We end up (for now)l by considering the dynamics of social systems, from bacterial quorum sensing to the development of an embryo.
Thinking about biology, we have to adopt a systems perspective. At each level of biological organization we can identify the objects that interact, how they interact, and the outcomes of their interactions. At the molecular level it is common to focus on the interactions between proteins and DNA (genes) that control gene expression (such as we considered in the context of the lac operon). These molecular level interactions play an important role in determining how cells behave. Interactions between cells influence the behaviors of the interacting cells, as well as the overall behavior(s) of biological communities and multicellular organisms. Interactions between organisms, ranging from mutual dependencies to host-pathogen and predator-prey interactions, underlie social and ecological systems. Interaction systems are complex. For example, interactions between cells will influence both lower (molecular level) and higher (organismic and social) systems. Moreover systems change over time and will respond to environmental perturbations in various, often unexpected ways. Systems thinking provides an analytical context to consider biological systems at all levels, from the gene to the ecosystem.