area) for new malaria infections, much like children here are repeatedly treated for colds and flu (except that malaria is a “worse” disease). Remember that people with sickle-cell are more resistant because when a malaria parasite enters a RBC, the RBC sickles, killing the parasite, thereby preventing it from multiplying and spreading. With our help, malaria is largely preventable: in many areas where malaria is endemic, whole families traditionally sleep together on a mattress, so by providing even one bed-net per family (at a cost of about $10 to donate to a relief agency), the whole family, including all the especially-vulnerable children and pregnant women, can be protected from mosquitoes biting them at night.
Division Zoomastigophora:
Trypanosoma gambienese
Trypanosoma gambienese
This Division contains some organisms which are free-living, others which are symbionts, and yet others which are parasites. An example of a symbiotic member of this Division is the protozoans which live in the gut of termites and digest cellulose in the wood the termites eat. An example of a parasitic form would be Trypanosoma gambiense, which causes African sleeping sickness and is spread by the bite of the tsetse fly. Symptoms include irregular fever, general swelling of the lymph nodes, skin eruptions, and areas of painful local swelling. Eventually CNS symptoms like tremors, headache, apathy, and convulsions appear and become worse, leading to eventual coma and death. Early on, the parasites are found in blood and lymph, but later only in the person’s cerebrospinal fluid.
Division Ciliophora:
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An example of an organism in this Division is Paramecium. These protozoans are solitary, fresh water organisms and use cilia to move. They have probably the most complex structure and organization of all cells. Rather than one nucleus, they have a larger macronucleus and several smaller micronuclei. They use a form of sexual reproduction called conjugation in which some of the micronuclei are exchanged between the two individuals involved.

Algal Protists

These protists are photosynthetic; their nutrition is plant-like (they do photosynthesis to provide their own food). Almost all of them have chlorophyll A, most have chlorophyll C, but only a few have chlorophyll B. They also have a variety of carotenoids and other pigments, and frequently they are grouped into Divisions based on similarities in pigments.

Division Dinoflagellata:
These are abundant in plankton, occasionally occurring in large numbers. They can occasionally become so numerous that the water looks red, thus this algal bloom (meaning there are large numbers of them, having nothing to do with flowers, which they do not have) is called Red Tide. Because Dinoflagellates are toxic to humans, it is not safe to eat “shellfish” (clams, etc.) collected where Red Tide is occurring (the Protists get inside the clam shell and cannot be easily removed). These Dinoflagellates are bioluminescent, that is, they are able to produce light like lightening bugs, and at night during Red Tide, the crests of the ocean waves appear to glow in the dark.
Division Euglenophyta:
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Euglena Movie
Probably the best-known example of this Division is genus Euglena. Each of these organisms has a flagellum on its anterior end, and this is used to propel the organism. They have chloroplasts and, when in the light, do photosynthesis. If they are not in the light, they can also obtain nutrition by phagocytosis. To help them sense light (which they then move toward), Euglena have a light-sensitive “eyespot” or stigma near their anterior ends. This is not a true eye, in that it cannot do any image formation, but rather it is a photoreceptor which senses the light level in the organism’s environment.
Division Chlorophyta:
These protists are also known as the “green algae.” Their chloroplasts and the pigments therein are similar to plants (this is about the only group of algae with chlorophyll B), thus it is thought that the green algae may be the evolutionary ancestors of plants. Various species of green algae may be found in a variety of environments including both fresh and salt water, damp soil, the surface of snow, and within other organisms (lichens, hydra, polar bear hair).
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Chlamydomonas are unicellular and contain an eyespot (stigma), a chloroplast, two flagella, and a nucleus.
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Volvox Movie
Volvox are colonial and often contain darker green daughter colonies inside. Each cell posesses two flagella, enabling the colony to be mobile. There is an intercellular matrix holding the colony of cells together.
Ulva is called Sea Lettuce. This is truely multicellular, with a division of labor among the various cells, and is macroscopic. The “body” is two cells thick, and there is a specially-modified “holdfast” to anchor the organism to the ocean floor. Its life cycle includes both 1n and 2n stages (see below).
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Closterium Movie
Closterium is a member of the sub-group called the Desmids. Some desmids form colonies, but Closterium is solitary. Its nucleus is in the center with a cone-shaped chloroplast on each side. Each chloroplast contains a series of starch-storage organelles called pyrenoids In living Closterium, each end of the cell bears a small vacuole containing several gypsum grains which “dance” by Brownian motion.
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Spirogyra Movie
Spirogyra are colonial, being organized into long filaments. Each cell contains a spiral chloroplast with pyrenoids (used to store starch) and a nucleus. They have conjugation — a type of sexual reproduction in which the contents of the male gamete cell go over into the female cell.
Alternation of Generations
Alternation of Generations
Many green algae, especially the multicellular ones, have both sexual and asexual stages in their life cycles, thus we must re-introduce the idea of Alternation of Generations we discussed along with meiosis. When we first discussed Alternation of Generations, we looked at a very simple diagram in which adults produced 1n gametes by meiosis, and those gametes joined by syngamy to form a new 2n generation. In reality in algae and plants, there are a few more stages in the process, thus we now need to re-visit this cycle. The 2n generation, which in humans is called an “adult,” in algae and plants is called a sporophyte because it produces spores. Within specialized reproductive structures in/on the bodies of the sporophyte, meiosis occurs to reduce the chromosome number from 2n to 1n, thus the spores which are produced are 1n. Each spore germinates and grows into a new, independent, 1n organism (which often looks totally different than the 2n generation). These 1n organisms are called gametophytes because they produce the gametes (eggs and sperm), which are still 1n. An egg and sperm unite by syngamy increasing the chromosome number from 1n to 2n, and forming a zygote which is 2n. The zygote grows into the sporophyte, and the cycle starts over. Various of the green algae go through this cycle as do members of the next two groups, the brown and red algae. Plants also go through this same cycle with some interesting modifications we will discuss later.
Division Phaeophyta:
These organisms are commonly known as the “brown algae.” They are multicellular and live in marine, temperate zone, costal areas. They all have a form of sexual reproduction with alternation of generations. One member of this Division with which you may be familiar is Kelp, which actually can be any of several species of seaweed in the genera Fucus and/or Laminaria. Brown algae are used in many cultures as human food, and are good sources of iodine. We need iodine for our thyroid glands, and if a person doesn’t enough iodine in his/her diet (most commonly in inland areas where iodine is not added to salt), the thyroid gland enlarges in an attempt to keep making enough thyroid hormone (which doesn’t do any good because what it’s lacking is the iodine needed to make the hormone). This enlarged thyroid is called a goiter. Laminaria also has an interesting gynecological use. If a woman is scheduled for some medical procedure for which the doctor needs access to the inside of her uterus, often a day or so beforehand, rolled-up, dried pieces of Laminaria are inserted into the opening of the woman’s cervix. As the seaweed absorbs water from her body fluids, it gently and slowly expands, gradually stretching the cervix. Thus, by the time her surgery is scheduled, her cervix has been dilated slowly and gently rather than the doctor having to forcibly and quickly (thus painfully) stretch the cervix open minutes beforehand.
Division Rhodophyta:
These are called the “red algae.” They also are multicellular and marine-dwelling, but are more typically found in tropical zones and deeper in the ocean. They also go through alternation of generations, Many of these (such as the Nori used in sushi) are used by humans as food, and are also good sources of iodine.

Fungus-like Protists

Division Myxomycota:
These organisms are called “slime molds.” They are fungus-like in their nutrition in that they absorb nutrients from their environment. Their “body” structure is unusual in that the nuclei undergo mitosis, but there is no cytokinesis — there are no individual cells with one nucleus each. Rather, the “body” is a giant, multinucleate mass of cytoplasm. Slime molds are mobile: they move by amoeboid movement, in other words, like a giant Amoeba with giant pseudopodia. They live in decayed wood and move around in between the fibers, ingesting bacteria, etc. by phagocytosis. Slime molds are often brightly-colored (yellow or orange).


Copyright © 1997 by J. Stein Carter. All rights reserved.
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