Protozoa- Locomotory organelles and locomotion methods

Locomotory Organelles in Protozoa

Protozoan shows different verities of locomotory organs, such as pseudopodia, pellicular contractile structure, flagella, cilia. 

1. Pseudopodia

Pseudopodia are false feet of some Sarcodina protozoans such as Amoeba. Pseudopodia are a temporary structure form by streaming of cytoplasm. It is comprised of ectoplasm and endoplasm both. Pseudopodia are four types based on form and structure.

a- Lobopodia

It is lobe-like pseudopodia with a round end, as in Amoeba. It moves by pressure flow mechanisms. Lobopodia is found in Amoebic protozoan.

b- Filopodia

Pseudopodia with the filamentous structure are called filopodia, these are usually tapering at the base and pointed at the tip. Filopodia are composed of only ectoplasm only. Example- Euglypha

c- Reticulopodia

It is also filamentous, which form branches, and branches of filaments are inter-connected profusely to form a network-like structure hence, also known as rhizopodia. Example- Globigerina. 

d- Axopodia

These are some-how straight, and each axopodium has a central axial rod covered by adhesive and granular adhesive cytoplasm. Example- Actinosphaerium.

2. Flagella

Flagella are the locomotory organ of flagellate protozoans, such as Trypanosoma, Euglena. Flagella are thread-like out projection on the body covering. Flagella of eukaryotic cells are microtubular in structure. Microtubules are arranged in 9+2 arrangement in flagella but 9+0 arrangement at basal body or origin point of flagella. Energy for the beating of flagella is mitochondrial ATP.

3. Cilia

Cilia are found in ciliate protozoan such as Paramecium. Cilia are resembling with flagella in the basic structure such as microtubular in structure, microtubules are 9+2 arrangement cilia and 9+0 arrangement at the basal body or origin point, but cilia are small in structure. Movement takes place by beating of cilia and energy for beating derive from mitochondrial ATP

4. Pelicular contractile structure

Some protozoans have the contractile structure in pellicle or ectoplasm or myonemes, such as contractile myofibril in large ciliates, ridge, and groove in Euglena.

Protozoa- Locomotory organelles and locomotion methods
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II. Method of locomotion

The above describe organ beat in a different way causing different types of movement in protozoans, so protozoans have several types of movement such as amoeboid, flagellar, ciliary, and metabolic movement. Some of the protozoans movements are described here –

1- Amoeboid movement

Sarcodina, certain Mastigophora, and Sporozoa have characteristic amoeboid movement. The process of amoeboid movement is done by pseudopodia formations, pseudopodia are formed by streaming flow of cytoplasm in the direction of movement. 

2- Flagellar movement

Flagellar movement is present in Mastigophora, which bears one or more flagellum. There are three types of flagellar movement that are recognized.

a- Paddle stroke

This type of flagellar movement is first described by Ulehla and Krijsman in 1925. They describe that in this flagellar movement of the flagellum is sideway consist of effective stroke or down-stroke in the opposite direction of movement and relaxed recovery stroke, during recovery stroke flagellum brought forward again and ready for next effective stroke. As flagella give effective stroke in water in backward direction then water propels organism in the forward direction.

b- Undulating motion

In this type of movement wave-like undulation takes place from base to tip or from tip to base. If wave-like undulation takes place from tip to base, the animal is pulled in the forward direction, and if wave-like undulation takes place from base to tip animal is pulled in the backward direction. And when undulation is spiral animals rotate.

c- Simple conical gyration

It is described in Butschli’s screw theory, this theory postulates spiral turning like a screw. This screw-like motion causes the pulling of the animal in the forward direction with spiral rotation as well as gyration of the animal. Although the exact mechanism for this type of flagellar beat is unknown, it is believed that axonemal fibers are involved in this process. Sliding tubules theory describe, doublet slide past each other, which is the cause of movement in flagella, and energy for this process is mitochondrial ATP.

3- Ciliary movement

In the case of ciliary movement, the cilia oscillate in a pendulum-like manner. In each oscillation, there is a fast effective stroke followed by the recovery stroke, like flagellar movement. During effective stroke cilia expel the water in the backward direction like an oar of the boat, and in response if this effective stroke water propels the animal in the forward direction. During recovery stroke, cilia come in forward direction ready for next effective stroke. Cilia neither beat simultaneously nor independently, cilia beat progressively in a characterized wave-like manner.

Mode of swimming by cilia

By ciliary movement animal directly does not follow the straight movement, they rotate spirally like a bullet of rifle in left-handed helix manner. It might be because cilia do not beat directly straight, beating is somehow obliquely toward the right and might be cilia at oral groove beat more obliquely and vigorously away from the mouth. This combined effect causes swimming movement in the animal.

4- Metabolic movement

This is due to the pellicular contractile structure. In this type of movement, organisms show gliding or wriggling, or peristalsis. Microtubules present in their pellicle is responsible for this type of movement.

References and Sources

  • A Text-Book of Zoology Invertebrates by R.L. Kotpal tenth edition.
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