2.13
plantlike
6 phyla
animal-like
4 phyla
PROTISTA
fungilike
1 phylum
Protists
In section 2.2, you used a dichotomous key to sort some pond-water protists
and, as a result, saw how varied these organisms are. The kingdom Protista is
like a junk drawer in that its 11 phyla appear to have little in common. The
members of these phyla differ greatly in shape, size, structure, complexity,
feeding habits, locomotion, and reproduction. Protists also display great
ecological diversity, occupying almost every known niche and habitat.
There are three distinct groups in the Protista kingdom: plantlike protists,
animal-like protists, and fungilike protists (Figure 1). Even with such diversity,
these organisms share the following features:
• Most are unicellular. Some form colonies of identical cells. A few are
multicellular but do not form tissues.
Figure 1
Grouping of protists
• Cells are eukaryotic. There is a membrane-bound nucleus, and most
contain vacuoles and mitochondria. Some contain chloroplasts.
• Cells reproduce asexually by binary fission. Some exchange DNA in a
form of sexual reproduction.
• Cells thrive in moist surroundings such as fresh water, salt water, animal
fluids, or very damp terrestrial environments.
Plantlike Protists
(a)
flagellum
eyespot
contractile
vacuole
(b)
mitochondrion
nucleus
chloroplast
pellicle
Figure 2
(a) A photomicrograph of a living
Euglena
(b) This longitudinal section shows
the internal organelles of
Euglena.
128
Unit 2
This group of protists is plantlike because the organisms contain chlorophyll,
the pigment that begins the process of photosynthesis. Energy is obtained
through this process, but during periods of darkness, cells become
heterotrophic and take in more complex nutrients, often by engulfing solid
food, a trait commonly associated with animals.
Euglena is a typical plantlike protist (Figure 2). Two striking features of
Euglena are the eyespot and the flagellum. The eyespot is believed to be part of
the organism’s sensory-motor system, used to detect light. The flagellum is
used to propel the organism through water in a whiplike fashion. Most species
of plantlike protists have two flagella.
The entire outer boundary of Euglena outside the membrane is surrounded
by a firm yet flexible covering called a pellicle. There is no cell wall. Other
conspicuous structures are the central nucleus, the large green chloroplasts,
and the vacuoles. The chloroplast pigments are identical to those in green
algae and land plants. The vacuoles are used to collect and remove excess
water. Food is stored in the form of starch granules, a common practice in
plants.
Euglena, like many flagellated protists, reproduces asexually. Following
nuclear division, the rest of the cell divides lengthwise. This process, called
longitudinal fission, involves growth in cell circumference while the organelles
are being duplicated. During unfavourable conditions, Euglena may form a
thickly coated resting cell.
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Section 2.13
Also in the plantlike-protist category are the green, brown, and red algae
(singular: alga). Most algae are multicellular but do not form tissues. This
distinguishes them from higher plants that are multicellular and form tissues.
Algae are extremely well adapted to wet or moist environments (Figure 3).
Normally considered to be aquatic organisms, they can also be found in soils,
on the lower trunks of trees, and on rocks. Both brown and red algae,
commonly called seaweeds, are generally large multicellular oceanic plants.
They contain chlorophyll and other coloured pigments that permit them to
carry out photosynthesis with the particular wavelengths of light that occur
with changes in water depth. Unicellular green algae, usually referred to as
phytoplankton, are found in both marine and freshwater environments,
floating on or near the water surface.
(a)
(b)
There are many variations in reproduction among the algae phyla.
Variations range from complex life cycles that include both sexual and asexual
reproduction to simple asexual reproduction by fragmentation—the organism
simply breaks apart. Most commonly, reproduction is asexual, by binary
fission. Spirogyra also reproduce by conjugation (Figure 4).
DID YOU
KNOW
?
Algae as Food
Algae are an important food
source for many marine mollusks.
One mollusk, Plakibranchus,
incorporates algae into its tissues.
The algae continue to function and
provide the animal with oxygen.
Figure 3
(a) Chlamydomonas, a common
single-celled green alga that
lives in fresh-water habitats
(b) Volvox, a colonial green alga,
also lives in fresh water. It is
composed of thousands of
interdependent flagellated cells
that closely resemble the freeliving cell Chlamydomonas.
A tube develops,
connecting the two
cells together.
Importance of Algae
Algae, especially green algae, play an important role in the overall global
environment. They are the primary food producers in aquatic food chains.
Through photosynthesis, algae supply about 80% of the global supply of
oxygen. It has become a concern that human wastes and industrial
contaminants may be reducing algal populations, particularly in oceans. For
example, the lakes near Sudbury are crystal clear—no algae can live in them—
as a result of the dumping of industrial contaminants. At the other extreme,
excessive growth of algae can also be a serious problem: by blocking sunlight,
algae can affect photosynthesis and oxygen uptake by marine life.
Humans use algae in a variety of ways. Some algae are consumed directly as
food, providing an excellent source of vitamins and trace minerals. Other algae
are used as fertilizers. Agar is a mucilaginous material extracted from the cell
walls of certain red algae. It is also used in the production of drug capsules,
gels, cosmetics, and culture medium. A similar substance, carrageenan, is used
in cosmetics, paints, ice cream, and pie fillings.
Algae help create petroleum resources. Brown algae store food as oils. Once
filled with oil, the algae die and sink to the bottom of the ocean where they may
be buried under mud and sand. Over millions of years and under extreme heat
and pressure within the Earth, the oil from the algae can be transformed into
crude-oil deposits.
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The contents of the
left-hand cell move
into the right-hand
cell.
The nucleus of
the left-hand cell
fuses with the
nucleus of the
right-hand cell.
The cytoplasm rounds off and
develops a thick wall, becoming
a zygospore.
Figure 4
Conjugation of Spirogyra, a
filamentous green alga that also
reproduces asexually by binary
fission and by fragmentation
Microbiology 129
Animal-like Protists (Protozoa)
Figure 5
An amoeba engulfing food
ectoplasm the thin, semirigid
(gelled) layer of the cytoplasm under
the cytoplasmic membrane
endoplasm the fluid part of the
cytoplasm that fills the inside of the
cell. The endoplasm is responsible
for an amoeba’s shape as it moves.
Figure 6
Four phyla of animal-like protists.
Protists range in size from 2 µm in
length to 5 cm in diameter.
(a) Sarcodina, pseudopod
organisms
(b) Mastigophora, flagellated
protozoans
(c) Ciliophora, ciliated protozoans
(d) Sporozoa, parasitic protozoans
oral groove
contractile
vacuole
micronucleus
macronucleus
gullet
trichocysts
(harpoons)
contractile
vacuole
anal pore
food vacuole
cilia
Figure 7
Paramecium is sometimes called the
“slipper animal” because of its
shape.
cyst a cell that has a hardened
protective covering on top of the
cytoplasmic membrane
130
Unit 2
In contrast to plantlike protists, all protozoa are heterotrophs. Generally, they
must move about to obtain food. Some engulf their food (bacteria and other
microorganisms); others absorb predigested or soluble nutrients directly
though their cell membranes.
One of the largest yet least complex protozoans is the amoeba (plural:
amoebae). An amoeba moves by repeatedly extending and retracting its
pseudopods. Its cytoplasm has two layers: the outer protective layer, the
ectoplasm, and the inner layer that contains most of the structures, the
endoplasm. The continuous movement of the endoplasm causes the amoeba
to change shape constantly as it moves. No wonder Linnaeus originally named
an amoeba Chaos chaos when he saw it for the first time!
An amoeba feeds by phagocytosis; its pseudopods simply flow around and
engulf food particles (Figure 5). The food eventually becomes enclosed in a
food vacuole, where the food is digested. Water taken in with the food, along
with water taken in via diffusion, collects in a contractile vacuole. As this
vacuole fills, it contracts, and the water is discharged though a pore in the
cytoplasmic membrane.
Amoebae reproduce by binary fission. Once the amoeba splits, the two
organisms then grow to their full size and may split again. Under proper
conditions, this could occur once a day.
(a)
(b)
(c)
(d)
Figure 6 shows that protozoans are classified according to their type of
locomotion. Ciliates (Figure 6(c)) use hairlike structures for mobility. In freemoving organisms such as Paramecium, cilia are armlike, synchronized for
swimming. Other protists use cilia to attach themselves to a surface.
Ciliates are the most advanced of the protozoans, living in both fresh and
salt water. The paramecium illustrates many of the group’s complex features
(Figure 7). The oral groove contains a mouth that leads into a cavity called the
gullet. Specialized cilia in the gullet sweep bacteria and other food particles
into the cavity. From the gullet, food enters a food vacuole where digestion
takes place. Wastes are expelled from the food vacuole through an anal pore.
Paramecia collect excess water in a contractile vacuole, from which the water
is expelled into the environment.
The paramecium has an interesting defence mechanism. Hundreds of
poison-laden barbs can be discharged either to drive away predators or to
capture prey. These structures are called trichocysts.
A paramecium has two nuclei: a large macronucleus that controls the cell’s
activities and a smaller micronucleus that is involved in reproduction.
Reproduction in protozoa is usually asexual, via binary fission. Periodically,
sexual reproduction in the form of conjugation may occur. Under adverse
conditions, protozoans may form resting cells called cysts. If favourable
conditions return, the organisms may emerge from their cysts.
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Section 2.13
Members of the Sporozoan phylum (Figure 6(d)) are unique among the
protozoans because they lack any means of independent locomotion; they
have no pseudopodia, cilia, or flagella. They are exclusively parasitic and
depend entirely on the body fluids of their hosts for movement. Also, they
have fewer organelles and specialized structures than other protozoans. A
distinguishing feature of sporozoans is the ability of their cells to multiply
asexually in animal tissues by forming reproductive cells, called spores, that
can develop into an individual without fertilization. Spores have a haploid, or
single, set of unpaired chromosomes.
Pathogenic Protozoans
The best-known sporozoans are members of the genus Plasmodium, which
causes malaria in humans. As you can see from the sequence in Figure 8, the
malaria parasite has a complex life cycle that involves two hosts: humans and
the mosquitoes of the genus Anopheles. Insects are frequently responsible for
transmitting sporozoans from one host to the next and are referred to as
insect vectors. Malaria can be treated with drugs, but so far the most effective
way to deal with the disease is to eliminate the vector. In many tropical
countries where malaria is common, pesticides were widely used during the
1950s and 1960s to eliminate the Anopheles mosquitoes. However, pesticideresistant strains have evolved, and today researchers are attempting to develop
a vaccine to control malaria. Meanwhile, the disease continues to be a serious
human health problem in many tropical areas.
Zygote forms a cyst; spores
develop inside the cyst.
Gametocytes develop into
gametes in mosquito's
digestive system;
fertilization occurs.
spore a reproductive cell that can
produce a new organism without
fertilization. Fungal spores have
thick, resistant outer coverings to
protect them.
haploid nucleus, cell, or organism
with one set of chromosomes. The
haploid number is designated as n.
CAREER CONNECTION
Field directors, working in areas
of the world where malaria is
prevalent, have backgrounds in
health sciences and epidemiology.
Malaria inspectors look for areas
of stagnant water providing a
breeding ground for mosquitoes.
DID YOU
KNOW
Cyst releases spores; spores travel
to mosquito's salivary glands.
zygote
?
Mosquito Food
Only female mosquitoes feed on
blood; male mosquitoes feed on
plant fluids.
Spores are
released when
mosquito bites
uninfected
humans.
gametes
Life Cycle in Mosquito
Life Cycle in Human
Spores travel
to liver through
the bloodstream.
Mosquito takes
in Plasmodium
gametocytes
when it bites an
infected human.
Some Plasmodium spores
develop into gametocytes.
gametocytes
Red blood cells burst and
release spores into blood
stream.
Spores invade and reproduce
in red blood cells.
Figure 8
The life cycle of the malaria parasite, Plasmodium vivax
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Microbiology 131
carrier an infected individual who
carries disease but does not
become infected herself/himself
undulating
membrane
nucleus
flagellum
Figure 9
The undulating membrane of
Trypanosoma allows it to move
through the human bloodstream
like an eel.
Some other serious human diseases are caused by pathogenic protozoans
that live in the human intestine. Entamoeba histolytica, unlike its free-living
cousin Amoeba proteus, is parasitic and causes amoebic dysentery. This
amoeba lives in the large intestine of humans and feeds on the intestine walls,
causing fever, chills, bleeding ulcers, diarrhea, and abdominal cramps. The
disease is spread when some amoebae form cysts that pass out of the infected
person in fecal matter. Another person becomes infected by eating food or
drinking water contaminated by these cysts. The cysts can survive for long
periods of time, making eradication of the parasite very difficult. In some
countries where amoebic dysentery is common, up to 50% of the population
have the disease-causing organism and can transmit it to others, but are not
infected themselves. They are carriers of the disease. Sanitary disposal of
human waste is the best way to prevent widespread transmission of dysentery.
To avoid dysentery in areas where human wastes are not properly handled, any
water used for drinking, cooking, or washing food must be boiled. Amoebic
dysentery can be treated with drugs, but several follow-up tests are required to
ensure that the intestine is cyst-free.
Some flagellated protozoans are also parasitic in humans. Giardia lamblia
(beaver fever) usually causes stomach upset and diarrhea, but can also have
more severe effects on some people. Another common parasite, Trichomonas
vaginalis, is passed through sexual intercourse and infects the urinary and
reproductive tracts. African sleeping sickness is caused by the Trypanosoma
protozoan (Figure 9), and is transmitted by the tsetse fly. This protozoan
multiplies in the blood of humans, releasing toxins that affect the nervous and
lymphatic systems. Symptoms include irregular fever, chills, headache, skin
eruptions, swollen lymph nodes, weakness, and fatigue. If diagnosed at an
early stage, African sleeping sickness can be treated with drugs.
Fungilike Protists
(a)
(b)
Figure 10
(a) A plasmodial slime mould
(b) Fruiting body of the slime
mould, where spores are
produced
132
Unit 2
Fungilike protists are also referred to as slime moulds (Figure 10). These
organisms prefer cool, shady, moist places and are usually found under fallen
leaves or on rotting logs. The name is derived from the slimy trail left behind
as the mould travels over the ground.
During some stage of their life cycle, slime moulds resemble protozoans and
become amoebalike or have flagella. At other times, they produce spores much
as fungi do. Unlike other primitive organisms, the slime moulds do not always
remain as single-celled organisms. One genus spends some of its life cycle as a
single-celled amoebalike organism; however, these cells can converge into a
large, slimy plasmodial mass. It then begins to act like a single organism. It
extends into a sluglike form and begins to creep, feeding on organic matter as
it goes. When it runs into an object, it retracts and slithers around it. The
coordinated movements are slow—sometimes only a few millimetres a day.
Biologists recognize that becoming multicellular is a tremendous advancement
for an organism. As individual cells begin to work together, the groundwork for
cell specialization is laid. In more advanced organisms in higher kingdoms, the
specialized cells develop structures or tissues suited for specific tasks.
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Section 2.13
Overall Importance of Protists
The impact of protists on the biosphere is enormous. Although they cause
some of the world’s most serious diseases, protists provide aquatic consumers
with food and supply both aquatic and terrestrial heterotrophs with oxygen.
Large populations of protists that drift in the ocean are called plankton.
Photosynthetic plankton, or phytoplankton, produce their own food and help
form the base of food pyramids, chains, or webs (Figure 11). Heterotrophic
protists, or zooplankton, are first-level consumers. If the population of
plankton were seriously affected by pollution or other environmental changes,
huge filter-feeding whales would starve. Aquatic carnivores such as tuna would
also be affected.
Aside from food, protists are involved in a variety of ecological relationships
with other organisms, including humans. One species of protozoans
(Trichomonas hominis) lives in the digestive tract of humans. It feeds on
intestinal contents that humans do not use, and it has never been known to be
harmful. Another protist lives in the digestive tract of termites, digesting the
wood that termites eat. Termites cannot digest cellulose without this protist.
Despite their generally small size and deceptively simple structure, members
of the protist kingdom are extremely important to all other life.
tuna
herring
crustaceans
zooplankton
phytoplankton
Figure 11
Plankton are an essential part of the
food pyramid for many aquatic
communities.
Section 2.13 Questions
Understanding Concepts
1. What is the main feature used to classify Euglena as
a plantlike protist? List five other identifying features
of this organism.
2. What is the most important structural difference
between algae and Euglena? between algae and land
plants?
3. List five ways in which algae are useful to humans.
4. “Amoeba and Paramecium are both animal-like
protists, yet their structure is very different.” Justify
this statement.
5. Use binomial nomenclature to name four pathogenic
protists. Explain how each enters the human body,
and describe the resulting symptoms.
6. Why do the fungilike protists belong in the kingdom
Protista, and how are they different from other protists?
7. Name a protist that feeds by phagocytosis and
explain how this happens.
8. Use the details presented in this section to describe
how the following organisms reproduce: Euglena,
Spirogyra, Paramecium, and Plasmodium.
9. Describe the mobility of each of the common protists
studied in this section: Euglena, alga, Amoeba,
Paramecium, and sporozoan.
10. Explain the differences between a cyst and a spore.
11. How are the sporozoans “parasitic”?
NEL
Applying Inquiry Skills
12. An amoeba reproduces by binary fission about once
every 24 h. How many amoebae would be produced
from an original cell in a month of 30 days, assuming
all survived and reproduced?
13. A sample of greenish scum is examined under the
microscope. The scum consists of threads made up of
identical cells. A single cell is isolated and examined
under high power. However, it is difficult to see
whether or not the cell has a true nucleus because it
contains so many oval-shaped, bright-green
organelles. To which kingdom would you assign this
organism, and why? Can the organism be classified
further without conducting additional tests? Explain.
Making Connections
14. A peregrine falcon in Washington State was
diagnosed with malaria. It was thought to be an
isolated case until the disease was discovered in
other birds of prey. How do you suppose malaria was
transmitted to the falcon? What implications could
this have for humans?
15. What are harmful algal blooms (HABs)? Draw a flow
chart to show their effect on the food chain. Describe
a human illness associated with HABs.
GO
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Microbiology 133