Download Amoeba - Edmonds

AMOEBA. A tiny blob of colorless jelly with a dark speck inside it--this is what an
amoeba looks like when seen through a microscope. The colorless jelly is cytoplasm, and
the dark speck is the nucleus. Together they make up a single cell of protoplasm, the
basic material of all living things.
Amoebas are usually considered the lowest and most primitive form of animal life. But
simple as they may seem, these tiny one-celled organisms carry out their activities
competently and efficiently.
The membrane is flexible, taking on the shape of the more watery cytoplasm inside,
which is continually moving and changing the body shape of the amoeba. The name
amoeba comes from a Greek word that means "change."
It is by changing its body shape that the amoeba travels. First it extends a lobe like
portion called a pseudopod, meaning "false foot." Then it slowly pours the rest of its
body into the pseudopodia, which enlarges and finally becomes the whole body. New
pseudopods form as old ones disappear. Their shapes range from broad and blunt to
long, thin, sometimes branching structures. Often many pseudopods form at the same
time in an uncoordinated way, as though the amoeba were starting off in all directions at
once. But most pseudopods exist only for a short time, and then flow back into the main
body.
The amoeba feeds mainly on other microscopic, one-celled plants and animals including
algae, bacteria, and tiny protozoa in the surrounding water. It has no mouth or other
body parts for taking in or digesting food. If it finds itself near something edible, it
may put out pseudopods to surround the food and flow over it. In this way, the food,
along with a tiny drop of water around it, is completely enclosed in a bubble like chamber
in the amoeba's body. The chamber is called a food vacuole, and an amoeba may have
several in its body at the same time. Once the food is digested, the vacuole disappears.
The outer membrane prevents any nutrients-- fats, proteins, carbohydrates, and
salts--from escaping. At the same time, it allows liquid wastes to flow out into the
surrounding water. The membrane also allows the surrounding water to pass into the
amoeba's cell. This process of water absorption is called osmosis. The amoeba must
work to keep its cytoplasm from becoming too watery: it gets rid of the excess water
by pumping it out through a tiny pore. Its pump is a clear vacuole inside its body called
the contractile vacuole. The amoeba "breathes" through its membrane. Oxygen from the
surrounding water passes in and carbon dioxide passes out.
If an amoeba is cut apart, it instantly forms a new membrane over the cut surfaces.
The part containing the nucleus may survive, but the other part cannot digest food and
eventually dies.
The nucleus is necessary for reproduction, which is asexual. The nucleus simply
pinches in two in the middle; the two halves pull apart; each takes half the cytoplasm;
and one amoeba has become two. The process, called fission, takes less than one hour.
Although the amoeba has no nerves, it reacts to its surroundings. With its whole body
it responds by moving toward or away from stimuli. It retreats from strong light, or
from water that is too hot or too cold. If touched or shaken, it rolls into a ball.
Various kinds of amoebas dwell in fresh and salt waters, in moist soils, and in moist
body parts of other animals. Common species are found in the debris of stagnant ponds
and puddles. The very common Amoeba proteus is found on decaying vegetation at the
bottom of freshwater streams and ponds. One ordinary, harmless species, Entamoeba
coli, is found in the human intestine. There are numerous parasitic amoebas, most of
which are harmless. But some amoebas are responsible for serious diseases: Entamoeba
histolytica, for instance, is the cause of amoebic dysentery. Frequently found in
unsanitary areas, these amoebas are carried by sewage, polluted water, and unwashed
food. They settle in the large intestine where they eat cells and tissues. The amoeba is
classified as a member of the subphylum Sarcodina of the phylum Protozoa.