An Online Introduction to the Biology of Animals and Plants





Key Concepts




Section 3

Chapter 1











Originally, the Kingdom Protista was splintered from the multicelled eukaryote Kingdoms because they didn't fit into them well;  too many single-celled groups had abilities of animals, plants, and fungi simultaneously.  In its current form, the Kingdom Protista also has problems - many biologists think it should be split up into several separate Kingdoms, since so many different types of organisms are "jammed" in here.  As an introduction to the multicellular animals, we are only going to concern ourselves with a subgroup of the Protistans, the Protozoans which are single-celled and act like animals.  Mostly we will describe the protozoan groups and give some examples;  going into how experts think they are all related could take an entire (and mind-numbingly technical) book of its own.

In general, the protozoans are unicellular animals (although some have some plant or fungal features) that, like cells in general, must be in liquid to be active.  Many can survive drying by sealing themselves in protective coverings, but they aren't really doing much in those stages.  This means that active protozoans are found in water - they can be found in the oceans, in fresh water, or even really wet land environments like the undersides of thick leaf litter, as well as inside the watery environment of multicellular organisms.  It is likely that you right now have protozoans living somewhere in you, although not as certain as the certainty that you have Monerans both on and in you.  Many of these "hitchhiker" protozoans do no or hardly any harm to their larger hosts, although some can produce various kinds of diseases.

As mentioned, many protozoans can seal themselves into drying-resistant coverings to get around in land environments.  This is especially important in protozoans that live in multicelled hosts, which need to be able to get from host to host.









There are 4 major subgroups of protozoans, two of which are often combined.  The ameba-like protozoans (called the Sarcodines) and the flagella-bearing protozoans (called the Mastigotes) are often linked into one inclusive group (called the Sarcomastigotes) because of some shared features.  The cilia-bearing protozoans (called Ciliates) are about as complicated as single cells can be.  The apicomplexans  (which used to be called the Sporozoans) tend to be tiny parasites, often living inside other cells, but are also pretty complicated.









Most people know what an ameba looks like:  blobby, with a shape that's always changing, kind of oozing along a surface.  And, in fact, that's what a lot of amebas look like.  Oddly enough, most of your white blood cells, which aren't amebas at all, also look and act like this - our human genes include codes to build a pretty convincing fake ameba.

have a feature that sets them apart from most other protozoans, a type of extension of the cell, called pseudopods (from the Latin for "phony feet").  The classic ameba has fairly thick pseudopods, produced by moving material inside the cell in such a way that part of it is pushed out.  The membrane of a pseudopod is capable of sticking to most surfaces, and the rest of the cell sort of "flows" into the new position.  The interior, the cytoplasm, of Sarcodines is often set off in two layers - the outer layer, the ectoplasm, full of contracting molecular cables, and an inner layer, the endoplasm, full of the looser cellular machinery, including the nucleus and most cell organelles.  Under a lab's light microscope, the ectoplasm usually looks clear and very uniform and the endoplasm looks very grainy.

Pseudopods can be more than just a thick extension for crawling.  In moving, they may be used as tentacles in almost the way an octopus would crawl.  A large number of thin pseudopods may radiate out from a Sarcodine and help it float.  They are often used for feeding.  They can be extended around food objects until the outer membrane fuses and the object is in a bubble of membrane, a food vacuole, inside the ameba;  they can be extended as nets or tentacles for capturing food as well.

Some amebas that live in water may have shell-like coverings, made of various materials, including glued-together sand grains.

types of amebas live in humans, most of them just "along for the ride."  These commensals (they benefit from the relationship but neither hurt nor harm their hosts) don't even steal food from us, living on materials that we wouldn't use anyway.

A few
amebas cause human diseases, some life-threatening, including amebic dysentery, but even with these species most hosts carry the amebas with no visible effects.  Hosting amebas may even make you resistant to getting sick from a new exposure - your current population of hitchhikers limit the effects of newcomers.  This is why natives can drink contaminated water while tourists can't.

One group
of amebas, the slime molds, has the unusual ability to become a multicelled form when necessary.  Typically, slime mold amebas exist totally independent from each other in areas such as the wet underside of leaf litter.  Under some conditions of food deprivation, many may fuse together and exist as a sort of giant ameba (giant only compared to their individual size).  If conditions get really bad, many amebas will get together and form a fruiting body that looks remarkably like a small fungus (that's where the "mold" part of the name comes from), with a base, a stalk, and a top from which spores in drying-resistant casings can be released into the air, moving possibly to a more hospitable environment.

You may wonder how the non-spore parts of the fruiting body benefit from this arrangement, since they just die after spore release.  The answer seems to be that, since most amebas reproduce asexually, the amebas in an area are likely to be genetically identical to each other and to the released spores, and so are contributing to the continuation of themselves even if they don't survive.









The flagellates are characterized by the possession of flagella:  one or more (but rarely more than a dozen) long, mobile extensions from the cell.  Unlike pseudopods, flagella have a rigidly-organized core of cross-connected microtubules that drive them.  Flagella move in various ways:  they may spin, or whip, or move like tentacles, among other things.  They may carry extra structures, like bristles, or combs, or stiff sections, or a flat outgrowth of membrane that acts like a fin.  Most flagellates are swimmers - being microscopic, they are not powerful swimmers, but many can get from place to place if the distances between are not very large.  In humans and most animals (and more primitive plants), sperm cells are very much like tiny flagellates;  again, humans have the genes to produce such forms.

Of the protozoans that have both animal and plant characteristics, most are flagellates - there are entire subgroups of flagellates that are considered algae.  If you've seen ponds or lakes where the water was yellow-green or kelly green, that was probably from a multitude of flagellate algae (dark blue-green water is usually from a type of bacterium).  Flagellates are important parts of plankton, life that drifts near the surface of large bodies of water, where they form the base of many aquatic food chains and may be producing most of the atmosphere's oxygen.

There are several forms of flagellates that can invade humans and produce disease:

is a teardrop-shaped cell that has two nuclei beneath a bit of a cup-shaped depression and eight flagella - under a microscope it looks a bit like a cross-eyed short-handled racket.  Giardia lives in the intestines of a large variety of animals, and seems to be able to pass between and survive in many different species, an ability which is not common in parasites (each species' digestive system is a unique type of ecosystem).  In humans, the flagellates reproduce in the intestine and usually cause no symptoms, although the host passes drying-resistant cysts in feces, through which other hosts can be infected.  Giardia is usually picked up from drinking water contaminated by feces from infected animals or humans.  When humans show symptoms, the disease is called giardiasis or beaver fever (it was connected to beaver ponds, which are popular watering holes and likely to be infected;  it isn't known for sure whether beavers can actually carry Giardia).  Upon introduction, these flagellates go through an explosive phase of reproduction and coat the intestinal lining, affecting food absorption and causing irritation that can lead to nausea, cramps, and diarrhea.  Giardiasis is almost never life-threatening, and even if untreated, the symptoms usually pass within a couple of weeks - at that time, the protozoa are still in there and the host is still passing cysts, but the population is low enough to not bother the host anymore (without reinfection, though, the population in many people will dwindle to nothing eventually).  And once in place, the animals effectively prevent reinfection from producing symptoms - it's a situation, again, where the "locals" can drink the water and be fine, but "tourists" (or returning locals who have lost their "buddies") are very likely to get sick from it.  This pattern can be seen in a few other water-borne microbes, too, including amebas and bacteria.  Hosts do not develop immunity to these parasites, mostly because immune systems can't react into the spaces of the digestive system - the surfaces are the response limit.

are a particularly nasty parasite, with different species that can affect different hosts and cause a number of fairly different diseases.  Trypanosomes use biting insects as both hosts and transport between vertebrate hosts.  Trypanosomes can cause diseases in humans, but can also affect game species and livestock.  In Africa, the parasites affect the nervous system, disturbing daily rhythms and sleep cycles - the disease (in two forms: chronic and acute) is called Sleeping Sickness (African trypanosomiasis).  The victims have trouble both with sleeping and staying awake, and eventually fall into comas and die. The African trypanosomes use tsetse flies as their other, or vector, host - vectors are sometimes called carriers.  In South America,  a type of biting bedbug is the vector and the disease is called Chagas Disease.  Chagas disease can take many years to produce symptoms, but the flagellates invade many of the body's tissues and cause irreversible damage.  Often problems with the heart are the first (and worst) sign of trouble.  It is quite likely, judging from descriptions of Charles Darwin's health in his later years, that he picked up Chagas disease during his trip on the Beagle, although he lived about 40 more years and didn't become ill until he was in his 50's.  Trypanosomes have a feature that helps them to live in a host for long periods of time without being killed by the host's immunity:  when invaders get into your tissues, your immune system recognizes that they don't belong by analyzing molecules on their surfaces and checking that against your own list of surface molecules (this is a huge oversimplification of what actually happens, but it will help you get the idea).  Foreign molecules (whether on dangerous invaders or not), called antigens, are responded to in these steps:  


- the antigen is recognized and processed by various types of white blood cells.

- antibodies to that particular antigen are made and released into the plasma.

- antibody molecules have 2 functions:
     - connect to some part of the antigen (often 2 antigens at a time).
     - once connected, activate an immune attack against the invader carrying the antigen.

Trypanosomes evade this response two ways:  some antigens are kept at the bottom of membrane furrows, where they cannot be recognized by the immune system;  other antigens are periodically changed, so there is nothing for the newly-made antibodies to attach to, and when antibodies are made to attach to the changed antigens, the antigens have changed again.  On a molecular level, the invader keeps changing its appearance so it can't be recognized and killed.

Since this pattern was discovered in trypanosomes, it has been found in diseases that follow similar patterns.  Even Acquired Immune Deficiency Syndrome - AIDS - does something similar to keep the immune system from wiping it out (although actually attacking the immune system certainly keeps it going as well).

There are several other important flagellate parasites, but discussing them wouldn't add much to the information here.

One reason why the amebas and flagellates are often placed in a combined group is that many species of amebas can generate flagella and swim under certain circumstances, and several flagellate species can crawl with pseudopods if the need arises.









This group used to be known as the Sporozoans, but it turns out that making spores just isn't that unique among protozoans, so the name wasn't really distinct enough (also, way too many were discovered that had the audacity to not produce spores).  The new name reflects a structure at one end of these cells, an apical complex, and so really applies much more specifically to these protozoans.

species in this group is parasitic (living at the expense of larger hosts), although the amount of damage they cause to the host varies quite a bit.  It is quite common that apicomplexans live as parasites inside other cells, giving them the ability to parasitize both single-celled and multi-celled organisms.  They also commonly go through alternation of generations, which in parasites creates different classes of hosts:  hosts in which the parasites reproduce asexually are intermediate hosts;  hosts in which sexual reproduction occurs are called definitive hosts.  Sexual reproduction also usually involves genders, with some cells small and mobile (male) and others large and stationary (female).

Several species of apicomplexans have major impacts on humans.  They may infect domestic livestock, from cows to chickens, pets, including dogs and cats, and people themselves.

the best-known apicomplexan is Plasmodium, which causes malariaFor this species, mosquitos are the definitive hosts (the place where sexual reproduction occurs, although in this case some asexual reproduction takes place in there as well as sexual) and humans are the intermediate hosts (where asexual reproduction takes place).  Plasmodium in humans reproduces first in liver cells, where it produces no real symptoms, and then in red blood cells, where the immune system's attempts to fight it produce most of the disease symptoms.  Mosquitos pick up the parasites with a blood meal, and the Plasmodium reproduces sexually in the mosquito's gut, migrates to the outer wall of the gut to make copies asexually, and then moves to the salivary glands, from which it will be "spit" into the next victim.

four species of Plasmodium can infect humans and cause disease, one species, Plasmodium falciparum, is the most dangerous or virulent.  It seems likely that these protozoans evolved as parasites of birds and "jumped" to humans at some point.  P. falciparum, being the nastiest, is assumed to have made that jump the most recently;  in general, the longer a parasite and host have a relationship, the better-adapted and less disruptive to the host's metabolism the parasite becomes.  After all, a healthier host is a better carrier for all the many offspring a parasite is going to make and try to spread beyond the host, so it's in the parasite's evolutionary interests to be as benign as possible.

An apicomplexan parasite of cats, Toxoplasma gondii, is passed in the cats' feces or in certain types or undercooked meats, and can sometimes jump to humans but rarely produces symptoms there.  It can cause problems when the new human host is pregnant and the parasites manage to invade the developing nervous system of the fetus and produce birth defects.  Each of these steps is fairly unlikely, but you may hear of pregnant women being advised to get rid of their cats - that is almost certainly too extreme a precaution, although reducing exposure to cat feces during a pregnancy is probably an excellent idea.  In the U.S., most toxoplasma exposures are from foods.









Although the characteristic feature for this group is cilia, there are several other unique or highly unusual features as well.  Ciliates are often organized in a way that seems more multicelled in complexity, almost more complexity than even seems possible with only one cell.

First, the cilia:  these small cell projections are constructed the same basic way that flagella are, with a core of cross-connected microtubules that interact to produce their motion.  However, there are several differences between the structures:





Larger / longer

Smaller / shorter

Rarely more than a dozen found on a cell

Always found in large numbers on a cell

May carry various additional structures

Do not carry additional structures (but may be fused together into structures)

When more than one is present, rarely act in a coordinated fashion

Almost always act with a high degree of coordination

Most common activity is a spinning / whipping motion

Most common activity is an oarlike stroke somewhat like a swimming human's arm


Once more, humans are capable of producing ciliated cells:  ciliated surfaces move dust-catching mucus out of the breathing tubes and egg cells from the ovaries to the uterus.  Our ciliated cells are not as complex as ciliated protozoans, however.

often have a depression or furrow on their surface down which a current drives food;  at the bottom of this feature, which is often called an oral groove,  food vacuoles form which often follow a set path through the cell (almost like the track of food through a digestive tract), ending at a special spot at which the undigested contents are dumped outside.  Freshwater ciliates, like freshwater protozoans in general, have contractile vacuoles that "bail" the water that continually floods in from the dilute surroundings (saltwater protozoans' dilution factors are balanced with their surroundings - there are equal concentrations of water inside and out - and they don't need to do this).  Amebas and flagellates often have simple bubbles for this function, but ciliates may have channels, and collection centers, and other complex patterns for their contractile vacuoles (many Paramecium species' contractile vacuoles look like daisies, for instance).

So complex
are ciliates that they have evolved an extra control level in the form of two different types of nuclei.  Ciliates each have a micronucleus, which, as it sounds, is usually small and may even be hard to find.  The micronucleus carries the "master copy" of the cell's DNA, which is used for making new nuclei, usually in preparation for reproduction.  Ciliates also each have one or more micronuclei, which are used as the "working" genetic codes - when the cell needs to make proteins, needs to perform some function, it accesses the DNA in a macronucleus for it.  It may even make extra DNA pieces with often-used genes on them, something rarely found in eukaryote cells.



Both asexual and sexual reproduction can be found in the ciliates, and in both cases the actual events are usually very complicated.  In asexual reproduction, many of the complex surface features need to be duplicated as the cell divides so that the two new cells are fully functional.  Some of the duplication is genetic, such as the construction of a second oral groove, and some is epigenetic, as new rows of cilia are produced from existing rows.  Micronuclei are very carefully copied and moved so that each daughter cell gets one, but once division is complete usually new macronuclei will be produced from each cell's micronucleus, so careful shepherding of the macronucleus is not necessary.  Sexual reproduction in ciliates, called conjugation, seems almost unreasonably complicated:


Cells stick to one another.

A "bridge" forms between the cells.

Meiosis of the micronucleus produces four haploid micronuclei in each cell.

Three of the four micronuclei degenerate.

The remaining haploid micronucleus goes through mitosis, producing two copies.

One micronucleus stays and the other moves across the bridge into the other cell.

The exchanged micronuclei fuse with the original micronuclei, forming genetically new diploid micronuclei.

The cells separate.  The original cells, if defined as individuals by their unique genetic makeup, are gone.  Each of the two new cells has the same genetic mixture derived from but different from the starting cells.

The original macronucleus degenerates.

The new micronucleus divides at least twice, with half or more of the new micronuclei enlarging and becoming macronuclei.

The cell itself divides, with each cell getting a micronucleus and macronucleus with the new genetic mix in it.

Although ciliates are common organisms living free in watery environments, few are found as parasites or even "hitchhiking" on larger organisms.  There is only one recognized ciliate parasite of humans, Balantidium coli, and a few that affect fish and amphibians, but this is a very minor group if one is looking for impact on people.



Informative Links



A page, with lots of nice pictures, to introduce marine (ocean / sea) protozoans.

A comprehensive page of freshwater protozoans, with lots of links.

Links to many many movies of protozoa in action.

Lots of pictures, organized by protozoan groups.

International Society of Protistologists.






Click on term to go to it in the text.
Terms are in the order they appear.



Environments Protozoans are Found In
Protozoan Subgroups
Amebas / Sarcodines
Food Vacuole
Amebic Dysentery
Slime Molds
Fruiting Bodies
Flagellates / Mastigophora
Giardia & Beaver Fever
Vector Hosts
Sleeping Sickness
Chagas Disease
Trypanosomes & Immune Response
Apicomplexans /  Sporozoans
Alternation of Generations
Intermediate Host
Definitive Host
Plasmodium & Malaria
Parasite Evolution
Cilia & Flagella
Oral Groove
Contractile Vacuole














Online Introduction to the Biology of Animals and Plants.


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