Metaphors for Biology and Engineering

It is common (and useful) to think of cells as intricate machines, with thousands of different parts dependent for their structure and function on those of the others. The metaphor encapsulates the experimental technique of reconstructing systems in vitro which is central to modern molecular biology, and reflects the Galilean/Newtonian/Cartesian approach which is deeply fundamental to modern science. Machine components and functions have been used frequently since the end of the last century to describe aspects of biological structure and function, beginning perhaps with Walter Cannon . The metaphor makes it easier to imagine deliberately engineering the properties of cells as we now do the technological artifacts of our culture, such as buildings or spacecraft, and encourages one to think what might constitute good engineering practices for such devices.

But the machine metaphor overlooks several crucial differences between organisms and our own artifacts. ``Biologically-based devices'' such as cells have thousands of kinds of parts, each having unique structural and functional properties; there is much less modularity and interchangeability than in many human artifacts. The parts make each other and the whole, and their material is precisely that of the manufacturing process; thus the system constructs itself and evolves as an ensemble (extreme just-in-time manufacturing). Though structure determines function, both aspects of the parts must be expressed explicitly, because a theory which could unequivocally predict the latter from the former does not (yet, and may never) exist. The upshot of all of this a set of relationships among the parts which are complex in time, space and organization. For both science and engineering, the task is to discover and articulate these relationships so that the effects of manipulating the parts can be accurately predicted.

Some of the complexity of the relationships and their evolution can be expressed by the weaving metaphor, and it was the web-like richness of the biology and the aptness of the metaphor that prompted Mary Berlyn to suggest it when we were struggling to name the project. But a more accurate metaphor is a fusion of these two, exemplified now only by the cells themselves. In this fusion, the parts are complex, active, and multidimensional: the warp and weft contains thousands of distinct threads, bound together in a rich network of functional and structural relationships. Each is actively making or degrading others to constantly create the whole. The cloth, which represents both the threads and their interrelationships, is multidimensional in space, time and organization.

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