Prokaryotic and eukaryotic

    Prokaryotic

      Bacteria are so small (1 micron in length) so they are subject to violent thermal agitation of the medium (Brownian motion). Yet they are able to move towards food sources or avoid adverse environmental conditions, moving to the considerable speed for their size, from 20 to 30 micrometers per second. Most often, they are using flagella, which function as a boat propeller. In the intestinal bacterium Escherichia coil, for example, each flagellum is a rigid filament 14 thousandths of micrometers in diameter and 10 micrometers long, which runs at the incredible speed of 200 revolutions per second with a small rotary engine inserted into the membrane and the cell wall (fig. 2). Flagella having a propeller straight shape, its rotation in the direction of clockwise when viewed from the bacterium to the flagellum, produces a thrust on the medium which causes the movement of the bacterium in the opposite direction positive thrust. Different flagella cooperate to produce a rectilinear movement of the bacterium, forming a single braid compatible with the helix in a straight form flagella and their individual rotation. Such a hard movement of the order of one second, after which the cell changes abruptly, and randomly, by pivoting direction on site, with a reversal of the direction of rotation of the flagella, which is incompatible with maintaining a organization of flagella in a single braid producing traction. A change in the frequency of changes in direction of the bacteria caused by signals from the environment to which it is sensitive allows him to move.
Some bacteria have developed a very different alternative means of moving the swim. These pathogenic bacteria, whose development in the body is intracellular. Thus, the bacterium responsible for listeriosis (Listeria monocytogenes) is able to give its flagella when it enters a cell and use it to his benefit systems molecular assemblies of the host cell to pass very quickly, and invade step by step, the cells in the body, thus escaping immune surveillance. Indeed, with some molecules it synthesizes and presents on its surface, this bacterium can govern the assembly of actin microfilaments, forming a so-called comet tail in its wake structure. This mechanism is now the subject of many studies. It is indeed convenient for understanding the assembly mechanisms at the origin of amoeboid movement type in front of the eukaryotic cell model system: it can be observed in a cell-free extract and interested physicists as well as biologists.

 Eukaryotic 

Numerous unicellular eukaryotic species move through ciliatures or flagellar apparatus of great variety, sometimes consist of several thousand flagella or cilia. However, all eyelashes and all flagella have, with few exceptions, the same basic structure, based primarily on microtubules. Cilia or flagella eukaryotic cells are fundamentally different bacterial flagella in size, at least ten times greater in diameter, and in their operation in that, while projecting outside of the cell body, they remain surrounded by the plasma membrane and intracellular organelles are therefore, unlike the bacterial flagellum (Fig. 2). The distinction between lash and flagella in eukaryotes based on their type of beat and not their structure. The difference between the beating of cilia and flagella explains that flagella are often significantly longer than the eyelashes. Flagella push or pull the cell body through trains symmetrical waves that propagate from one end to the other. Cilia function more like oars, or like the arms of a swimmer, and an asymmetrical movement in which one can distinguish an active support of the surrounding liquid phase and a recovery phase which produces no movement . These ciliary or flagellar beating are not completely stereotyped as protozoa are capable of a wide variety of behaviors depending on environmental conditions. Consider two examples, one in ciliates and flagellates in the other.
Swimming cells occurs through a coordinated beating of cilia or flagella. The movement of the cell in a given direction is often accompanied by a slow rotation of the cell body. This is the case for the paramecium, ovoid cell of more than 100 microns in length which has several thousand lashes evenly distributed in longitudinal rows (along meridians). Beat is synchronous with a row of eyelashes to each other in the transverse direction but has a small phase shift along a row, which causes a métachronale spiral wave at the surface of the cell which results in a slow rotation. Paramecium face an obstacle can change course when reversing, under the control of a calcium signal, the beat of her lashes, which has the effect of reducing. Then the ciliary beat stops temporarily on the entire surface of the cell, except in the buccal cavity, and very rich in eyelashes located in an asymmetric position on the front of the cell. This produces a rotation of the axis of the cell body. When the normal beat resumes, the direction of forward motion forms an angle of tens of degrees with the previous management. Paramecium also has answers for many environmental signals, including the gravity field.
For its part, the unicellular green alga Chlamydomonas biflagellate reinardtii moves forward with an asymmetric beat (ciliary type) from its two flagella, whose bases are oriented at 450 with respect to the axis of the cell body, and the direction of movement. The beating of the two flagella occur symmetrically with respect to the plane of symmetry of the flagellar apparatus. There is however a slight shift between the beats of the two flagella, causing again a slow rotation of the cell body. This rotation allows him to explore the environment, particularly to locate the source of light through a photoreceptor structure in an asymmetric position, the stigma (or eye spot), which can, by means of transduction in the plan a governing of the membrane to calcium influx base flagella, result in modification of one beat only two flagella. As a rower using oars asymmetrically to tack the cell and changes its direction and will seek the light they need to photosynthesize. This alga is also able to retreat, especially if it is subjected to bright light, thanks to a particularly elaborate behavior: the flagellar bases (kinetosomes) normally oriented at 900 with respect to each other, can become parallel with the contraction under control calcium small structures that connect them. Two flagella then adopt a flagellar beating type and produce a rearward thrust comparable to that of a propeller. Other solutions have been selected during the evolution of unicellular algae, which can be four or even eight flagella, to ensure the transition between moving forward and backward movement.
There are many other examples. Thus, some unicellular ciliates have a large ciliature extremely differentiated by region of the surface. Groups of eyelashes can be physically associated with each other, forming tendrils whose beat is up, and can be used not only to swim but also to "walk" on surfaces. Other examples can be found in unicellular algae that use their two flagella to capture their prey.