THOUGHTS ON THE USE OF INFORMATION-THEORY IN BIOLOGY

For living beings, information is as fundamental as matter or energy. In this paper we show: a) inadequacies of quantitative theories of inf ormation, b) how a qualitative analysis leads to a classification of i nformation systems and to a modelling of intercellular communication. From a quantitative point of view, the application in biology of infor mation theories borrowed from communication techniques proved to be di sappointing. These theories ignore deliberately the significance of me ssages, and do not give any definition of information. They refer to q uantities, based upon arbitrarily defined probabilistic events. Probab ility is subjective. The receiver of the message needs to have 'meta-k nowledge' of the events. The quantity of information depends on langua ge, coding, and arbitrary definition of disorder. The suggested object ivity is fallacious. In common language, the word 'information' is syn onymous with knowledge of order. Following common sense a message (let ters, coded signals, etc.) is information just in case it is interpret able, i.e. if it fits to a previously acquired meaning (the words of a n available language, etc.). The consequence is that calculation of qu antities in the sense of Shannon can be used for transmissions, but it is itself meaningless (has no significance).In linguistics and semant ics, information is composed of a 'signifier', a physical medium (lett ers, coded signals, etc.), and a 'signified' or significance.The natur e of information is complex. The laws of linguistics and semantics are valid not only at the human, organismic level, but also at the cellul ar and molecular level. The physiology of sensations gives us many exa mples for application of a concept of information An electromagnetic w ave of 0,7% give us the sensation of a red colour. Sensations have no physical reality. They are purely subjective. At the cellular level co mmunication operates by means of chemical messengers (first messengers ), which generally do not penetrate the plasmic membrane. Specific cap tors operate as transductors: external factors are converted into seco ndary messengers (cyclic AMP, Ca ion, etc.). Sometimes, electric signa ls (like depolarization waves) may also play a part in the intercellul ar communication. Such processes are characterized by changes in a seq uence of different molecules carried by a physical signal. What is tra nsmitted is the meaning of the message (significance) which can be mem orized by the cell, providing a possible following use. At the molecul ar level one can find also the processes of linguistic nature. We know that the significance of a word is changed with changing the order of letters (ADD --> DAD, etc.). In the same way bases C and U are coding for serine (UCC), leucine (CUC) or proline (CCU). Here, amino-acids e xpress the significance. In spite of the fact that this key-lock mecha nism may explain many reactions, the examples prove that other element s are necessary for understanding the information. The living cell is the receiver. The message actualizes only previously learned and memor ized significances or actions (trigger effect). Significance is not an emergent property of the shape of the message. It depends on the rece iver and the transmitter. A word can have more than one meaning. Simil arly, a messenger can order different physiological responses: muscula r tension, hormonal secretion, etc.. Thus a chemical messenger is a si gnal which is identified and interpreted by the receiver, depending up on specific languages and previous learning. These views are in harmon y with immunological and Jerne's theory (of idiotypical net). The abov e mentioned considerations led the author to propose the theory of dat a transfer, which takes into account significance. In this theory the quantity of information is the product of the probabilistic recognitio n of message and the value of significance as determined by its semant ic level. (See: Acta biotheoretica vol. 41 No 1/2 June 1993.) The comp lex nature of information asks to propose a qualitative classification with respect to the material support and the significance. a) The mat erial support may be linear in lime (sequential reading, ADN translati on) - The material support may be referred to non-temporally (drawings , logos, holograms) (Reading is instantaneous) - The material support may be in circulation, or in stock. b) The significance may be local ( tissues, organs) or general (organisms). A significance may be a comma nd to be executed (imperative, conditional order) or knowledge to be m emorized. The purpose of significance may be a coding for space (for m orphology) or for time (ontogeny, ageing). Conclusion: Information can not any longer be regarded as an object. Its nature is complex, at all levels of a living being. At the molecular level to memorize an infor mation by modification of a molecule is comparable with writing words on a diary. The key-lock process does not suppress the question of the interpretation, i.e. relations existing between the shape of a micros copic element as a molecule, and its macroscopic effect, as an antenna or a leg. There are still many unclear points in these relations, e.g . the sweet taste of molecules of tomatine and monelline. The abstract nature of significance which at the human level is concerned to menta l processes, is not only a philosophical problem. In fact, there is a hypothesis based on quantum mechanics which allows to consider a physi cal nature or significance. In any case, the important conclusion is t hat significance in bio-information must be considered in relation to the message-receiver. The receiver must no longer be considered a pass ive one. The qualitative classification of information will allow an u nderstanding of circulation of information in organisms and between ce lls.