Locomotion in Protozoa: Amoeboid Movement

Locomotion implies progression of an organism through the medium in which organism changes its place and position. A protozoan moves about in search of food material, protection and partners. Moreover, the progression is effected so as to adopt themselves to the new environment which is due to external stimuli. Such locomotion is called reflex locomotion. This type of locomotion is generally backward or forward in direction.

Protozoans also exhibit another type of locomotion, called automatic type of locomotion, which seems to begin from within the cell body.

The nature and distribution of the locomotory organelles constitutes the basis of classification. Locomotion in Protozoa is of four types, each of which is performed by means of different locomotory or motile organelles.

I -   Amoeboid Locomotion -    by means of pseudopodia

II -  Ciliary Locomotion  -    by means of cilia

III – Flagellar Locomotion -    by means of flagella

IV – Metabolic or Contractile Locomotion- by means of myonemes

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I – AMOEBOID LOCOMOTION

Progression with the help of pseudopodia is considered to be the most primitive type of locomotion. Pseudopodia are generally the temporary organelles for locomotion as well as feeding. These are the outgrowths of protoplasm of protozoa which are either naked or possess a very thin pellicle. They are mainly formed of ectoplasm in which endoplasm flows and forms a core. There is a considerable variation in their size, shape, structure and mode of working, on the basis of which they can be grouped as follows:

a) Lobopodia - These are blunt relatively short or finger-like, rarely branched pseudopodia. These are formed very quickly from the ectoplasm and endoplasm flows into it. Dellinger has
described the walking movement in Amoeba proteus. These have been also reported in Arcella limax. A. vulgaris Difflugia, etc.

b)   Filopodia – These are Filamentous pseudopodia often with rounded ends. They are made of exclusively hyaline ectoplasm.
They slightly anastomose with each other. (Amoeba radiosa, Euglypha etc.)

c)   Rhizopodia — The rhizopodia or reticulopodia or myxopodia are thin filamentous structures that frequently anastomose and form a complex network. This net work is mainly responsible to serve as a trap for food capturing rather than locomotion. The protoplasmic cylindrical axis if surrounded by constantly streaming granular cytoplasm. (Elphidium, Allogromia, Chlamydoprys).

The forameniferans have a characteristic pseudopodial arrangement of a network of fine pseudopodial strands, which are termed “reticulopodia”. Microtubular strands/fibril support these reticulopodia (McGee- Russel and Allen 1971).

d) Axopodia - The axopodia or actinopodia are stiff protoplasmic filaments. They are somewhat semi-transparent filaments radiating from the cell body. They possess a hare central axis of endoplasm and an outer covering of cytoplasmic sheath. It can be easily formed and equally quickly absorbed. These axopodia are mainly used to capture food particles as in case of Actinophrys. Each axopod has a central structure, the axial rod or axoneme that is made up from a large number of microtubules which arc arranged parallel to the long axis of axopod. The number of microtubules per axoneme is not constant.

The Helizoida, which are often called “sun-organisms’ because they resemble the Sun, have numerous, long, stiff extensions called axopodia. These organisms move very slowly, rolling along by shortening and lengthening of axopodia. The forward axopodia lengthen and become attached at the same time the posterior axopodia detach and retract.

pseudopodia formation in Amoeba

Amoeboid movement is characteristic feature of  Rhizopoda and some Sporozoa. This type of locomotion has been explained by a number of theories. Hyman (1917) proposed the change of viscosity theory or sol-gel theory. Pantin (1923-26) later advanced this theory. According to this theory the consistency of protoplasm is constantly changing.

The light microscopic structure reveals that ordinarily there are two layers – the ectoplasm and the endoplasm. The ectoplasm can be further  physiologically differentiated into-

(i)   Hyaline area the outer most part which serves as a limiting membrane and (ii)  Inner plasmalemma.

The endoplasm consists of the solated central portion called (i) Plasmasol, which is surrounded by a gelated protoplasm (ii) Plasmagel. Most (1925) in Amoeba has reported that four processes take place one after the other in the following succession -

i)   Plasmalemma attaches to the substratum.

ii)  Plasmasol flows forward and undergoes gelation at the anterior end

iii) Plasmagel of hinder end, at the same time, undergoes solation.

iv) Plasmagel tube at the posterior end contracts and moves forward.

The repeated succession of the four processes results into the forward movement of the organism. The speed of locomotion depends upon nature of substratum, temperature, osmotic pressure of the media, salt concentration and pH of the medium. The speed varies from 0.2 to 2 micron per second. Goldacre and Lorch (1950) have expressed their doubt regarding the gelation and solation of cytoplasm can results into a force, which is capable of moving Amoeba.

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