Multicellular organisms have a number of eukaryotic cells. Contrary to protozoa colonies, their cells are varied; it is a single whole, the next level of living matter organization. Although there are intermediate forms, in adverse conditions a population of amoeba Dictyostelium discoideum can form from separate specimens of Plasmodium, having embryos with organs of movement and reproduction.
Tendency to form multicellular organisms caused problem of concentration of all hereditary information in a single cell nucleus. This information must include all the program of the individual's development. Modern views are closer to the concept of 'preformation' of A. Leeuwenhoek (1632-1723), than to 'epigenesis' of R. Descartes (1596— 1650). Inheritance began to combine with 'ontogenesis' — the process of organism's formation from a gametal cell.
Differentiation of cells is guided by autocatalytic molecular reactions, similar to mechanisms caused by the evolutionary progress. It is an explanation of the biogenetic Haeckel-Muller law (1866) about the repetition of phylogenesis by ontogenesis.
The common genetic code of all cells of an organism creates prerequisites for their cooperative behavior. Intercellular interaction oversteps the cellular egoism, which is considered as pathology and can produce such diseases as cancer. The being of a cell is directed for prosperity of its organism, for protection, and spreading of the common genetic code. All cells have a specialization (neurons in the neural tissue, myocytes in muscles, etc.). There are programs ofcellular self-destruction; their lifetime (for human erythrocyte - 100-120 days) is defined by interactions of the organism and species. Special 'stem cells' (0.2-1% of the total number of cells) have been discovered lately; they are not really specialized and have no restriction for divisions. Furthermore, they can form any kind of tissues; so they are used for treatment of many diseases.
Sexual reproduction is very typical for multicellular organisms. Its advantage is an increase of genetic variation of offspring in comparison with asexual reproduction. It stimulates evolution and is especially important in times of intensive species formation. For stable conditions, it is not so important, and now for such successful group as flowering plants there is a tendency to turn to asexual forms of reproduction.
A multicellular organism's cell usually contains two complete genetic sets ('genomes'); it is 'diploid'. Since sexual reproduction involves participation of hereditary material from two gametes, the number of genomes has to be halved in the gametes. As a part of the reproductive process, organisms of males and females produce 'haploid' cells (containing single genome) from diploid ones in a process of'meiosis'.
At the new level, multicellular organisms have repeated the way of unicellular ones in coordination of internal processes. It was necessary to realize at least four main functions of an organism: homeostasis (first of all, maintenance of stability of internal environment, in accordance with the law of K. Bernard, 1872); growth (in a certain sense conflicting with homeostasis); adaptation; and reproduction. All these processes require united control systems for their regulation. In this way, the 'hormonal system' (of chemical nature, extension of the cellular one) was formed. In organisms, there is a system of interacting endocrine glands, producing special effectors ('hormones') and reacting on incoming chemical agents. Hormones can influence cells and organs as inductors, derepressor, etc. The cell membrane has receptors, reacting to chemical agents. In accordance with these signals, the cell coordinates its activity with the whole organism. Later the 'neural system' of animals was originated. It can influence cells both directly (electrically) and through the hormonal system. A special organ
'hypothalamus' plays the role of the coordinating center of the neural and hormonal systems.
The neural system is the most developed control system of life. It is organized on cybernetic principles, and it is small wonder, because the development of the nervous system gave ultimately the birth to science and, particularly, cybernetics. Control of all the biological processes has an information basis. Each reaction of the organism is based on two kinds of information: external from organs of sense, and internal from the organism's memory. An elementary unit of behavior is test-operate-test-exit (TOTE); it consists in comparison of real (from external information) and necessary (from internal information) states before and after the operation.
Memory should be big (there are a great number of possible influences and reactions) and fast (sometimes speed of response is critical). Usually organisms have both these types of memory separately. An organism deals with four kinds of information: (1) structural (fixed in its structure); (2) genetic; (3) mnemonic (from its memory); (4) external (from organs of sense).
Information flow to human organism is estimated as 107 bps; its lifetime is 109 s; the brain cortex contains about 10 cells. Very probably, that information is stored at molecular level, for example, in RNA molecules. Some experiments confirm this theory. Theoretically, it can be explained by different probabilities of different nucleo-tide types' joining to forming RNA under the influence of external neural impulse.
Was this article helpful?
You Might Start Missing Your Termites After Kickin'em Out. After All, They Have Been Your Roommates For Quite A While. Enraged With How The Termites Have Eaten Up Your Antique Furniture? Can't Wait To Have Them Exterminated Completely From The Face Of The Earth? Fret Not. We Will Tell You How To Get Rid Of Them From Your House At Least. If Not From The Face The Earth.