Physical principles of cellular motion

Cellular movement
Movement is a fundamental characteristic of life. All living cells are capable of movement in some respect. This movement could either be movement of the cell through the environment, movement of the cytoplasm and internal organelles or movement of surrounding fluid across the cell surface. The ability of an eukaryotic cell to resist deformation, to transport intracellular cargo and to change shape during movement depends on the cytoskeleton, a dynamic network of filamentous proteins. Cell movement is based on the existence of the so-called motile molecules which, on reaction with cytoskeletal structures, generate movement at molecular level.

1. Cellular movement related to microfilaments
This type of movement is related to the interaction of microfilaments with molecules of the myosin type. A typical example of this type of movement is the muscle cell and it is expressed as its contraction. Movement as a result of the interaction of actin and myosin is exhibited by many other cells as well.

1.1 Amoeboid movement
This crawling movement is the result of cytoplasmic streaming into cellular extensions called pseudopods. Careful examination of these pseudopods reveals two kinds of cytoplasm. The thin, peripheral ectoplasm is a relatively clear viscous gel, whereas the more internal endoplasm is a very granular and more fluid sol. At the forward end of the advancing pseudopod, it is possible to discern a sol-gel transformation occurring. As the fluid endoplasm flows up to the tip of the cellular extension, it parts to flow down the periphery of the pseudopod toward the main body of the cell. As it moves posteriorly, the fluid sol converts to a more solid gel. Therefore, at the very tip of the pseudopod, the cytoplasm is a sol similar to cellular respiration. In terms of the cytoskeleton, pseudopodia extend and contract through the reversible assembly of actin subunits into microfilaments and of microfilaments into networks that convert the cytoplasm from sol to gel. This movement strategy is not limited to the amoeba but is seen in many cells in the animal body, such as white blood cells.

1.2 Cell adhesion
In many cells their movement is determined by their attachment to the substrate, whereby complicated structures are formed in the contact areas between the cell and its substrate. They are made up of special proteins in the cell membrane, which connect with other proteins on the inner and outer side of the cell. An example of the binding proteins in the membrane are the integrins. On the outer side of the cell, integrins bind with the proteins of the substrate, while on the inner side they connect with actin filaments.

1.3 Cytoplasmic flow/streaming
This is a typical form of cell movement in cells incapable of amoebic movement. A periodic circular flow of cytoplasm within cells. This movement speeds the distribution of materials and organelles within the cell. Cytoplasmic streaming has two major functions: it can act to move cells through the environment, and it can move molecules and organelles inside the cell. Again, the probable cause of this type of movement is the presence of actin filaments in the cytoplasm.

1.4 Cytokinesis
Cell division does not merely consist of asymmetrical division of chromosome into daughter nuclei, but includes a physicial separation of the cytoplasm into two compartments as well. The process is called cytokinesis and is accomplished by a bundle of actin and myosin filaments forming a conctractile ring in the cortical cytoplasm. The ring is an example of purpose-built temporary actin-myosin structures.

2. Cellular movement related to microtubules
This category includes both intra-cellular movement of substances and the movement of some eukaryotic cells by means of cilia and flagella. Microtubules serve as a „roads“ or tracks that guide the intracellular movement of membrane-type organelles. As in the above types of movement, mechanical energy recquired for this movement is obtained from ATP through motive molecules, in this case represented by a group of proteins called kinesin and dynein. Dyneins and kinesins are the motor proteins that use ATP energy to transport molecular „cargo“ along microtubules. This organization of movement is confirmed in Golgi complex and endoplasmic reticulum.

Eukaryotic flagella and cilia are very similar in overall structure but typically, cilia are shorter and more numerous. The common characteristics of both moving projections is their structure, the basis of which is the main cylinder (axonema) arranged of microtubuels and surrounded by a projection of the cytoplasmic membrane. The principle odf the movement of cilia and flagella is the sliding action of microtubules in the central cylinder. A single flagellum or cilium is composed of a sheathed cylinder containing regularly spaced microtubules that extend along its entire length. These microtubules are arranged in a ring of nine pairs surrounding a central pair. A system of spokes and links holds the 9+2 arrangement together. Each 9+2 array, or axoneme, arises from acentriole. The centriole remains at the base of the completed array and is called ambasal body. There is atransition zone between the basal body and the axoneme where the arrangement of microtubules within the basal body begins to take on the characteristic structure of the axoneme. Eukaryotic cilia and flagella are covered by an extension of the cell membrane, which is why they are also considered a specialization of the plasma membrane.

2.1 Cilia
Cilia (sg. cillium) are about 0,25 mm in diameter, 2-10 mm long and occur on the cell surface in great number. Each cillium is formed by a projection of the plasmatic membrane and is, therefore, an intracellular structure. Cilia are found both in isolated cells and in multi-cellular populations. Single-cell organisms, such as protozoa, use cilia both for movement and to obtain food. In multi-cellular organisms, cilia serve to generate movement of the environment around the cells. E.g. the ciliary epithelium in the respiratory tract directs the stream of the air and sweeps along dust, phlegm and foreign bodies, etc. from the respirtory tract. Cilia exhibit coordinated oar-like beat that either propels the cell through the medium or moves the medium across the cell surface, acting in parallel with the surface of the cell. In a living cilium, dynein heads periodically make contact with the adjacent microtubule, thereby producing the force for sliding between microtubules. Other proteins serve to hold the bundle of microtubules together and to convert the sliding motion of microtubules produced by dyneins into bending of cilium. Cycle of movements: power stroke and recovery stroke; cycle typically requires 0.1–0.2 second and generates a force perpendicular to the axis of the cilium. Fast power stroke: cilium fully extended and fluid is driven over the surface of the cell; Slower recovery stroke: cilium curls back into position with minimal disturbance to the surrounding fluid.

2.2 Flagella
Flagella (sg.flagellum) have the same diameter as cilia but are much longer (10-200 mm) and a cell has only one flagellum or just a few. As with cilia, flagella are formed by a projection of the plasmatic membrane. Flagella differ from cilia by the nature of the movement which is either parallel with its long axis and the cell is propelled forward by the flagellum using an undulating wave action. Only exceptionally can the cell be pulled by the movement of the flagellum.

3. Bacterial flagellum rotation
This type of movement occurs in prokaryotes. Regardless of the similarity of the name, the mechanism of the movement in bacteria is completely different.