For Bsc 1st sem
ZOOH Genome
Amoeboid movement
The pseudopodia are finger-like temporary processes given out from any part of
the body and withdrawn. The pseudopodium is formed by the projection of
ectoplasm in which endoplasm flows. Thus, the cytoplasm which enters into
pseudopodia is naturally drawn from the other parts of the body.
Amoeba moves by the formation of pseudopodia, characteristic of this animal, is
known as amoeboid movement. Various theories have been put forth to explain
the way of formation of pseudopodia and amoeboid movement.
Sol-gel theory or Change of viscosity theory:
This theory was proposed by Hyman (1917) and strongly supported by Pantin
(1923-26), Mast (1925-31) and others.
According to this theory the body of the Amoeba is made up of 4-regions:
(a) The outer most thin and elastic cell membrane or plasma membrane,
(b) Plasmagel, an outer stiffer jelly-like region of the ectoplasm,
(c) The plasmasol, an inner more fluid region of the endoplasm and
(d) A hyalin fluid which is a clear ectoplasmic area between plasma membrane
and plasmagel.
This theory assumes that the pseudopodium is formed by the change of sol to gel
and gel to sol states in the peripheral cytoplasm. The tip of the pseudopodium
controls the change.
During the formation of the pseudopodium the plasma membrane of Amoeba gets
attached to the substratum by means of an adhesive secretion. A local reversion
of plasmagel to plasmasol takes place at the anterior end by internal chemical
reaction.
The gel at the anterior end becomes thinner and weak. The rest of the plasmagel
exerts pressure on the weakened area. The contracting plasmagel of the posterior
end is continuously changed into plasmasol and it flows forwards and breaks the
weak gel.
Anteriorly the plasmagel tube is continuously regenerated by gelation of
plasmasol and a new pseudopodium is formed. The animal then progresses
forward with the help of the pseudopodium.
Four steps occurring simultaneously in the body of Amoeba:
(i) Attachment of the body of Amoeba to the substratum.
(ii) Conversion of plasmasol into plasmagel, i.e., gelation at the anterior
advancing tip.
(iii) Conversion of plasmagel into plasmasol, i.e., solation at the posterior
opposite end of the body.
(iv) Contraction of the plasmagel at the posterior end of the body to push the
plasmasol forward.
These processes are repeated again and again and, thus, Amoeba moves ahead.
The speed of amoeboid movement varies from 2 microns to 3 microns per second.
This is supposed to be the most primitive type of animal movement.
Remark:
Though the theory is most popular to the zoologists but the actual mechanism of
reversion of gel to sol or vice versa could not be explained properly.
Molecular mechanism:
The amoeboid movement is related to the sol-gel transition of cytoplasm and
various non-muscle contractile proteins, Ca++ ions and membrane receptors are
involved in this.
Under normal or resting condition the ectoplasm remains in gel state in which the
actin filaments are cross-linked with one another to form a complex network-like
structure and sol-state condition of the endoplasm contains non cross-linked actin
filaments.
