So how are animal-like, plant-like, and fungi-like protists distinguished from each other? Mainly through how they get their carbon. Of course, carbon is essential in the formation of organic compounds: carbohydrates, lipids, proteins, and nucleic acids. You get it from eating, as do other animals.
For such simple organisms, protists get their food in a complicated process. Although there are many photosynthetic protists (such as the algae discussed in the Plant-like Protists section below) that get their energy from sunlight, many others still must swallow their food through a process like endocytosis. Endocytosis was discussed in the Cell Functions chapter.
When a protist is ready to eat, it will wrap its cell wall and cell membrane around its prey, which is usually bacteria. In doing so, it creates a food vacuole or a sort of “food storage compartment." Next the protist produces toxins which paralyze its prospective dinner. Once paralyzed, the food material simply moves by force of gravity through the vacuole and into the cytoplasm of the hungry protist. Other protists are parasitic, and absorb nutrients meant for their host, harming the host in the process.
Animal-like Protists
Animal-like protists are called protozoa. Protozoa are unicellular eukaryotes that share certain traits with organisms in the animal kingdom. Those traits are mobility and heterotrophy. Animal-like protists are heterotrophs which mean they get their carbon from outside sources—in other words, they eat organic materials. Animal-like protists are very tiny measuring only about 0.01–0.5mm. Animal like protists include the zooflagellates, ciliates, and the sporozoans (Figure below).
Figure 9.3
Euglena are animal-like protists. Over 1000 species of Euglena exist and are used in industry in the treatment of sewage.
Although most protists obtain nutrition through pinocytosis, some protists literally “eat with their tails”. The tail of a protist is a flagellum and these protists are called flagellates. Flagellates acquire oxygen and nitrogen by constantly whipping the flagellum back and forth in a process of filter-feeding. The whipping of the flagellum creates a current that brings food into the protist.
A flagellum (plural: flagella), is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and it usually functions in helping the cell move. A flagellum is a cellular structure and not an organelle. Prokaryotic cells may also have flagella.
Different Kinds of Animal-like Protists
Are there different types of animal-like protists? How are they distinguished? You can distinguish one from the other based on how they get around or rather, by their method of locomotion. For example, flagellates have long flagella or tails. Flagella rotate in a propeller-like fashion. An example of a flagellate is the Trypanosoma, which causes African sleeping sickness. Other protists have what is called a “transient pseudopodia” or a moving fake foot. Here’s how it works. The cell surface extends out a membrane and the force of this membrane propels the cell forward. An example of a protist with a pseudopod is the amoeba. Another way to move if you are a protist is by the movement of cilia. The paramecium has cilia that propel it. Cilia are thin, tail-like projections that extend about 5–10 micrometers outwards from the cell body. Cilia beat back and forth, propelling the protist along. A few protists are non-mobile such as the toxoplasma. Protists such as the toxoplasma form spores and are known as sporozoans; these protists but do not have any mobility themselves.
Plant-like Protists
Plant-like protists are autotrophs. This means that they produce complex organic compounds from simple inorganic molecules using a source of energy such as sunlight. Plant-like protists live in soil, in seawater, on the outer covering of plants, in ponds and lakes (Figure below). Protists like these can be unicellular, or multicellular. Some protists, such as kelp live in huge colonies in the ocean. Plant-like protists are essential to the environment; they produce oxygen (a product of photosynthesis) which sustains other organisms and they play an essential role in aquatic food chains. Plant-like protists are classified into a number of basic groups (Table (below)).
Plant-like Protists Phylum Description Number (approximate) Example
Chlorophyta green algae - related to higher plants 7,500 Chlamydomnas, Ulva, Volvox
Rhodophyta red algae 5,000 Porphyra
Phaeophyta brown algae 1,500 Macrocystis
Chrysophyta diatoms, golden-brown algae, yellow-green algae 12,000 Cyclotella
Pyrrophyta dinoflagellates 4,000 Gonyaulax
Euglenophyta euglenoids 1,000 Euglena
Figure 9.4
Red algae are a very large group of protists making up about 5,0006,000 species. They are mostly multicellular, live in the ocean. Many red algae are seaweeds and help create coral reefs.
Fungus-like Protists
Fungus-like protists are heterotrophs that have cell walls and reproduce by forming spores. Fungus-like protists are mostly immobile but some develop movement at some point in their lives. There are essentially three types of fungus-like protists: water molds, downy mildews, and slime molds (Table (below)). Slime molds represent the characteristics of the fungus-like protists. Most slime molds measure about one or two centimeters, but a few slime molds are as big as several meters. They are often bright colors such as a vibrant yellow. Others are brown or white. Stemonitis is a kind of slime mold which forms small brown bunches on the outside of rotting logs. Physarum polycephalum lives inside rotting logs and is a gooey mesh of yellow “threads” that are a several centimeters long. Fuligo, sometimes called “vomit mold,” is a yellow slime mold found in decaying wood.
Fungus-like Protists Protist Source of Carbon Environment Characteristics
omycetes: water molds (Figure below) decomposed remains, parasites of plants and animals most live in water Causes a range of diseases in plants; common problem in greenhouses where the organism kills newly emerged seedlings; have been employed as biocontrol agents; includes the downy mildews, which are easily identifiable by the appearance of white "mildew" on leaf surfaces.
Mycetozoa: slime molds (Figure below) dispose of dead plant material, feed on bacteria common in soil, on lawns, and in the forest commonly on deciduous logs Includes the cellular slime mold, which involves numerous individual cells attached to each other, forming one large "supercell," essentially a bag of cytoplasm containing thousands of individual nuclei. The plasmodial slime molds spend most of their lives as individual unicellular protists, but when a chemical signal is secreted, they assemble into a cluster that acts as one organism.
Figure 9.5
An example of a slime mold.
Figure 9.6
An aquatic insect nymph attacked by water mold.
Importance of Protists
Earth would be uninhabitable if it were not for the 80 different groups of organisms called protists. Protists produce almost one-half of the oxygen on the planet, decompose and recycle nutrients that humans need to live, and make up a huge portion of the food chain. Many protists are commonly used in medical research. For example, medicines made from protists are used in treatment of high blood pressure, digestion problems, ulcers, and arthritis. Other protists are used in molecular biology and genetics studies. Slime molds are used to analyze the chemical signals used in directing cellular activities. Protists are also valuable in industry. Carrageenan, extracted from red algae, is used as a gel to solidify puddings, ice cream, and candy. Chemicals from other kinds of algae are used in the production of many kinds of plastics.
Lesson Summary
Protists are highly diverse organisms that belong to the kingdom Protista.
Protists are divided into three subgroups: animal-like protists, plant-like protists, fungus-like protists.
Animal-like protists are unicellular eukaryotes that share certain traits with organisms in the animal kingdom such as mobility and heterotrophy.
Plant-like protists are unicellular or multicellular autotrophs that live in soil, in seawater, on the outer covering of plants, in pond
s and lakes.
Fungus-like protists, such as water molds, downy mildews, and slime molds, are heterotrophs that reproduce by forming spores.
Review Questions
List the unifying characteristics of protists.
List two ways that protists obtain food.
Describe the characteristics of an animal-like protist.
Describe the characteristics of a plant-like protist.
Describe the characteristics of a fungi-like protist.
Name three kinds of fungi-like protists.
Write a convincing essay demonstrating the importance of protists to life on Earth.
Imagine that you are a scientist delivering a paper called “Protists: the Junk-Drawer Kingdom” What would you say in your paper to explain your choice of title?
Further Reading / Supplemental Links
King, Katie and Ball, Jacqueline, Protists and Fungi. 2003 Gareth Stevens Publishing.
Marguilis, L., Corliss, J.O., Melkonian, M.,and Chapman, D.J. (Editors) 1990. Handbook of Protoctista. Jones and Bartlett, Boston.
Jahn,T.L., Bovee, E.C. & Jahn, F.F. 1979 How to Know the Protozoa. 2nd ed. Wm. C. Brown Publishers, Div. of McGraw Hill, Dubuque, Iowa.
Patterson, D.J. 1996. Free-Living Freshwater Protozoa: A Colour Guide. John Wiley & Sons, NY.
Streble H., Krauter D. 1988. Life in a waterdrop. Microscopic freshwater flora and fauna. An identification book.
http://www.ucmp.berkeley.edu/alllife/eukaryotasy.html
http://www.funsci.com/fun3_en/protists/entrance.htm
http://www.biology.arizona.edu/cell_bio/tutorials/pev/main.html(http://www.biology.arizona.edu/cell_bio/tutorials/pev/main.html)
http://waynesword.palomar.edu/trfeb98.htm
http://www.na.fs.fed.us/fhp/ded/
http://www.ehow.com/facts_5919260_differences-baker_s-_amp_-brewer_s-yeast.html
Vocabulary
autotroph
Organism that produces complex organic compounds from simple inorganic molecules using a source of energy such as sunlight.
cilia
Thin, tail-like projections that extend about 5–10 micrometers outwards from the cell body; beat back and forth, propelling the protist along.
filter-feeding
Characteristic of flagellates; acquire oxygen and nitrogen by constantly whipping the flagellum back and forth; creates a current that brings food into the protist.
flagellum
A tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and it usually functions in helping the cell move.
heterotroph
Organism which obtains carbon from outside sources.
protist
Eukaryotic organism that belongs to the kingdom Protista; not a plant, animal or fungi.
protozoa
Animal-like protists
transient pseudopodia
A moving fake foot; the cell surface extends out a membrane and the force of this membrane propels the cell forward.
Points to Consider
Fungi comprise one of the eukaryotic kingdoms. Think about what might distinguish a fungi-like protist from a true fungus?
Given the vast differences between the protists discussed in this lesson, think about the possibilities of dividing this kingdom into additional kingdoms. How might that division be accomplished? Is that a good idea or would it just lead to confusion?
Lesson 9.2: Fungi
Lesson Objectives
Describe the characteristics of fungi.
Identify structures that distinguish fungi from plants and animals.
Explain how fungi can be used in industry.
Check Your Understanding
What is a significant difference between a protist and other eukaryotic organisms?
What are some of the distinguishing characteristics of fungus-like protists?
Introduction
Ever notice blue-green mold growing on a loaf of bread? Do you like your pizza with mushrooms? Has a physician ever prescribed an antibiotic for you? If so, then you have encountered fungi. Fungi are organisms that belong to the kingdom Fungi (Figure below). Our ecosystem needs fungi. Fungi help decompose matter and make nutritious food for other organisms. Fungi are all around us and are useful in many ways to the natural world and to humans in industry.
Figure 9.7
These many different kinds of organisms demonstrate the huge diversity within the kingdom Fungi.
What are Fungi?
If you had to guess, would you say fungus is a plant or animal? Scientists used to debate about which kingdom to place fungi in. Finally they decided that fungi were plants. But they were wrong. Now scientists know that fungi are not plants at all. Fungi are very different from plants. Fungi belong to their own kingdom called the kingdom Fungi.
Plants are autotrophs, meaning that they make their own "food" using the energy from sunlight. Fungi are heterotrophs, which means that they obtain their "food" from outside of themselves. In other words, they must "eat" or ingest their food like animals or many bacteria do.
Yeasts, molds, and mushrooms are all different kinds of fungi (Figure below). There may be as many as 1.5 million species of fungi. You can easily see bread mold and mushrooms without a microscope, but most fungi you cannot see. Fungi are either too small to be seen without a microscope or they live where you cannot see them easily - such as deep in the soil, under decaying logs, or inside plants or animals. Some fungi even live in or on top of other fungi.
Figure 9.8
The blue in this blue cheese is actually mold.
Fungi and Symbiotic Relationships
If it were not for fungi, many plants would go hungry. In the soil fungi grow closely around the roots of plants. Then as they form that close relationship, the plant and the fungus “feed” one another. The plant provides glucose and sucrose to the fungus that the plant makes through photosynthesis which the fungus cannot do. The fungi then provides minerals and water to the roots of the plant. This form of helping each other out is called mycorrhizal symbiosis. Mycorrhizal means “roots” and symbiosis means “relationship” between organisms (Figure below).
Figure 9.9
This mushroom and tree live in symbiosis with each other.
Lichens
Have you ever seen an organism called a lichen? Lichens are crusty, hard growths that you might find on trees, logs, walls, and rocks. Although lichens may not be the prettiest organisms in nature, they are unique. A lichen is really two organisms that live very closely together—a fungus and a bacteria or algae. The cells from the algae or bacteria live inside the fungi. Each organism provides nutrients for the other. Consequently, a lichen is the result of the symbiosis between a fungus and an another organism.
The earliest scientist to study lichens was Beatrix Potter. You might have heard of her as the author and illustrator of the Peter Rabbit stories. Before Beatrix Potter became a famous author, she was a botanist and studied hundreds of different kinds of fungi. She was the first person to explain the symbiotic relationship between bacteria and fungi. She even presented a scientific paper to the British scientific community in 1897.
Fungi and Insects
Many insects have a symbiotic relationship with certain types of fungi. For example, ants and termites grow fungi in underground “fungus gardens” that they create. When the ants or termites have eaten a big meal of wood or leaves, they eat some fungus from their gardens. The fungus helps them digest the cellulose in the wood or leaves. The two species are actually dependent on each other for survival. Ambrosia beetles live in the bark of trees. Like ants and termites, they grow fungi inside the bark of trees and use it to help digest their food.
Fungi as Parasites
Although lots of symbiotic relationships help both organisms, sometimes one of the organisms is harmed. When that happens, the organism that benefits and is not harmed is called a parasite. Have you ever heard of Dutch Elm Disease? Beginning in 1950, elm trees
in the United States began to die. Since then much of the species has been eliminated. The disease was caused by a fungus that acted as a parasite. The fungus that killed the trees was carried by beetles that inoculated the tree with the fungus. The tree tried to stop the growth of the fungus by blocking its own ability to gain water. However, without water the tree soon dies.
Some parasitic fungi cause human diseases such as athlete’s foot and ringworm. These fungi feed on the outer layer of warm, moist skin.
Fungi as Predators
It might seem that fungi growing on a tree trunk or a mushroom in your yard are passive and participating in very little activity, but did you know that some fungi are actually hunters? Some fungi trap nematodes. A nematode is a kind of a worm and can be dinner for fungi. These hungry fungi live deep in the soil where they set traps for unsuspecting nematodes by making a circle with their hyphae. Hyphae are sort of the "arms and legs" of a fungus; they look like cobwebs and can be sticky. Fungi set out circular rings of hyphae with a lure inside which brings the nematode inside the fungus (Figure below).
Figure 9.10
Hyphae are the cobweb-like arms and legs of fungi.
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