Model of the initiation of signal transduction by ligands in a cell culture: simulation of molecules near a plane membrane comprising receptors

Trapping of diffusing particles by short absorbing spikes periodically protruding from reflecting base

We study trapping of diffusing particles by a periodic non-uniform boundary formed by absorbing spikes protruding from a reflecting flat base. It is argued that such a boundary can be replaced by a flat uniform partially absorbing boundary with a properly chosen effective trapping rate. Assuming that the spikes are short compared to the inter-spike distance, we propose an approximate expression which gives the trapping rate in terms of geometric parameters of the boundary and the particle diffusivity. To validate this result, we compare some theoretical predictions based on the expression for the effective trapping rate with corresponding quantities obtained from Brownian dynamics simulations.

Green's function for reversible geminate reaction with volume reactivity

The kinetics of a diffusing particle near a reversible trap may be described by an extension of the Feynman-Kac equation to the case of reversible binding, which can occur within a finite reaction sphere. We obtain the Green's function solution for the Laplace transform of this equation when the particle is initially either bound or unbound. We study the solution in the time-domain by either inverting the Laplace transform numerically or propagating the partial differential equation in the time-domain. We show that integrals of this solution over the reaction sphere agree with previously obtained solutions.

Lauriston S. Taylor Lecture on radiation protection and measurements: what makes particle radiation so effective?

The scientific basis for the physical and biological effectiveness of particle radiations has emerged from many decades of meticulous basic research. A diverse array of biologically relevant consequences at the molecular, cellular, tissue, and organism level have been reported, but what are the key processes and mechanisms that make particle radiation so effective, and what competing processes define dose dependences? Recent studies have shown that individual genotypes control radiation-regulated genes and pathways in response to radiations of varying ionization density. The fact that densely ionizing radiations can affect different gene families than sparsely ionizing radiations, and that the effects are dose- and time-dependent, has opened up new areas of future research. The complex microenvironment of the stroma and the significant contributions of the immune response have added to our understanding of tissue-specific differences across the linear energy transfer (LET) spectrum. The importance of targeted versus nontargeted effects remains a thorny but elusive and important contributor to chronic low dose radiation effects of variable LET that still needs further research. The induction of cancer is also LET-dependent, suggesting different mechanisms of action across the gradient of ionization density. The focus of this 35th Lauriston S. Taylor Lecture is to chronicle the step-by-step acquisition of experimental clues that have refined our understanding of what makes particle radiation so effective, with emphasis on the example of radiation effects on the crystalline lens of the human eye.