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A Glossary for Systems Biology Contents Index - Glossary - SENSITIVITY Glossary RELIABILITY Submit - Comment ROBUSTNESS see also adaptation, integral control, modularity, reliability Meaning The insensitivity of system properties to parameter variation and other uncertainties in components and the environment [15]. Biology: Mostly used with regard to changes in environment, but also as robustness against structural failure (reliability). Systems Theory: Mostly used with regard to insensitivity against parameter uncertainties. Explanation Biologists and Systems Theorists infer the same basic meaning to the term robustness: A robust system is insensitive to certain changes, allowing it to exhibit a constant behavior in spite of these changes. Differences in meaning look small but are still significant. Biologists usually mean that the system is robust against variations in its environment, e.g. in concentration levels of nourishing substrates (see examples). Systems theorists mostly think of uncertainties in system parameters, either because they could not be measured with sufficient accuracy (for use in a model) or because they really do fluctuate over time. These are different kinds of robustness, so there is a potential for misunderstandings even though the basic meaning is identical. KITANO [34] sees these two kinds of robustness as two of three classes of behavior of robust systems, the third being graceful degradation (see below). He cites four types of mechanisms that are employed in technical and biological systems alike to achieve those classes of robust behavior: system control, redundancy (modularity), structural stability and modularity. Recent papers suggest that robustness is achieved through complexity: Complex regulatory networks enable the cell to exhibit a simple behavior consistently over a wide range of parameter and/or environmental values [39]. (systems biology) This is in sharp contrast to more well known chaos and complexity theories, which associate complexity with fractals and edge-of-chaos, originating in simple systems [15,16]; see also Chapter 'Recommended Readings'. The systems that are the object of complexity theory are simple in structure but exhibit complex behaviors. One typical characteristic of those systems is their sensitivity to smallest changes in parameters, their tendency to instability. Biological systems usually try to avoid instability; therefore they use a different approach and gain simplicity and robustness (of behavior) through complexity (in their structure) [39]. This approach has its downsides as well, though, as along with robustness ``complexity brings a type of fragility.'' [DOYLE in [50]]. If all the mechanisms of robustness are ever overwhelmed, such systems tend to cascading effects of breakdown, the first subsystem taking the others down with it. In this case the third class of robust behavior shows its importance: graceful degradation, meaning ``the characteristic slow degradation of a system's functions after damage [or catastrophic disturbance], rather than catastrophic breakdown'' [34]. This characteristic is important in technical systems like medical equipment as well, where it is referred to as the 'principle of first error', meaning that the first error which occurs must not be dangerous or even fatal [52]. Examples Biology: bacterial chemotaxis: ``the precision of adaptation in bacterial chemotaxis is robust to dramatic changes in the levels and kinetic rate constants of the constituent proteins'' [63,3]; see also Chapter 'Recommended Readings'. catabolite repression: multiple nourishing substrates can be utilized, making the organism independent of one special substrate. [13] Systems Theory: liquid-holding tank: must not run dry, must not overflow despite significant changes in in- and outflow system with changing mass, e.g. course control for a rocket which burns off fuel [20] (adaptation: adaptive control)   Contents   Index Glossary Submit Comment

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