Cell motility

Cell motility

The major characteristic of living organisms is movement, and can take the form either of movements of cells or the movements within cells themselves. Some bacteria which are lowerprokaryotic cells are able to swim within their environment with the help of fine appendages, but the motile repertoire exhibited by higher eukaryotic cells is much greater. Higher cells may also have to beat appendages in the form of numerous short cilia or fewer longer flagella and in the unicellular protozoa these function in a coordinated way, not only to propel the organism but also to assist feeding. The cilia are also present on cells of certain differentiated tissues of metazoans, where their beating serves to move the environment relative to the stationary cells. But not all protozoa move by means of cilia and flagella. Some, such as freshwater and soil amoebae, crawl over their substrates, and the variety of cells, such as leucocytes, in multicellular organisms show similar crawling movements. Higher cells, as well as being much larger than bacteria, have separate nuclear and the cytoplasmic compartments and often show considerable asymmetry. This has necessitated the development of mechanisms for the moving components intracellularly, a property that is well illustrated by plant cells, many of which show different forms the exaggerated cytoplasmic streaming. Intracellular movement is also an important feature of animal cells in which the organelles, vesicles, and the genetic messages are transported to and from the specific sites within the cell. Replicated genetic material within chromosomes is also separated into two prospective daughter cells by similar mechanisms of that.

Swimming Cells:

The bacteria, Ciliates and Flagellates, Sperm Many bacteria can sense various aspects of their surroundings and respond by moving towards or away from stimuli, for example by migrating up and down the chemical gradients of attractants and repellents. Some motile bacteria glide over surfaces. Other bacteria, such as the Escherichia coli, swim by means of a number of fine helical filamentous flagella approximately 20 nm in diameter and the several micrometres in length. These act as propellers, rotating rapidly at about 300 Hz, driven by a rotary motor at their base. The energy for the driving the motor derives from the transmembrane proton gradient. The rotatory motors on the one cell are capable of turning in the both directions, and the collective direction determines the swimming path. The observations have shown that when all the flagellar motors are rotating in the same direction the flagella form a bundle and the bacterium is propelled . Swimming Cells: Bacteria, Ciliates, and Flagellates, Sperm . Crawling Cells: Amoebae, the Leucocytes . The internal Movements: Cytoplasmic Streaming, Vesicle Transport . Dividing Cells, Contracting Cells . Movements in Tissues: Fibroblasts, Growth Cones, Cancer Cells When, however, one or more rotates in the opposite direction the flagella are splayed, the movement is more irregular and the cell is seen to ‘tumble’. A cell normally alternates between the swimming patterns, performing what is known as a random walk. In response to a stimulus the emphasis on the particular direction of the flagellar rotation changes, and the pattern of swimming is altered. The flagellar rotation is genetically complex, with around 50 genes required for flagellar assembly and functioning, so that it is unsurprising that the mechanism of torque production is not understood. The certain higher cells throughout the plant and the animal kingdoms also possess the motile appendages on their cell surfaces and use these either for the swimming or for the moving fluid over their surfaces. The protozoa, for example, covered by numerous short cilia, whereas others possess the fewer longer flagella, and that are used to propel the unicell through its environment. The considerable variation exists in the pattern of the ciliary and the flagellar movement. However, in general, a cilium beats via a rapid bending of the effective stroke – and this is followed by the slower recovery stroke in which the wave passes from its base to the tip to return the cilium to its original position.

Crawling Cells

Amoebae,the leucocytes A with the wide variety of cells move by means of crawling over substrates rather than swimming through their environment. Since the microscopes began to be used to look at cells, observers have been fascinated by the movements of the free-living protozoans, such as the freshwater Amoeba proteus. The observations of A. proteus have shown that it is the advances over its substrate by extending the large processes known as pseudopodia and that their direction of extension from the cell surface may be in the response of food. Within an extending pseudopodium, endoplasm (known as Plasmasol) can be seen to flow forward through a more rigid outer layer of ectoplasm. When the forward-flowing Plasmasol reaches the so it is called hyaline cap at the pseudopod tip, it is dispersed in a fountain-like streaming pattern before being converted to the stationary the granular ectoplasmic Plasmagel. At the tail or uroid of A. proteus, the converse occurs and Plasmagel is recruited to the Plasmasol which flows forward rapidly to complete the cycle. Many different amoeboid cells living in freshwater, seawater and also the soil migrate in this fashion, the main difference being the size of the pseudopodial extensions which vary from broad Lobopodia to much finer filopodia.

Internal Movements

Cytoplasmic Streaming, Vesicle Transport Plant cells are often large with substantial central vacuoles and there are numerous instances of cytoplasmic streaming associated with this. Patterns of streaming vary considerably, but certain examples are particularly spectacular and have been chosen for experimental investigation. One such is the stamenal hairs of the flowers of Tradescantia. Stamenal hairs are comprised of elongated cells joined end to end. Cytoplasmic strands traverse the central vacuole and not only does the deployment of the strands constantly change but streaming in individual strands shows a complex pattern at rates of the order of 20 mm s 2 1 . A quite different type of streaming occurs in the exceptionally large internodal cells of the alga Nitella. In this case, there is a circulatory rotational streaming of the endoplasmic layer inside a stationary layer of ectoplasm. Here streaming is unidirectional and at a rate of approximately 100 mm s 2 1 . Both types of cytoplasmic streaming have been shown to be based on myosin-driven movements along actin filaments appropriately arranged in the plant cell cytoplasm. Compared with cytoplasmic streaming, animal cells tend to show a slower translocation of organelles and vesicles along another cytoskeletal component: microtubules. In undifferentiated cells, microtubules derive from a region close to the nucleus and extend towards the cell periphery. In differentiated cells, they are invariably found parallel to axes of asymmetry, and indeed are responsible for maintaining asymmetrical cell shape. While occurring throughout all cells, vesicle translocation is often greatly emphasized in cell extensions or processes. One of the most extensively studied examples is the vertebrate neuron, which has short dendrites and a long axon extending from the cell body Components synthesized in the cell body pass outwards in an anterograde direction along the axon to the synapse, and do so at a fast rate of approximately 5 mm s 2 1 . This is superimposed on a slower,

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