2. • An amoeba (also ameba, amœba or amo
eboid) is a type of cell or organism which
has the ability to alter its shape, primarily
by extending and
retracting pseudopods. Amoebae do not
form a single taxonomic group, but are
found in every
major lineage of eukaryotic organisms
(domain Eukaryota).
3. • Amoeoboid cells occur not only
among the protozoa, but
also fungi, algae and animals.
• Among microbiologists, the terms
"amoeboid" and "amoebae" are
often used interchangeably for
any organism that
exhibits amoeboid movement.
4. • The best known amoeboid protists are the
"giant amoebae" Chaos
carolinense and Amoeba proteus, both of
which are widely cultivated and studied in
classrooms and laboratories. Other well
known species include the so-called "brain-
eating amoeba" Naegleria fowleri, the
intestinal parasiteEntamoeba histolytica,
which causes amoebic dysentery, and the
multicellular "social amoeba" Dictyostelium
discoideum.
9. • Amoebae move and eat by using pseudopods,
which are bulges of cytoplasm formed by the
coordinated action
of actin microfilaments pushing out the plasma
membrane that surrounds the cell.
• Groups of amoebae are distinguished by the
appearance and internal structure of their
pseudopods.
• Amoebozoan species typically have bulbous
pseudopods, rounded at the ends and roughly
tubular in cross-section (lobose)
10. • Cercozoan amoeboids, such
as Euglypha and Gromia, have slender,
thread-like (filose)
pseudopods. Foraminiferan emit fine,
branching pseudopods that merge with
one another to form net-like (reticulose)
structures. Some groups, such as
the Radiolaria and the amoeboids loosely
called Heliozoa, have stiff, needle-like,
radiating actinopods supported from
within by bundles of microtubules
12. • Free-living amoebae may be "testate"
(enclosed within a hard shell), or "naked"
(lacking any hard covering). Testate amoebae
shells are composed various substances,
including calcium, silica, chitin, or
agglutinations of found materials like small
grains of sand and the frustules of diatoms.
• To regulate osmotic pressure, most
freshwater amoebae have a
contractile vacuole which expels excess water
from the cell.
14. • Some are detritivores and eat dead organic
material. Amoebae typically ingest their food
by phagocytosis, extending pseudopods to
encircle and engulf live prey or particles of
scavenged material.
• Amoeboid cells do not have a mouth
or cytostome, and there is no fixed place on
the cell at which phagocytosis normally
occurs. Some amoebae also feed
by pinocytosis, imbibing dissolved nutrients
through vesicles formed within the cell
membrane.
16. • Amoebae in multicellular organisms: animals
and slime molds
• Some multicellular organisms have amoeboid
cells only in certain phases of life, or use
amoeboid movements for specialized
functions.
• In the immune system of humans and other
animals, amoeboid white blood cells pursue
invading organisms, such as bacteria and
pathogenic protists, and engulf them
by phagocytosis.
17. • Amoeboid stages also occur in
the multicellular fungus-like protists,
the so-called slime molds.
• Both the plasmodial slime molds,
currently classified in the
class Myxogastria, and the cellular
slime molds of the
groups Acrasida and Dictyosteliida,
live as amoebae during their feeding
stage.
18. • The cells of the former form a
giant multinucleate amoeboid
organism,while the cells of the
latter live separately until food
runs out, at which time the
amoebae aggregate to form a
multicellular migrating slug which
functions as a single organism
