Cell locomotion may involve movement of the entire cell or a portion of it, to the benefit of the organism. This is a biological phenomenon displayed o enable many roles such as feeding, digestion, reproduction, circulation and protection. A cell’s or unicellular organism’s locomotion is accomplished by the use of flagella, cilia, and, in particular, a muscular system.
Biological movement are also performed by the cytoplasm of the cells such as cytoplasmic streaming in plants cells or cyclosis in amoeba.
As there is no exterior cell wall in animal cells, finger like blunt protrusions form the cell may protect, which help to locomotion and also feeding.
In amoeba, which is a unicellular protozoan, such protrusions are called pseudopodia which are formed at any points on the surface. Pseudopodia are temporary process formed when an amoeba is moving on a solid substratum.
Actin filaments and myosin molecules have been identified in amoeba. When they interact, action-mycosis complex is formed, providing the basis for locomotion. Due to this interaction, contraction, is possible with hydrolysis of ATP.
Various hypothesis has been postulated for explaining the formulation of pseudopodia and locomotion in amoeba. The ectoplasm layer of an amoeba is more rigid and gel-like, whereas the endoplasm layer more diluted and sol-like. When the ectoplasm contracts in a certain region, the liquid endoplasm streams towards the region, which can be observed under a polarized microscope. When the endoplasm is propelled in the ectoplasmic tube, it also changes its viscosity and become rigid. Automatically, contraction is created in the direction opposite to that of streaming of endoplasm.
According to Mast, during the formation of pseudopodium, the plasma membrane gets attached to the substratum followed by a local and partial liquefaction in the plasmagel at that point. Rest of the plasma gel flows out in that area to produce a bulge. At the posterior end, contracting plasmagel in converted to plasmasol and anteriorly, an ectoplasmic tube is continuously regenerated by gelation of plasmasol.
The sol-gel interconversion are actually the contraction-relaxation events, which are enforced by the osmotic pressure and ither ionic changes. It is considered that ca++ ions play an important role in regulation the ameboid movements. Free ca++ ions in the plasmasol induce contractions and bring about conversion of sol into gel.
Flagellar and ciliary locomotion:
With the help of specialized locomotor organelles on the surface, viz., flagella and cilia, many unicellular organisms exhibit locomotion.
Flagella are long, slender thread-like extensions from the cell’s surface. The base of the flagellum is anchored in the cytoplasm on a motion-controlling kinetosome region of granule. A cell usually has one flagellum but tin many cases there may be more anchored in the kinetosome granules.
Fig: Movement of Flagella
Cilia are shorter and each cilium has its own kinetosomes at its base. A cilium is about 1 to 15 um long, whereas a flagellum can be about 150um long.
Cilia and flagella are made up of a cytoplasmic core surrounded by a double membrane that is an extension of the cytoplasm. Inside the cytoplasm pairs of elongated fibers are present in the center. In all three are nine paired and two unpaired fibrils attached to a basal plate.
Fig: Structure of Motile and Non-motile Cilium
The cilium originates from a basal body or kinetosome which lies embedded in the ectoplasm. The tip of the flagellum or cilium tappers to a point where the number of fibrils is reduced. In the certain cases the basal region contains a secondary fiber.
In some cells fine ciliary rootlets are also found arising from the basal granule. Cross section of cilium shows that the axial microtubular structure of axonema consists of nine pairs of longitudinal tubules, arranged usually around two centrals unpaired filaments. The central tubules may or may not always be present, hence it is suggested that the essentials motile elements are the peripheral tubules. The paired peripheral microtubes are ellipsoidal, whereas the central ones are circular.
Each paired microtubule or doublet has a subfibre A and a subfibre B, with subfibre B being the bigger of the two. Each subfibre A generates dynein arms, which are all orientated clockwise.
Subfibre A of the doublet gives rise to radial links or spoke like structures and they reach the central sheath of unpaired fibers. The radial links end up in a dense knob like structure. The dynein arms contain dynein or a high molecular weight ATPase, requirements Mg+2 and ca2+ for its activity. The basic mechanism of ciliary or flagellar motion, as proposed by Gibbons in 1917, is due to interaction between tubulin and dynein.
Basal body or kinetosomes is similar to the centrioles of the mitotic spindle. The basal body in a cross section in seen to consist of 9 groups of tubules arranged in a circle.
The tubules are connected with each other and also to the central fiber through radial connections. All tubular is enveloped in a membrane, and from the basal body in may cases two thin filaments or rootless are found to the extend. From the basal plate of the cilium two dense perpendicular process arise which seem to originate form the triplets. These processes have been termed the basal feet, which are composed of microfilaments.
Movement of cilia is coordinated when they beat together, and the rhythm is called isochronal. But certain times each cilium moves a fraction of a second after the preceding one, producing a wave-like movement and this rhythm is called metachronal.
Fig: Structure of motile Cilia
Ciliary movement is studied in two parts: the effective stroke caused by the simultaneous contraction of five of the paired microtubules, causing the cilium to bend like a hook; the second part is the recovery stroke, which is slower than the effective stroke and is supposed to be causes by the contraction of the other four tubular filaments. In the recovery stroke the cilium is not stiff, but exhibits a contractile motion from the base to tip. The central two filaments act to extend support. However, their role is not clear.