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1.13
Unicellular Organisms
You are a multicellular organism. You have many specialized cells that
work together to carry out all of life’s functions. However, many living
things are composed of just one cell. These unicellular organisms,
referred to as microorganisms or microbes because they are only visible
under a microscope, must also carry out all of life’s functions. The
single cell is responsible for feeding, digestion, excretion, and
reproduction.
The Importance of Microorganisms
Most people become aware of microorganisms when they get sick.
However, it is unfair to think of microorganisms just in terms of
disease. It’s true that they cause many diseases, but most are harmless
and many are even helpful, as you can see in Figure 1. Dairy products
such as buttermilk, cottage cheese, and yogurt are produced by the
action of microorganisms.
Bacteria
Bacteria (singular form is bacterium) are among the most primitive and
also the most plentiful organisms on the planet. They are said to be
very successful because they have survived and changed little over
several billion years (Figure 2). Some, like plants, can make their own
food. Others are parasites. (Parasites can live by invading the body of
an animal or a plant.) Some bacteria can even live with little or no
oxygen. There are bacteria in every Earth environment, even in hot
springs. Bacteria are different from animal and plant cells in that
they have no nucleus, no mitochondria, and no ribosomes.
pili: These hairlike structures help bacteria
to attach to each other and to surfaces.
They also help bacteria to move.
a Each droplet sprayed into
the air during a sneeze
could contain thousands
of microorganisms.
b Microorganisms
decompose dead plants
and animals into chemical
building blocks that can
be recycled by plants into
food for humans and
other animals.
Figure 1
Some microorganisms make
us sick, but without the
others we could not survive.
flagellum: Some bacteria
have whiplike tails to help
them move.
chromosome: The genetic
material of bacteria is organized
in one chromosome. There is no
nucleus.
cell membrane: Regulates
movement of materials
into and out of the cell
Figure 2
A typical bacterium
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Unit 1
cell wall: Provides
rigid support
capsule: A sticky coating surrounds disease-causing
bacteria. The capsule makes it difficult for animals’
white blood cells to destroy these bacteria.
Protists
If you look into a drop of pond water, you
will find an incredible collection of protists.
Almost anywhere there is water, even in moist
soil or in rotting leaves, you will find protists.
Unlike bacteria, protists have a nucleus and
contain organelles such as mitochondria,
ribosomes, and lysosomes.
Plantlike Protists
Diatoms
Diatoms are found in both fresh and salt
water. They contain chlorophyll and can make
their own food. Diatoms are encased in two
thin shells joined together. Figure 3 shows
some diatoms.
Euglena
Euglena (Figure 4) is like both a plant and an
animal cell. If there is lots of sunlight, euglena
acts like a plant and makes its own food. With
reduced sunlight, euglena acts like an animal
and begins feeding upon smaller cells.
Figure 3
Each species of diatom has a unique shape.
They all have symmetrical grooves and pores.
flagellum: Helps the cell to move
eyespot: Directs euglena
toward the light
mitochondria: Provide the energy
for the flagellum and other needs
Golgi apparatus:
Stores chemicals
chloroplasts: Contain chlorophyll,
the green pigment used to make
food by photosynthesis
contractile vacuole:
Expels excess water
Figure 4
Euglena has features of both
plant and animal cells.
nucleus:
Contains the
genetic material
pellicle: A firm covering that
surrounds the cell membrane.
Unlike plant cell walls, this
covering is flexible.
Cells, Tissues, Organs, and Systems
43
Animal-like Protists
Paramecium
Animal-like protists cannot make their own
food and must feed on things that are living
or were once alive. They have all of the
organelles of an animal cell, and, like
euglena, they have a contractile vacuole.
Amoeba
As the amoeba moves, it changes shape
(Figure 5). These bloblike organisms move by
stretching out a branch of cytoplasm, called a
pseudopod (false foot). The pseudopod
anchors to an object, and the rest of the cell is
dragged toward it. This method of movement
is also used by animal white blood cells,
including the ones in your blood vessels. The
crawling motion of the amoeba is also used
for feeding.
The paramecium (plural paramecia), like
the amoeba, uses structures designed for
movement to help it feed (Figure 6). Tiny
hairlike structures, called cilia, beat together
to create water cur rents that move the
paramecium. There are also cilia around the
paramecium’s oral groove. These cilia draw
food into the groove. Bacteria and other
smaller cells are the main food source
for paramecia.
anal pore
Expels waste
cilia
Hairlike structures
that beat together
to move water
Figure 5
The amoeba crawls and feeds
at the same time.
nucleus
pseudopod
contractile
vacuole
food
oral groove
Lined with cilia that
sweep in food
a The amoeba is a single
animal cell. It feeds by
wrapping itself around food.
food
vacuole
b Amoeba slowly engulfs food
particles by extending its
pseudopods.
c The membrane around the food forms
a food vacuole. Digestion takes place
inside the vacuole.
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Unit 1
Figure 6
Understanding Concepts
The paramecium is also a single animal
cell that must perform most of the
functions that your body performs.
macronucleus
Contains genetic material
that regulates cell function
micronucleus
Contains genetic information
that is exchanged between
paramecia during reproduction
food vacuole
Where food is
slowly digested
1. Why do you think bacteria are
considered to be more primitive
than other cells that you have
studied?
2. Why are diatoms called plantlike
protists?
3. Compare euglena to a plant cell.
Make a list of similarities and
differences.
4. Compare the process of feeding
in the paramecium and amoeba.
Making Connections
5. Why do many people associate
microorganisms with disease?
6. Using the information that you have
gained about osmosis, indicate why
euglena, paramecia, and amoebas
need contractile vacuoles.
gullet
A cavity at the end of the oral
groove. Here the food enters
a food vacuole.
contractile vacuole
Pumps out excess water;
prevents the paramecium
from bursting
7. Penicillin is an antibiotic that
weakens the cell wall of bacteria.
The sugars and proteins in the
cytoplasm of the bacteria are in
higher concentration than they are
in their environment. Draw a series
of diagrams showing how penicillin
kills bacteria.
Exploring
Fungus
Fungi (singular fungus) include many organisms that
are multicellular. Bread mould, mushrooms, and puff
balls are well-known fungi. Harmful fungi include those
that cause ringworm, Dutch elm disease, and athlete’s
foot. However, there are some unicellular fungi.
8. What effect do chemicals such
as caffeine have on unicellular
organisms? State your hypothesis,
and design an experiment to test it.
With your teacher’s approval,
conduct your experiment.
Yeast, the Unicellular Fungus
Yeast is one of the few unicellular fungi (Figure 7).
There are many different species of yeast. Like animal
cells, yeast cells do not have chlorophyll and must rely
on other organisms for their source of energy.
Figure 7
Using a microscope,
it is difficult to tell
that yeast are living.
There are cells in the tubes that lead
to your lungs that have cilia much like
those of paramecium. Human white
blood cells, like amoeba, engulf and
digest foreign particles. Examine the
structures of the unicellular organisms
carefully. Would any of these features
be useful in your model cell? Consider
looking to primitive cells for
assistance with your Challenge.
Cells, Tissues, Organs, and Systems
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