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Unicellular Organisms
By Cindy Grigg
1
You know that living things are made of cells. Some
living things are made of only one cell. They are called
unicellular. Uni- means one, as a unicycle has only one wheel.
Most of us don't think about these unicellular creatures very
much, but they are all around us. Most are invisible without a
microscope, but a few can be seen with the naked eye.
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The main groups of unicellular organisms are bacteria, protozoa, unicellular
algae, and unicellular fungi or yeasts. Each species is unique. They can be interesting
to study because of their intricate and beautiful patterns. Some are sensitive to light.
They may move toward it or away from it. They have novel ways of moving around.
They also respond to touch. Some can live in very extreme environments.
3
Single-celled organisms have been on Earth for 3.8 billion years. They have had a
long time in which to adapt. Scientists believe they were the first forms of life on
Earth. Although they are only one cell, they are complex. The more that we learn
about them, the more marvelous we find them to be.
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Their shapes may be spheres, oblong, or spiral. A spherical shape helps keep them
from drying out. That would be fatal to a cell. An oblong shape increases the cell's
surface area for chemical processes. A spiral shape makes it easier for the organism to
move.
5
Unicellular organisms have unusual ways of moving. The euglena have tiny
hair-like structures called flagella. The flagella beat in the water and move the cell
forward. The word flagellum means whip in Latin. Flagella (pronounced fla-jell-ah) is
the plural form. Some have groups or rows of structures that move together like oars.
These are called cilia (pronounced silly-ah). Cells with cilia move fast! One
unicellular organism that moves with cilia is the paramecium.
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Some unicellular organisms are able to change their shapes. Amoebas are
"shape-shifters" that can extend parts of themselves into what are called pseudopodia
and then flow in the direction in which they wish to go. Pseudopodia (pronounced
sue-do-pode-ee-ah) means false feet. If they find something to eat, they can engulf, or
surround, the particle with their pseudopodia. Then they pull themselves around it to
digest it.
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Just as the amoeba uses its pseudopodia for both movement and feeding, some
unicellular organisms with cilia also use the cilia to catch food. The stentor has cilia
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that beat rhythmically, which draws food into its mouth. This adaptation allows
stentors to spend most of their lives attached to a surface. They can then save their
energy for catching food.
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Some unicellular organisms live in extreme environments. They are called
extremophiles. They have been found in such extreme habitats as Antarctic ice and in
very hot deep-sea vents. The ability of organisms to adapt to extremely hot or cold
environments gives scientists hope that signs of life will eventually be found on other
planets. They believe that Mars and Europa, an icy moon of Jupiter, have conditions
that could support some sort of living thing.
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People use yeast, one kind of unicellular organism, to make bread and beer.
Others help us by eating harmful bacteria and are food for fish and other animals. At
least one kind of unicellular organism can cause illness in people. Amoebic dysentery,
whose main symptom is bloody diarrhea, is transmitted by water containing one type
of amoeba. It is well known as traveler's dysentery, or Montezuma's Revenge,
particularly in Mexico. Liver infection and abscesses can occur also. It can be treated
with antibiotic drugs.
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Unicellular organisms are all around us. They have lived on Earth for many
millions of years and have developed unique ways to survive. Although small, they are
complex and beautiful. The next time you look at pond water, just think what you
might discover there!
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Name _____________________________
Date ___________________
Unicellular Organisms
1. What are unicellular organisms?
Not alive
Plants with many cells
Protozoa, algae, fungi, or
bacteria made of only one cell
Animals with many cells
3. How long do scientists think
unicellular organisms have lived on
Earth?
3.8 trillion years
3.8 million years
Millions of years
3.8 billion years
5. What is the advantage of having a
spiral shape?
Easier to move
Keeps it from drying out
More surface area for chemical
processes
Can get more food
7. What is one kind of unicellular
organism that is helpful to people?
Paramecium
Yeast
Amoeba
Euglena
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2. What does unicellular mean?
Unicycle
One cell
Many cells
Multicellular
4. What is the advantage of having a
spherical shape?
Keeps it from drying out
Easier to move
Can get more food
More surface area for chemical
processes
6. Where have extremophiles been
found?
On Mars and Europa
In Antarctica and the deep sea
On the moon
On the sun
8. What does pseudopodia mean?
False name
Encyclopedia
Made-up name
False feet
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Name _____________________________
Date ___________________
Unicellular Organisms
Choose one unicellular organism and write about it. Include as many details as you can.
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Name _____________________________
Date ___________________
Unicellular Organisms
What are the differences between unicellular and multicellular organisms? Include as many
details as you can.
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Unicellular Organisms - Answer Key
1
2
3
4
5
6
7
8
Protozoa, algae, fungi, or bacteria made of only one cell
One cell
3.8 billion years
Keeps it from drying out
Easier to move
In Antarctica and the deep sea
Yeast
False feet
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Unicellular Organisms
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