Systems biologists often distance themselves from reductionist approaches and formulate their aim as understanding living systems "as a whole." Yet, it is often unclear what kind of reductionism they have in mind, and in what sense their methodologies would offer a superior approach. To address these questions, we distinguish between two types of reductionism which we call "modular reductionism" and "bottom-up reductionism." Much knowledge in molecular biology has been gained by decomposing living systems into functional modules or through detailed studies of molecular processes. We ask whether systems biology provides novel ways to recompose these findings in the context of the system as a whole via computational simulations. As an example of computational integration of modules, we analyze the first whole-cell model of the bacterium M. genitalium. Secondly, we examine the attempt to recompose processes across different spatial scales via multi-scale cardiac models. Although these models rely on a number of idealizations and simplifying assumptions as well, we argue that they provide insight into the limitations of reductionist approaches. Whole-cell models can be used to discover properties arising at the interfaces of dynamically coupled processes within a biological system, thereby making more apparent what is lost through decomposition. Similarly, multi-scale modeling highlights the relevance of macroscale parameters and models and challenges the view that living systems can be understood "bottom-up." Specifically, we point out that system-level properties constrain lower-scale processes. Thus, large-scale modeling reveals how living systems at the same time are more and less than the sum of the parts. Editorial introduction: This contribution from Fridolin Gross and Sara Green focuses on the promises and possible pitfalls of large-scale modelling in systems biology, from both a practical and a theoretical point of view. As readers will be aware, molecular biology has often been accused of being "reductionist." Systems biology has been presented as a potential response to this reductionism. Gross and Green distinguish two types of reductionisms: "modular reductionism" (the claim that living systems consist of individual functional subunits) and "bottom-up reductionism" (the claim that biological phenomena can and should be studied at the molecular level, understood as the "fundamental" biological level). They show how systems biology questions both forms of reductionism. To this end, they use two examples: whole-cell modelling of a very "simple" bacterium (from a piece published in 2012), and multi-scale computer modelling of the human heart. Without masking the limitations of these models, Gross and Green show the insights they can yield for anyone interested in ontologies of the living world.
KeywordsAs with some of the other contributions in this special issue (e.g. Fagan's, and Kendig and Eckdahl's), a virtue of this paper is to illustrate how research strategies-particularly modelling strategies-can in...