Protozoa www.biologycorner.com
Organisms that are single celled, swim in water and consume food are generally
called protozoa. They belong to the
Kingdom Protista and are classified into different phyla based on how they
move. If you take a drop of pond water
and observe it under the microscope, you can often see tiny little organisms
swimming around. In fact, Anton van
Leeuwenhoek, one of the first scientists to observe these creatures under the
microscope gave them the name “animalcules”, as if they were a combination of
animals and molecules. Though protozoa
may be tiny and unicellular, they have fascinating complexity.
Take for instance, the amoeba, which belongs to the Phylum Sarcodina. This single-celled protist can be any shape it wants because its membrane is flexible and it can push its cytoplasm around to change its shape. The word “amoeba” means “to change”. When you first look into the microscope for an amoeba, you may miss it because it does move slowly. It seems to take a lazy approach to life by casually stretching out its cytoplasm into extensions called pseudopodia. These extensions can also trap smaller protists within them, which create a food vacuole where the amoeba can digest them. In this case, slow doesn’t mean harmless – the smaller protists really don’t even sense the danger.
Another interesting protozoan is the paramecium. It moves using tiny hair-like structures on
its surface called cilia. In
fact, the paramecium belongs to a whole group of protists that move using
cilia, the Phylum Ciliophora. Compared
to the amoeba, the paramecium is fast swimmer. It is so fast that when looking for it under the microscope it may zoom
right over your viewing field before you have a chance to really even see
it. For this reason, biologists add a
thickening agent to the water to slow the paramecium down so it can be seen
more clearly. You can also place
obstacles on the slide to get in its way, such as cotton fibers. Once you have the paramecium slowed or
trapped, you can see many amazing features within it.
The paramecium has two nuclei. One nucleus controls the cells activities, and the other functions in sexual reproduction. As the paramecium swims forward, it will roll its body so you can see both sides. On one side is an indentation called the oral groove. The paramecium sweeps food into this opening, which then forms a food vacuole within the cell where digestion occurs. Like the amoeba, paramecium generally eat protists that are smaller than they are. The oral groove is also used in sexual reproduction, where two paramecia join together and exchange DNA. Once they separate and divide by mitosis, the new paramecia are different from the original parent.
Both the amoeba and the paramecium live in fresh water, and due to osmosis, water will tend to enter their cells. These two protists must have a strategy for removing the excess water (or they might explode!). The organelle called the contractile vacuole does the job. It serves as a water pump to remove the extra water that builds up in the cell. Under the microscope, the contractile vacuole will often look like a clear air bubble within the cell.
The amoeba and paramecium are just two of the many protozoa you can find living in pond water. There are other groups like the Zoomastigina phylum which include protists that move using a tail like structure called a flagella. There is even a group of protists that are parasitic and live within a host. Malaria is an illness caused by a protist that infects the blood through the bite of a mosquito. Generally, most protists are harmless and can be studied safely in a biology laboratory.
1. What organelle is
used to remove excess water in protozoa?
a. food vacuole b. contractile vacuole c. nucleus
2. Which of these
protists moves the fastest?
a. amoeba b. paramecium
3. Where do the
amoeba and the paramecium live?
a. pond water b. sea
water c. within the blood
4. The word “amoeba”
means:
a. to change b. unicellular c. slow mover
5. To what Kindgom
and Phylum does the paramecium belong?
a. Protista, Sarcodina b. Protista, Ciliophora c. Ciliophora, Sarcodina
6. A pseudopodia is
a(n)
a. row of cilia b. type
of protist c. extension of the cytoplasm
7. Which is an
illness caused by a protist:
a. swine flu b. small pox c. malaria
8. Food is digested
within:
a. the oral groove b. food vacuoles c. the nucleus
9. Tiny hairlike
structures located on the surface of the cell are called:
a. cilia b. flagella c. pseudopodia
10. Which of the
following is unicellular?
a. amoeba b. paramecium c. both
11. Mitosis is a type
of:
a. movement b. cell
division c. amoeba
12. Protozoa are
grouped into different phyla based on:
a. their color and size b. how they move c. where they live
13. If you are
studying a paramecium, what should you do to the slide?
a. heat it b. add cotton
fibers c. turn it upside down
14. How does an
amoeba catch its food?
a. by trapping it within its pseudopodia b. by sweeping food into its oral groove
15. The Blepharisma
is protist related to the paramecium. It
has cilia to help it move. How would you
classify the blepharisma?
a. Phylum Sarcodina b. Phylum Zoomastigina c. Phylum
Ciliophora