Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Protists

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pseudopodia (false feet) to move, while others are sessile. They

may be autotrophs, producing their own food from sunlight, or

heterotrophs, requiring an outside source of nutrition. It is

unknown whether protists were the precursors to plants, ani-

mals, or fungi. It is possible that several evolutionary lines of

protists developed separately. Biologists consider the protists

as a polyphyletic group, meaning they probably do not share a

common ancestor. The word protist comes from the Greek

word for the very first, which indicates that researchers assume

protists may have been the first eukaryotes to evolve on Earth.

Despite the great diversity evident in this kingdom, sci-

entists have been able to classify the protists into several

groups. The protists can be classified into one of three main

categories, animal-like, plant-like, and fungus-like. Grouping

into one of the three categories is based on an organism’s

mode of reproduction, method of nutrition, and motility. The

animal-like protists are known as the protozoa, the plant-like

protists are the algae, and the fungus-like protists are the slime

moldsand water molds.

The protozoa are all unicellular heterotrophs. They

obtain their nutrition by ingesting other organisms or dead

organic material. The word protozoa comes from the Latin

word for first animals. The protozoans are grouped into vari-

ous phyla based on their modes of locomotion. They may use

cilia, flagella, or pseudopodia. Some protozoans are sessile,

meaning they do not move. These organisms are parasitic

because they cannot actively capture food. They must live in

an area of the host organism that has a constant food supply,

such as the intestines or bloodstream of an animal. The proto-

zoans that use pseudopodia to move are known as amoebas,

those that use flagella are called flagellates, those that use cilia

are known as the ciliates, and those that do not move are called

the sporozoans.

The amoebas belong to the phylum Rhizopoda. These

protists have no wall outside of their cell membrane. This

gives the cell flexibility and allows it to change shape. The

word amoeba, in fact, comes from the Greek word for change.

Amoebas use extensions of their cell membrane, called

pseudopodia, to move as well as to engulf food. When the

pseudopodium traps a bit of food, the cell membrane closes

around the meal. This encasement forms a food vacuole.

Digestive enzymesare secreted into the food vacuole, which

break down the food. The cell then absorbs the nutrients.

Because amoebas live in water, dissolved nutrients from the

environment can diffuse directly through their cell mem-

branes. Most amoebas live in marine environments, although

some freshwater species exist. Freshwater amoebas live in a

hypotonic environment, so water is constantly moving into the

cell by osmosis. To remedy this problem, these amoebas use

contractile vacuoles to pump excess water out of the cell. Most

amoebas reproduce asexually by pinching off a part of the cell

membrane to form a new organism. Amoebas may form cysts

when environmental conditions become unfavorable. These

cysts can survive conditions such as lack of water or nutrients.

Two forms of amoebas have shells, the foraminiferans and the

radiolarians.

The foraminiferans have a hard shell made of calcium

carbonate. These shells are called tests. Foraminiferans live in

marine environments and are very abundant. When they die,

their shells fall to the ground where they become a part of the

muddy ocean floor. Geologists use the fossilized shells to

determine the ages of rocks and sediments. The shells at the

ocean floor are gradually converted into chalky deposits,

which can be uplifted to become a land formation, such as the

white cliffs of Dover in England. Radiolarians have shells

made of silica instead of calcium carbonate. Both organisms

have many tiny holes in their shells, through which they

extend their pseudopodia. The pseudopodia act as a sticky net,

trapping bits of food.

The flagellates have one or more flagella and belong to

the phylum Zoomastigina. These organisms whip their flagella

from side to side in order to move through their aquatic sur-

roundings. These organisms are also known as the zooflagel-

lates. The flagellates are mostly unicellular with a spherical or

oblong shape. A few are also amoeboid. Many ingest their

food through a primitive mouth, called the oral groove. While

most are motile, one class of flagellates, called the

Choanoflagellates, is sessile. These organisms attach to a rock

or other substrate by a stalk.

The ciliates are members of the phylum Ciliopa. There

are approximately 8,000 species of ciliates. These organisms

move by the synchronized beating of the cilia covering their

bodies. They can be found almost anywhere, in freshwater or

marine environments. Probably the best-known ciliate is the

organism Paramecium. Paramecia have many well-developed

organelles. Food enters the cell through the oral groove (lined

with cilia, to “sweep” the food into the cell), where it moves

to the gullet, which packages the meal into a food vacuole.

Enzymes released into the food vacuole break down the food,

and the nutrients are absorbed into the cell. Wastes are

removed from the cell through an anal pore. Contractile vac-

uoles pump out excess water, since paramecia live in freshwa-

ter (hypotonic) surroundings. Paramecia have two nuclei, a

macronucleus and a micronucleus. The larger macronucleus

controls most of the metabolic functions of the cell. The

smaller micronucleus controls much of the pathways involved

in sexual reproduction. Thousands of cilia appear through the

pellicle, a tough, protective covering surrounding the cell

membrane. These cilia beat in a synchronized fashion to move

the Paramecium in any direction. Underneath the pellicle are

trichocysts, which discharge tiny spikes that help trap prey.

Paramecia usually reproduce asexually, when the cell divides

into two new organisms after all of the organelles have been

duplicated. When conditions are unfavorable, however, the

organism can reproduce sexually. This form of sexual repro-

duction is called conjugation. During conjugation, two para-

mecia join at the oral groove, where they exchange genetic

material. They then separate and divide asexually, although

this division does not necessarily occur immediately following

the exchange of genetic material.

The sporozoans belong to the phylum Sporozoa. These

organisms are sessile, so they cannot capture prey. Therefore,

the sporozoans are all parasites. As their name suggests, many

of these organisms produce spores, reproductive cells that can

give rise to a new organism. Sporozoans typically have com-

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