- Email: [email protected]
The cell as unit
J. Theoret. Biol. (1963) 5, 389-397
The Cell as Unit PAUL
WEISS
The Rockefeller Institute, New York, N. Y., U.S.A. (Received 1 July 19633 It is plainly impossible to deal with the problem of ‘the cell as a unit’ more than perfunctorily in the limited space of a brief essay. The most one can hope to achieve is to set the problem in the proper perspective by an examination, however cursory, of whether ‘cell biology’ has any factual claim to the status of an autonomous discipline in the organizational hierarchy of nature, or whether it is merely a temporary station on the way to its complete resolution into terms of ‘molecular biology’. Having been responsible for introducing this categorical distinction, the author feels an obligation to review its merits, which are based on both facts and logic. Rated by its composition, a cell is obviously no more than the sum total of the molecules composing it, and these in turn can, if one wants to, be described in terms of their component atomic and subatomic elements. But since this process of progressive decomposition yields essentially the same result for a live and for a dead cell, and indeed even for the homogenate of a physically disintegrated cell, one realizes that such a reductionist description loses some highly relevant ‘information content’ on the way down; it loses the criteria which distinguish the live cell from the dead one, the dead one from its homogenate, the structured macromolecule from a random scramble of its constituent atoms and ions, and so forth. In trying to derive the more complex systems from their elements, therefore, one must make up for this deprivation somehow by restoring the lost properties. The practice of doing this through verbal symbols, such as ‘organization’ or ‘integration’, is an old one, but seldom makes explicit as to whether these symbols are meant to be final logical postulates to compensate for the limitations of pure reductionism or merely provisional promissory notes that they will ultimately yield to analytical resolution. Having been batted around for ages in an odd mixture of scientific reasoning and emotional preconceptions, the argument between these two alternatives is at last losing some of its steam under the critical scrutiny of modern ‘operationalism’. One honestly cannot deny that hierarchical ‘order’ and ‘organization’ as superordinated principles sui generis would have gained T.B.
389
25
390
PAUL
WEISS
scientific acceptance more readily if they had not been suspected of theological implications. Divested of all such bywork, what then is the true operational meaning of the above alternatives? Does it not simply lie in the difference between a mental reconstruction of a higher system from symbols representing elements on the one hand, and the physical reconstruction of such a system from separate components on the other-in either case, without the cheating intervention of an already ordered ‘model’ or ‘organizer’ as integrator ? In other words, the true test of a consistent theory of reductionism is whether or not an ordered unitary system (a cell being such a system) can, after decomposition into a disordered pile of constituent parts, resurrect itself from the shambles by virtue solely of the properties inherent in the isolated pieces. If not, the symbolic terms, which permit us to execute in mental imagery what physical reality is impotent to reproduce, would acquire the logical and scientific respectability of axioms. Conversely, spectacular recent progress in achieving true ‘synthesis’ of higher-order systems from lower-order elements, in general with the input of energy, has rather fanned the hopes of believers in the eventual triumph of an absolute reductionism. These latter no longer doubt that it will be possible to ‘synthesize’ a living cell from a mixture of molecules; they just ask when it will come to pass, and some pretend the feat to be just around the corner. Unfortunately, such optimism is mostly in direct proportion to the lack of first-hand and penetrating acquaintance with the living cell as a whole, which is a unit rather than a sheer summative assemblage or conglomerate. For however familiar and expert one may be with one particular feature of a cellular system, be it genie replication, contractility, respiration, selective permeability, impulse conduction, enzyme action, membrane formation, etc., he misses the essence of the problem of cellular unity unless he takes due account of the indispensable cooperative coexistence of all these features; that is, that every single one must contribute to the maintenance and operation of all the others in such a way that collectively they achieve a relatively stable and durable group existence. Just bear in mind, for instance, that while models of contractility must fall back on ordered macromolecular structures as synchronizers and coordinators of enzyme activity, enzyme action, in turn, is instrumental in the establishment of structural assemblies. While the mechanisms of photosynthesis and respiration have been convincingly connected with arrays of macromolecular complexes in definite sequential order, this very order depends for its establishment and maintenance on photosynthetically or respiratorially provided energy. As membranes perform selective screening functions between the media to either side, their very formation and growth depend on the uninterrupted presence within the cell
THE
CELL
AS
391
UNIT
of highly discriminative powers as to what it lets in, retains, converts, assimilates, compounds and localizes. Thus, by the time we have laid out the pattern of the reproductive and functional performances of a cell in a total, rather than sectorial, view, we recognize that the basic criterion of cell life lies in the intricate web of interactions and interdependencies among all of its component activities. True, each one of these particular components can be successfully analyzed in its own right in relative sectional isolation, but only if one takes for granted and then borrows some ready-made cellular derivates, such as enzymes, membranes, chromosomes, from the other sectors. In contrast to the analyzing scientist, however, the living cell does it all by itself in its own household; its know-how, with only an ‘uninformed’ environment to draw upon, residing not in the static composition of its chemical endowment, but in the dynamics of its interactions harmoniously coordinated. Coordinated by what ? Entelechy ? Feedbacks? Information? Fields? Should we not desist from coining allusive labels until we have described in sober operational terms the factual content of the phenomena thus to be labelled? Translated to such terms, what is the meaning of ‘interdependence’ of component events in a system such as a cell? It evidently refers to a relation between a group of events, a, b, c, . . . n, in a physical continuum such that the omission of any one of them would preclude the occurrence of all the others. Since in dynamics events are merely spot samples of continuous processes, our formulation must be expanded to imply dependencies among concurrent processes, that is, between time courses a’ + a” -+ a”’ -+ . ..d. b’ -+ b” -+ b”’ -+ ...b’. c’ + c” + c”’ + ...I?. so that at no point on the time line will any one of these series be out of correspondence with the others. All the component processes must mesh like gears in a machine or civic activities in a community. The crucial alternative raised above can now be phrased more succinctly: can such interlocking systems be taken apart and put together again stepwise, like a machine or jigsaw puzzle, by adding one piece at a time, or is the very existence of the system as a whole predicated on the simultaneous presence and operation of all components? In the former instance, an eventual ‘synthesis’ of artificial cells could be envisaged; in the latter case, it could not. Let us look at the record. A few years ago, the United States National Academy of Sciences held a symposium, organized by F. 0. Schmitt (1956), on “Biomolecular Organization and Life Processes”, in which the conditions for the complexing first of molecules into macromolecules, and then of macromolecules into viruses, were reviewed. In the concluding address on “The Compounding of Complex Macromolecular and Cellular Units into Tissue Fabrics”, the author presented 25-z
392
PAUL
WEISS
experimental evidence that would permit the principle of stepwise assembly of ordered complexes to be extended to even higher levels of organization. Just as individual connective tissue fibers are not just random aggregates of molecules of the protein, collagen, but strictly ordered strands of such molecules aligned in tandem and register, so the population of fibers need not remain a random feltwork, but in combination with a mucopolysaccharide matrix, can assume the regular geometric arrangement of an ordered fabric; and even a population of cells that has been isolated from complex organs, dispersed and scrambled, can rearray itself upon reassembly into the typical pattern of the organ of origin (Weiss, 1956, pp. 819-830). Here then are striking examples that “organization on a higher level may emerge from ordered interactions of organized elements” (p. 830). Although our scant factual information on this kind of ordered complexing of heterogeneous elements is largely empirical, certain underlying principles can be dimly perceived. One is the thesis that the orderly grouping of macromolecules into ‘macromolecular assemblies’ (Schmitt, 1963) might be based on the steric matching of complementary sites among the components; failing this, some other ordering mechanism would have to be postulated to account for the apparent strict sequential order in which macromolecules (e.g. enzymes) are packed into subcellular structures. Another principle to emerge is what may be called ‘macrocrystallinity’, that is, the self-ordering of mixed macromolecular populations in definite grids, or space lattices, with periodicities of several hundred Angstroms (Weiss, 1956, p. 825), determined presumably by the optimum equilibrium positions of the domains of the interacting components in those particular combinations. Perhaps one day both principles will yield to a description in terms of ‘minimum free energy’ of the resultant compound systems with regard to a given common environment, suboptimal for the stability of any one of the participating components alone. Yet despite the intriguing support which the indicated ‘synthesis’ of subcellular systems from separate elements seems to lend to an extension of the concept to the cell itself, hence to the prospect of a reductionist explanation of cell life, there are two major and fundamental objections to its uncritical acceptance. The first one is the qualification that in order for macromolecules to be able to congregate in higher-order patterns, they must themselves possess conforming patterns of organization, that is, properties which prematch them for mutual conjugation. The implication of this has been stated by the author (Weiss, 1961) and the following quotation is taken from the conclusion to the introductory chapter (p. 66): “We have arrived at last at a point which comes rather close to what might be defined as ‘molecular control of cellular activity’, only to discover that the
THE
CELL
AS
UNIT
393
‘controlling’ molecules have themselves acquired their specific configurations, which are the key to their power of control, by virtue of their membership in the population of an organized cell, hence under ‘cellular control’. And this indeed has been the whole purpose of my long discourse; to document by practical examples that the distinction between molecular control of cellular activity and cellular control of molecular activity is based on the semantic ambiguity of the term ‘control’, hence fades in the light of true understanding of the phenomena involved. A cell is nothing but the population of component entities that constitute it. But these entities are not just of molecular rank, nor can their ordered behavior in the group be fully appreciated and understood solely by studying them in isolation, out of context. As I have tried to carry the principle of self-organization of higher organizational units by the free interaction of elements of lower order as far as present factual evidence-not hopes, nor belief-would honestly entitle us to do, I had to add at every turn that elements endowed for such ordered group performance have always been prefitted for it by properties previously imparted to them as members of just such an organized group unit, whether cell parts, cell, or germ. This circular argument contains one of the most fundamental truths about the nature of organisms and, as one can readily understand, does not augur well for an eventual affirmative answer to our introductory question, in its ndive form, of whether a cell will ever be synthesized de nova without the active intervention of another cell.” A second reservation regarding the concept of ‘self-assembly’ stems from the essentially static character of the examples we have chosen to prove that true ‘compounding without systemic guidance does occur; for we have concentrated our attention on structural features, neglecting the inseparable complementarity between structure and process in the living system, in which processed structure is but an outcome of structured processes. The fact that diverse activities of a definite pattern can coexist and go on concurrently in the space continuum of the cell even in the absence of tight compartmentalization, reveals that although only a fraction of the cellular estate is strictly structured in a mechanical sense, there still is coordination among the diverse biochemical processes, which evidently must remain relatively segregated and localized. So, here we are back again at the question asked before: Coordination, how and by what? Now, it is intriguing to speculate in the interest of consistency that perhaps the structured portion of the cell might itself also subserve this function of coordinating the unstructured fraction of the cell content by establishing and maintaining differential topographic distributions within the otherwise unsegregated molecular populations of the intracellular pools. The author is quite partial to this view and has strongly advocated it in the opening
394
PAUL
WEISS
address on “Structure as the Coordinating Principle in the Life of the Cell”, delivered at the Welch Foundation Symposium in 1961. The trend of thought is this. Ordered patterns of cellular and subcellular structures are definitely capable of inducing a corresponding patterning in the adjacent layers of the ambient liquid space by selective adsorption, chemical bonding and the concomitant local physico-chemical changes; obviously the fact that differential enzyme localization is part of this process would explain that not only the segregation, but even the regulation of the kinetics, of the heterogeneous activities could likewise be referred back in the last analysis to some ordered structural mosaic. Most of these considerations would seem to be consonant with current notions of ‘coding’ of cellular activity. The modern transliteration of the older term ‘organization’ to ‘information’ is acceptable, though inconsequential, for this discussion; after all, both are just words. The simile of a ‘code’ for non-random sequential order, as of letters in a word, likewise is satisfactory to illustrate certain aspects of an organized system. Genes can be viewed as coded sequences of nucleotides (deoxyribonucleic acids); different segments of these linear chains can be faithfully copied onto ‘messenger’ molecules; these can transfer the ‘code’ to still other molecules (both ribonucleic acids); and the sequential code of the latter can act as the model for a corresponding specific sequence of amino acids in the construction of a specific protein. Up to this point, the scheme provides a plausible mechanism for the transmission, hence conservation, of specific space order. In passing, it is worth noting a major distinction between this manufacture of macromolecules and the elementary kind of molecular ‘synthesis’ with which we are familiar from simple inorganic systems. The former requires the presence of organized end products as models, ‘templates’ or ‘primers’ whereas, for instance, H and Cl combine to HCl unaided. This distinction may of course turn out to be fundamental, but this need not concern us here. By far more critical is the further problem : suppose we do know how genes beget proteins-and surely, this knowledge is a spectacular achievementhow do we get from there to the knowledge, let alone the synthesis, of a living ceN? In principle, by just more of the same? The standard affirmative answer, that after all proteins in the form of enzymes do hold the key to the synthesis of all other, non-protein, compounds in the cell, begs the question; for it still leads only to a random bag of compounds, instead of the highly coordinated chemical machinery that is the cell. Now, as indicated before, the cell is neither this sort of random scramble of molecules, nor, at the other extreme, a rigid stereotyped composite of microstructures, but something in between; part fluid, part consolidated. And the fact that it can exist at all, considering the enormous variety of
THE
CELL
AS
UNIT
395
molecular species and groupings it contains, cannot be simply passed off by just referring to the progressive complexity of molecular interactions, but calls for an exact specification of the principle that ‘coordinates’ these interactions so that their combined performances will ensure, as if by concert, a high degree of stability in the total system. Logically, this ‘coordinating’ principle cannot be of the same categorical order as individual reactions themselves-just one more of them. The common habit of personifying compounds by calling them ‘regulators’, ‘integrators’, ‘organizers’, etc., and crediting them verbally with the ‘regulatory, integrative and organizing’ effects which one observes but cannot explain analytically, either intends to endow chemicals with spiritual powers up and above their ordinary properties, or else is wholly meaningless. To state it bluntly, it would be rather a reversion to the prescientific age if on observing, for instance, the spinning of a whirl of fluid, one were to invoke a special compound as ‘spinner’. By reasons of logic and scientific honesty, the problem of coordinated unity of the cell must therefore be acknowledged as a real one. It cannot be hedged by assuming that starting from gene reduplication and the first steps of protein synthesis, all further developments would run off collaterally and uninterrelated; for this would imply that once having been mapped out microprecisely down to the most minute details, they would then actually be capable of pursuing with absolute rigidity their individual courses so predesigned as to yield blindly, but unfailingly, a viable product-a modern version of Leibnitz’ ‘prestabilized harmony’. The unpredictability of the vicissitudes of the environments in which those courses materialize rules out any such concept of absolute predetermination as utterly unrealistic and absurd; ‘environment’ to be regarded as anything in the world surrounding given items-organisms, cells, organelles, molecules, etc.-which can interact with the latter or affect their mutual interactions. In fact, the whole mental picture of cells as if they were stamped out identically like tin soldiers is false and misleading. Cells of the same type vary far more widely in detailed composition, conf&uration and activity, among one another and from moment to moment, than is usually realized and taken into account. Therefore, the actual course of a given train of interactions cannot be predicted by either the cell or its observer deterministically, but can be defined only in terms of the probability of its going to lie somewhere within a given range. Thus, obviously, if there is a very large number, n, of independent processes, each subject to a large number, m, of random serial excursions from an average course, the cumulative variance among them would become increasingly greater, as time goes on. This would render it highly improbable that any two such bundles of processes would ever lead to
396
PAUL
WEISS
recognizably similar results, or even that any one of them would retain essential identity with itself for any length of time of activity. However, notwithstanding this measure of relative indeterminacy of the component processes and the metabolic flux of their substance, living cells of a given kind do resemble each other and do retain essential invariance for long periods of time. We can express this fact symbolically in the formula : v, < c (v, + Z’b+ UC+ . . . z$, where V, denotes the total variance within a population of cells of a given type (or between successive stages of the same cell), whereas aO,Q, uC, . . . are the variances of component cell activities. The formula represents an ‘operational’ description of what it is that makes the cell as a unit ‘more than the sum of its parts’. In order that this formula be satisfied, one must evidently postulate that the component processes, when operating in the common integral system, are interdependent in such a manner that as any one of them strays off the norm in one direction, this entails an automatic counteraction of the others. In electric control systems, such compensatory stabilization devices are built in as ‘negative feedback’ loops. But other kinds of systems achieve the same effect without circuitry. The cell is one of these. The principle involved is often referred to as ‘homoeostasis’. The question now remains whether it is conceivable that such a system of cooperative dynamics could be assembled from its parts in steps, one at a time, or whether it can exist only in its entirety or not at all. To be specific, consider the requirements a number of biochemical processes a, b, c .. . 11,etc., must satisfy in order to be able to go on side by side, as they do in the cell. If they are to coexist, they must either be mutually supporting, each yielding (and receiving) needed products and energy to (and from) the surrounding ones, or at any rate be at least mutually compatible, none yielding products or effects that would interfere with the others. Such families of processes must be harmoniously adjusted to each other not only as to the kinds of reactions, but as to their rates and time courses as well, and furthermore they must keep adjusting continuously to the fluctuations of environmental conditions referred to a while ago. Now, it is relatively simple to set up a model for a pair of reciprocally matched processes a and b chosen as of such properties that whatever a needs in specific compounds and energy will be furnished on schedule by b, and vice versa. In essence, what a laboratory biochemist does when he reproduces an isolated metabolic reaction (e.g. an enzymatic one) in vitro, is nothing but playing the part of b for a, or of a for b, to the best of his ability, providing each reaction with the necessary conditions and ingredients (e.g. substrates, accessories, pH buffers, etc.) ubiquitously and optimally. In the cell (or any organized fragment recovered from a cell), the same reaction depends of
THE
CELL
AS
UNIT
397
course on whether or not the same conditions and sources are made available locally in the immediate micro-environment as the result of commensurate neighboring reactions. Now, it can readily be seen that the feasibility, in principle, of compounding such coupled interaction systems according to the scheme of 1 + 1 = 2 ends with a doublet, for if there are more than two components depending integrally upon one another, such a system presents us with logical attributes akin to the ‘many-body’ problem in physics. If a is indispensable for both b and c; b for both a and c; and c for both a and b; no pair of them could exist without the third member of the group, hence any attempt to build up such a system by consecutive additions would break down right at the first step. In other words, a system of this kind can exist only as an entity or not at all. Operationally, the cell falls in this category; to call it a ‘unit’ is merely a shorthand reference to this operational description. By implication, this also reaffirms the principle of unbroken continuity of organization in living systems, which the author once expressed as “omnis organisatio ex organisatione”, with the understanding, however, that higher degrees of organization can emerge from the free interaction of organized and prematched systems of lower order of complexity (e.g. specific macromolecular assemblies from pools of macromolecules; organs from dispersed cell populations). In conclusion, on the basis of facts and logic, it seems unwarranted, and indeed unsound, to expect that it will ever be possible to describe cell behavior solely in reductionist terms of properties of its component elements, that is, without giving a due account of its ‘system character’. Hopefully, a scientific systems theory and methodology, as currently applied to the interpretation of brain function, group behavior, engineering, communication, economics, and so on, will also provide conceptual tools for describing and treating the cell as a system more rigorously than heretofore. At any rate, the disciplined study of the systemic properties of the cell-of ‘cell biology’that is, the manner in which its molecular components, which are the prime objects of ‘molecular biology’, are subordinated to ordered group coexistence in a system of ‘molecular ecology’-is one of the major challenges and tasks of modern science. To meet it, we must face it. To face it, we must see it. To see it, we may even at times have to put on blinders so as to reduce the dimming effect of contrast engendered by all the brilliant light that emanates from ‘molecular biology’. REFERENCES Proc. Nat. Acad. Sci., Wash. 42, 789. Devel. Biol. 7, 546. Nat. Acad Sci., Wash. 42, 819. WEISS, P. (1961). “From Cell to Molecule” in “The Molecular Control of Cellular Activity” (ed. J. M. Allen), pp. l-72. New York: McGraw-Hill. SCHMITT, F. 0. (1956). SCHMITT, F. 0. (1963). WEISS, P. (I 956). Proc.
The Cell as Unit PAUL
WEISS
The Rockefeller Institute, New York, N. Y., U.S.A. (Received 1 July 19633 It is plainly impossible to deal with the problem of ‘the cell as a unit’ more than perfunctorily in the limited space of a brief essay. The most one can hope to achieve is to set the problem in the proper perspective by an examination, however cursory, of whether ‘cell biology’ has any factual claim to the status of an autonomous discipline in the organizational hierarchy of nature, or whether it is merely a temporary station on the way to its complete resolution into terms of ‘molecular biology’. Having been responsible for introducing this categorical distinction, the author feels an obligation to review its merits, which are based on both facts and logic. Rated by its composition, a cell is obviously no more than the sum total of the molecules composing it, and these in turn can, if one wants to, be described in terms of their component atomic and subatomic elements. But since this process of progressive decomposition yields essentially the same result for a live and for a dead cell, and indeed even for the homogenate of a physically disintegrated cell, one realizes that such a reductionist description loses some highly relevant ‘information content’ on the way down; it loses the criteria which distinguish the live cell from the dead one, the dead one from its homogenate, the structured macromolecule from a random scramble of its constituent atoms and ions, and so forth. In trying to derive the more complex systems from their elements, therefore, one must make up for this deprivation somehow by restoring the lost properties. The practice of doing this through verbal symbols, such as ‘organization’ or ‘integration’, is an old one, but seldom makes explicit as to whether these symbols are meant to be final logical postulates to compensate for the limitations of pure reductionism or merely provisional promissory notes that they will ultimately yield to analytical resolution. Having been batted around for ages in an odd mixture of scientific reasoning and emotional preconceptions, the argument between these two alternatives is at last losing some of its steam under the critical scrutiny of modern ‘operationalism’. One honestly cannot deny that hierarchical ‘order’ and ‘organization’ as superordinated principles sui generis would have gained T.B.
389
25
390
PAUL
WEISS
scientific acceptance more readily if they had not been suspected of theological implications. Divested of all such bywork, what then is the true operational meaning of the above alternatives? Does it not simply lie in the difference between a mental reconstruction of a higher system from symbols representing elements on the one hand, and the physical reconstruction of such a system from separate components on the other-in either case, without the cheating intervention of an already ordered ‘model’ or ‘organizer’ as integrator ? In other words, the true test of a consistent theory of reductionism is whether or not an ordered unitary system (a cell being such a system) can, after decomposition into a disordered pile of constituent parts, resurrect itself from the shambles by virtue solely of the properties inherent in the isolated pieces. If not, the symbolic terms, which permit us to execute in mental imagery what physical reality is impotent to reproduce, would acquire the logical and scientific respectability of axioms. Conversely, spectacular recent progress in achieving true ‘synthesis’ of higher-order systems from lower-order elements, in general with the input of energy, has rather fanned the hopes of believers in the eventual triumph of an absolute reductionism. These latter no longer doubt that it will be possible to ‘synthesize’ a living cell from a mixture of molecules; they just ask when it will come to pass, and some pretend the feat to be just around the corner. Unfortunately, such optimism is mostly in direct proportion to the lack of first-hand and penetrating acquaintance with the living cell as a whole, which is a unit rather than a sheer summative assemblage or conglomerate. For however familiar and expert one may be with one particular feature of a cellular system, be it genie replication, contractility, respiration, selective permeability, impulse conduction, enzyme action, membrane formation, etc., he misses the essence of the problem of cellular unity unless he takes due account of the indispensable cooperative coexistence of all these features; that is, that every single one must contribute to the maintenance and operation of all the others in such a way that collectively they achieve a relatively stable and durable group existence. Just bear in mind, for instance, that while models of contractility must fall back on ordered macromolecular structures as synchronizers and coordinators of enzyme activity, enzyme action, in turn, is instrumental in the establishment of structural assemblies. While the mechanisms of photosynthesis and respiration have been convincingly connected with arrays of macromolecular complexes in definite sequential order, this very order depends for its establishment and maintenance on photosynthetically or respiratorially provided energy. As membranes perform selective screening functions between the media to either side, their very formation and growth depend on the uninterrupted presence within the cell
THE
CELL
AS
391
UNIT
of highly discriminative powers as to what it lets in, retains, converts, assimilates, compounds and localizes. Thus, by the time we have laid out the pattern of the reproductive and functional performances of a cell in a total, rather than sectorial, view, we recognize that the basic criterion of cell life lies in the intricate web of interactions and interdependencies among all of its component activities. True, each one of these particular components can be successfully analyzed in its own right in relative sectional isolation, but only if one takes for granted and then borrows some ready-made cellular derivates, such as enzymes, membranes, chromosomes, from the other sectors. In contrast to the analyzing scientist, however, the living cell does it all by itself in its own household; its know-how, with only an ‘uninformed’ environment to draw upon, residing not in the static composition of its chemical endowment, but in the dynamics of its interactions harmoniously coordinated. Coordinated by what ? Entelechy ? Feedbacks? Information? Fields? Should we not desist from coining allusive labels until we have described in sober operational terms the factual content of the phenomena thus to be labelled? Translated to such terms, what is the meaning of ‘interdependence’ of component events in a system such as a cell? It evidently refers to a relation between a group of events, a, b, c, . . . n, in a physical continuum such that the omission of any one of them would preclude the occurrence of all the others. Since in dynamics events are merely spot samples of continuous processes, our formulation must be expanded to imply dependencies among concurrent processes, that is, between time courses a’ + a” -+ a”’ -+ . ..d. b’ -+ b” -+ b”’ -+ ...b’. c’ + c” + c”’ + ...I?. so that at no point on the time line will any one of these series be out of correspondence with the others. All the component processes must mesh like gears in a machine or civic activities in a community. The crucial alternative raised above can now be phrased more succinctly: can such interlocking systems be taken apart and put together again stepwise, like a machine or jigsaw puzzle, by adding one piece at a time, or is the very existence of the system as a whole predicated on the simultaneous presence and operation of all components? In the former instance, an eventual ‘synthesis’ of artificial cells could be envisaged; in the latter case, it could not. Let us look at the record. A few years ago, the United States National Academy of Sciences held a symposium, organized by F. 0. Schmitt (1956), on “Biomolecular Organization and Life Processes”, in which the conditions for the complexing first of molecules into macromolecules, and then of macromolecules into viruses, were reviewed. In the concluding address on “The Compounding of Complex Macromolecular and Cellular Units into Tissue Fabrics”, the author presented 25-z
392
PAUL
WEISS
experimental evidence that would permit the principle of stepwise assembly of ordered complexes to be extended to even higher levels of organization. Just as individual connective tissue fibers are not just random aggregates of molecules of the protein, collagen, but strictly ordered strands of such molecules aligned in tandem and register, so the population of fibers need not remain a random feltwork, but in combination with a mucopolysaccharide matrix, can assume the regular geometric arrangement of an ordered fabric; and even a population of cells that has been isolated from complex organs, dispersed and scrambled, can rearray itself upon reassembly into the typical pattern of the organ of origin (Weiss, 1956, pp. 819-830). Here then are striking examples that “organization on a higher level may emerge from ordered interactions of organized elements” (p. 830). Although our scant factual information on this kind of ordered complexing of heterogeneous elements is largely empirical, certain underlying principles can be dimly perceived. One is the thesis that the orderly grouping of macromolecules into ‘macromolecular assemblies’ (Schmitt, 1963) might be based on the steric matching of complementary sites among the components; failing this, some other ordering mechanism would have to be postulated to account for the apparent strict sequential order in which macromolecules (e.g. enzymes) are packed into subcellular structures. Another principle to emerge is what may be called ‘macrocrystallinity’, that is, the self-ordering of mixed macromolecular populations in definite grids, or space lattices, with periodicities of several hundred Angstroms (Weiss, 1956, p. 825), determined presumably by the optimum equilibrium positions of the domains of the interacting components in those particular combinations. Perhaps one day both principles will yield to a description in terms of ‘minimum free energy’ of the resultant compound systems with regard to a given common environment, suboptimal for the stability of any one of the participating components alone. Yet despite the intriguing support which the indicated ‘synthesis’ of subcellular systems from separate elements seems to lend to an extension of the concept to the cell itself, hence to the prospect of a reductionist explanation of cell life, there are two major and fundamental objections to its uncritical acceptance. The first one is the qualification that in order for macromolecules to be able to congregate in higher-order patterns, they must themselves possess conforming patterns of organization, that is, properties which prematch them for mutual conjugation. The implication of this has been stated by the author (Weiss, 1961) and the following quotation is taken from the conclusion to the introductory chapter (p. 66): “We have arrived at last at a point which comes rather close to what might be defined as ‘molecular control of cellular activity’, only to discover that the
THE
CELL
AS
UNIT
393
‘controlling’ molecules have themselves acquired their specific configurations, which are the key to their power of control, by virtue of their membership in the population of an organized cell, hence under ‘cellular control’. And this indeed has been the whole purpose of my long discourse; to document by practical examples that the distinction between molecular control of cellular activity and cellular control of molecular activity is based on the semantic ambiguity of the term ‘control’, hence fades in the light of true understanding of the phenomena involved. A cell is nothing but the population of component entities that constitute it. But these entities are not just of molecular rank, nor can their ordered behavior in the group be fully appreciated and understood solely by studying them in isolation, out of context. As I have tried to carry the principle of self-organization of higher organizational units by the free interaction of elements of lower order as far as present factual evidence-not hopes, nor belief-would honestly entitle us to do, I had to add at every turn that elements endowed for such ordered group performance have always been prefitted for it by properties previously imparted to them as members of just such an organized group unit, whether cell parts, cell, or germ. This circular argument contains one of the most fundamental truths about the nature of organisms and, as one can readily understand, does not augur well for an eventual affirmative answer to our introductory question, in its ndive form, of whether a cell will ever be synthesized de nova without the active intervention of another cell.” A second reservation regarding the concept of ‘self-assembly’ stems from the essentially static character of the examples we have chosen to prove that true ‘compounding without systemic guidance does occur; for we have concentrated our attention on structural features, neglecting the inseparable complementarity between structure and process in the living system, in which processed structure is but an outcome of structured processes. The fact that diverse activities of a definite pattern can coexist and go on concurrently in the space continuum of the cell even in the absence of tight compartmentalization, reveals that although only a fraction of the cellular estate is strictly structured in a mechanical sense, there still is coordination among the diverse biochemical processes, which evidently must remain relatively segregated and localized. So, here we are back again at the question asked before: Coordination, how and by what? Now, it is intriguing to speculate in the interest of consistency that perhaps the structured portion of the cell might itself also subserve this function of coordinating the unstructured fraction of the cell content by establishing and maintaining differential topographic distributions within the otherwise unsegregated molecular populations of the intracellular pools. The author is quite partial to this view and has strongly advocated it in the opening
394
PAUL
WEISS
address on “Structure as the Coordinating Principle in the Life of the Cell”, delivered at the Welch Foundation Symposium in 1961. The trend of thought is this. Ordered patterns of cellular and subcellular structures are definitely capable of inducing a corresponding patterning in the adjacent layers of the ambient liquid space by selective adsorption, chemical bonding and the concomitant local physico-chemical changes; obviously the fact that differential enzyme localization is part of this process would explain that not only the segregation, but even the regulation of the kinetics, of the heterogeneous activities could likewise be referred back in the last analysis to some ordered structural mosaic. Most of these considerations would seem to be consonant with current notions of ‘coding’ of cellular activity. The modern transliteration of the older term ‘organization’ to ‘information’ is acceptable, though inconsequential, for this discussion; after all, both are just words. The simile of a ‘code’ for non-random sequential order, as of letters in a word, likewise is satisfactory to illustrate certain aspects of an organized system. Genes can be viewed as coded sequences of nucleotides (deoxyribonucleic acids); different segments of these linear chains can be faithfully copied onto ‘messenger’ molecules; these can transfer the ‘code’ to still other molecules (both ribonucleic acids); and the sequential code of the latter can act as the model for a corresponding specific sequence of amino acids in the construction of a specific protein. Up to this point, the scheme provides a plausible mechanism for the transmission, hence conservation, of specific space order. In passing, it is worth noting a major distinction between this manufacture of macromolecules and the elementary kind of molecular ‘synthesis’ with which we are familiar from simple inorganic systems. The former requires the presence of organized end products as models, ‘templates’ or ‘primers’ whereas, for instance, H and Cl combine to HCl unaided. This distinction may of course turn out to be fundamental, but this need not concern us here. By far more critical is the further problem : suppose we do know how genes beget proteins-and surely, this knowledge is a spectacular achievementhow do we get from there to the knowledge, let alone the synthesis, of a living ceN? In principle, by just more of the same? The standard affirmative answer, that after all proteins in the form of enzymes do hold the key to the synthesis of all other, non-protein, compounds in the cell, begs the question; for it still leads only to a random bag of compounds, instead of the highly coordinated chemical machinery that is the cell. Now, as indicated before, the cell is neither this sort of random scramble of molecules, nor, at the other extreme, a rigid stereotyped composite of microstructures, but something in between; part fluid, part consolidated. And the fact that it can exist at all, considering the enormous variety of
THE
CELL
AS
UNIT
395
molecular species and groupings it contains, cannot be simply passed off by just referring to the progressive complexity of molecular interactions, but calls for an exact specification of the principle that ‘coordinates’ these interactions so that their combined performances will ensure, as if by concert, a high degree of stability in the total system. Logically, this ‘coordinating’ principle cannot be of the same categorical order as individual reactions themselves-just one more of them. The common habit of personifying compounds by calling them ‘regulators’, ‘integrators’, ‘organizers’, etc., and crediting them verbally with the ‘regulatory, integrative and organizing’ effects which one observes but cannot explain analytically, either intends to endow chemicals with spiritual powers up and above their ordinary properties, or else is wholly meaningless. To state it bluntly, it would be rather a reversion to the prescientific age if on observing, for instance, the spinning of a whirl of fluid, one were to invoke a special compound as ‘spinner’. By reasons of logic and scientific honesty, the problem of coordinated unity of the cell must therefore be acknowledged as a real one. It cannot be hedged by assuming that starting from gene reduplication and the first steps of protein synthesis, all further developments would run off collaterally and uninterrelated; for this would imply that once having been mapped out microprecisely down to the most minute details, they would then actually be capable of pursuing with absolute rigidity their individual courses so predesigned as to yield blindly, but unfailingly, a viable product-a modern version of Leibnitz’ ‘prestabilized harmony’. The unpredictability of the vicissitudes of the environments in which those courses materialize rules out any such concept of absolute predetermination as utterly unrealistic and absurd; ‘environment’ to be regarded as anything in the world surrounding given items-organisms, cells, organelles, molecules, etc.-which can interact with the latter or affect their mutual interactions. In fact, the whole mental picture of cells as if they were stamped out identically like tin soldiers is false and misleading. Cells of the same type vary far more widely in detailed composition, conf&uration and activity, among one another and from moment to moment, than is usually realized and taken into account. Therefore, the actual course of a given train of interactions cannot be predicted by either the cell or its observer deterministically, but can be defined only in terms of the probability of its going to lie somewhere within a given range. Thus, obviously, if there is a very large number, n, of independent processes, each subject to a large number, m, of random serial excursions from an average course, the cumulative variance among them would become increasingly greater, as time goes on. This would render it highly improbable that any two such bundles of processes would ever lead to
396
PAUL
WEISS
recognizably similar results, or even that any one of them would retain essential identity with itself for any length of time of activity. However, notwithstanding this measure of relative indeterminacy of the component processes and the metabolic flux of their substance, living cells of a given kind do resemble each other and do retain essential invariance for long periods of time. We can express this fact symbolically in the formula : v, < c (v, + Z’b+ UC+ . . . z$, where V, denotes the total variance within a population of cells of a given type (or between successive stages of the same cell), whereas aO,Q, uC, . . . are the variances of component cell activities. The formula represents an ‘operational’ description of what it is that makes the cell as a unit ‘more than the sum of its parts’. In order that this formula be satisfied, one must evidently postulate that the component processes, when operating in the common integral system, are interdependent in such a manner that as any one of them strays off the norm in one direction, this entails an automatic counteraction of the others. In electric control systems, such compensatory stabilization devices are built in as ‘negative feedback’ loops. But other kinds of systems achieve the same effect without circuitry. The cell is one of these. The principle involved is often referred to as ‘homoeostasis’. The question now remains whether it is conceivable that such a system of cooperative dynamics could be assembled from its parts in steps, one at a time, or whether it can exist only in its entirety or not at all. To be specific, consider the requirements a number of biochemical processes a, b, c .. . 11,etc., must satisfy in order to be able to go on side by side, as they do in the cell. If they are to coexist, they must either be mutually supporting, each yielding (and receiving) needed products and energy to (and from) the surrounding ones, or at any rate be at least mutually compatible, none yielding products or effects that would interfere with the others. Such families of processes must be harmoniously adjusted to each other not only as to the kinds of reactions, but as to their rates and time courses as well, and furthermore they must keep adjusting continuously to the fluctuations of environmental conditions referred to a while ago. Now, it is relatively simple to set up a model for a pair of reciprocally matched processes a and b chosen as of such properties that whatever a needs in specific compounds and energy will be furnished on schedule by b, and vice versa. In essence, what a laboratory biochemist does when he reproduces an isolated metabolic reaction (e.g. an enzymatic one) in vitro, is nothing but playing the part of b for a, or of a for b, to the best of his ability, providing each reaction with the necessary conditions and ingredients (e.g. substrates, accessories, pH buffers, etc.) ubiquitously and optimally. In the cell (or any organized fragment recovered from a cell), the same reaction depends of
THE
CELL
AS
UNIT
397
course on whether or not the same conditions and sources are made available locally in the immediate micro-environment as the result of commensurate neighboring reactions. Now, it can readily be seen that the feasibility, in principle, of compounding such coupled interaction systems according to the scheme of 1 + 1 = 2 ends with a doublet, for if there are more than two components depending integrally upon one another, such a system presents us with logical attributes akin to the ‘many-body’ problem in physics. If a is indispensable for both b and c; b for both a and c; and c for both a and b; no pair of them could exist without the third member of the group, hence any attempt to build up such a system by consecutive additions would break down right at the first step. In other words, a system of this kind can exist only as an entity or not at all. Operationally, the cell falls in this category; to call it a ‘unit’ is merely a shorthand reference to this operational description. By implication, this also reaffirms the principle of unbroken continuity of organization in living systems, which the author once expressed as “omnis organisatio ex organisatione”, with the understanding, however, that higher degrees of organization can emerge from the free interaction of organized and prematched systems of lower order of complexity (e.g. specific macromolecular assemblies from pools of macromolecules; organs from dispersed cell populations). In conclusion, on the basis of facts and logic, it seems unwarranted, and indeed unsound, to expect that it will ever be possible to describe cell behavior solely in reductionist terms of properties of its component elements, that is, without giving a due account of its ‘system character’. Hopefully, a scientific systems theory and methodology, as currently applied to the interpretation of brain function, group behavior, engineering, communication, economics, and so on, will also provide conceptual tools for describing and treating the cell as a system more rigorously than heretofore. At any rate, the disciplined study of the systemic properties of the cell-of ‘cell biology’that is, the manner in which its molecular components, which are the prime objects of ‘molecular biology’, are subordinated to ordered group coexistence in a system of ‘molecular ecology’-is one of the major challenges and tasks of modern science. To meet it, we must face it. To face it, we must see it. To see it, we may even at times have to put on blinders so as to reduce the dimming effect of contrast engendered by all the brilliant light that emanates from ‘molecular biology’. REFERENCES Proc. Nat. Acad. Sci., Wash. 42, 789. Devel. Biol. 7, 546. Nat. Acad Sci., Wash. 42, 819. WEISS, P. (1961). “From Cell to Molecule” in “The Molecular Control of Cellular Activity” (ed. J. M. Allen), pp. l-72. New York: McGraw-Hill. SCHMITT, F. 0. (1956). SCHMITT, F. 0. (1963). WEISS, P. (I 956). Proc.