Stochastic Transport in Biology: From Molecules to Ecosystems

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Intellectual Merit: Transport phenomena are relevant to all levels of biological organization and span length and time scales ranging over many orders of magnitude. Examples on one end of this range are molecular processes such as the transport of ions across cellular membranes and on the other end, migrations of birds and animals over inter-continental distances. A common feature of these phenomena is their intrinsically stochastic (random) character and the structurally complex and dynamic environments in which they occur. A single-celled organism moving in search of favorable conditions relies on stochastic molecular-level, receptor-binding events to determine its direction of motion at each moment, which introduces randomness to its overall trajectory. Thus, randomness at the molecular level produces randomness at the cellular level. Furthermore, the complexity of dynamic environments leads to surprising transport behaviors. Such observations suggest the hypothesis that stochastic processes at one level of organization can manifest themselves in stochastic transport phenomena at higher levels of organization, in predictable ways. This project will develop a versatile theoretical framework, based on the physics of diffusion, to investigate these ideas at three distinct levels of biological organization. At the molecular level, this project will explore how stochasticity influences transport by the molecular motor kinesin and also the collective transport behavior of multiple motors pulling the same supra-molecular cargo. At the cellular level, the project will characterize the motor-driven transport of cargo across the cell along cytoskeletal networks which, to first approximation, are considered static or, more realistically, are changing over timescales comparable to the transport process itself. Finally, at the multi-cellular level, this project will develop models to explore the role of cellular communication and spatial complexity in transport-driven pattern formation in multi-cellular communities of cyanobacteria moving toward a light source. Broader Impacts: A fundamental understanding of the basic physics of transport at the molecular, cellular and multi-cellular levels of organization will have broad significance for biological research and biotechnology, in particular for the optimal design and control of transport processes. In addition, the general theories arising from this research will have implications for such problems as information transfer and routing on dynamic computer networks and the spreading of populations on large-scale, dynamic ecological and human networks. The PI and Co-PI will also engage in a variety of outreach activities within their institutions and in the local region. The PI will continue to develop and teach a successful and innovative calculus-based, freshman-level physics sequence for biological science majors, that uses biological examples to motivate and illustrate physical concepts. The course will be further developed with the addition of new computational labs and a flash-animation based, interactive website that allows the students to explore physical concepts and their biological connections on their computers. The PI will partner with local community colleges to train instructors in the use of these new course materials, which are designed to add an exciting, quantitative focus to the introductory life science curriculum. Additional outreach activities are targeted at local high schools and, in conjunction with the Co-PI, at high schools in the Bay Area, especially for women and students from underprivileged socioeconomic backgrounds.