Unicellular Organisms
By Cindy Grigg
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.
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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.
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Although they are only one cell, they are complex. The more
that we learn about them, the more marvelous we find them
to be.
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.
4
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
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cilia also use the cilia to catch food. The stentor has cilia 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.
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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Copyright © 2012 edHelper
Unicellular Organisms
1. 2. What are unicellular
What does unicellular mean?
organisms?
One cell
Plants with many cells
Many cells
Protozoa, algae, fungi,
Unicycle
or bacteria made of only one
Multicellular
cell
Not alive
Animals with many
cells
3. 4. How long do scientists think What is the advantage of
unicellular organisms have having a spherical shape?
lived on Earth?
Can get more food
3.8 trillion years
Keeps it from drying out
3.8 million years
Easier to move
Millions of years
More surface area for
3.8 billion years
chemical processes
5. 6. What is the advantage of
having a spiral shape?
Can get more food
More surface area for
chemical processes
Keeps it from drying
out
Easier to move
Where have extremophiles
been found?
On Mars and Europa
On the sun
In Antarctica and the deep
sea
On the moon
7. 8. What is one kind of
What does pseudopodia mean?
unicellular organism that is
False name
helpful to people?
Made-up name
Yeast
False feet
Euglena
Encyclopedia
Amoeba
Paramecium