Cell movement
The ability of cells to move in a liquid or aeriform environment occurs through direct or indirect movement. The indirect movement occurs completely passively, by means of the wind (in the case of pollen), by means of the "water, or with the circulatory stream. A special type of indirect movement is the Brownian movement, which is carried out with the" collision of the cells with colloidal molecules contained in a medium; this type of movement is very irregular (zig-zag). The direct movement is characteristic of certain cells which must possess some peculiarities to carry it out: amoeboid cells, hair cells, muscle cells.
The movement of amoeboid cells is characterized by the "emission of offshoots of cellular substance (the pseudopodia). These offshoots can be emitted at any point of the cell wall, but when they are everted in a certain direction and always in that, they allow small movements of the cell . With this mechanism, leukocytes, migrating connective cells, histiocytes and monocytes move. The speed of movement is not more than a few microns per minute. The ciliated and flagellate cells are instead able to perform the so-called vibratile movement, for means of filament organelles stably implanted in the cells, called flagella and cilia. The flagella are a classifying element of a "whole class of Protozoa called precisely Flagellates: in man they are found only in the spermatozoon; the cilia are instead much more frequent in the cells , both of the animal and vegetable kingdoms: in man they are found on the free part of carpeted cells zanti the respiratory tract, the uterus, the tube, the efferent ducts of the testicle. Each cilia has a small body on which it is fixed, called the basal corpuscle, inside the cytoplasm.
The filaments perform two types of movement: a rotary one, whereby the flagellum twists on itself, and an oscillatory one, similar to that of the fish tail; the result of these movements can be either a propulsion of the cell or a retraction.
The last type of movement to which the cells are subject is muscle movement: only the smooth and striated muscle cells are subject to this motion and consists in the contraction of particular differentiated elements contained within the cell called myofibrils. The contraction of the myofibrils, and consequently of the entire muscle cell, is never spontaneous but always occurs following an excitement due to nerve impulses.
Cellular adaptability
With this term we mean the ability of a cell to be able to react to the stimuli of the external environment and to be able to adapt to it in order to achieve the best vital conditions. The stimuli can be of various kinds and not necessarily harmful to cellular life; depending on the harmfulness or otherwise of the stimulus, the cell responds with a movement that can be either one of orientation (tropism) or one of distancing (taxi). Both tropism and rate can be negative if the cell moves away refusing the stimulus , or positive if the element approaches the source of the stimulus. The chemotaxis deserves a particular mention, that is the cellular movement towards a specific chemical substance that is found in greater useful concentration (positive chemotaxis) or the removal from it (negative chemotaxis).
Cellular reproduction
Cell division is an essential process for the continuation of the species: in fact, in all living beings, both animals and plants, cells cannot originate except by the division of previous mother cells. For an individual already conceived, cell division leads to his morphogenesis, that is to say that all the embryonic sketches that will give rise to the individual organs are built on the fertilized egg: it is the means of its growth through which from a newborn individual of small size it comes to the mature individual. Finally, cell division is the only means available to the living being for repairing losses occurring for physiological reasons or for trauma. There are two modes of cell reproduction: direct division or amitosis and indirect division or mitosis or karyokinesis.
