Figure 1b: The spherical coordinate system used in the analysis of diffusion in three dimensions, where a source or sink of protein mass Q_s, is created at the cell centre. Simplified protein redistribution by diffusion is illustrated in the text by two examples: the first by solving at spherical grid points defined on the radial vector r, assuming angular symmetry, i.e., no variations in polar angle or azimuthal angle, and the second by solving at spherical grid points on r and polar angle grid points on, but assuming azimuthal angular symmetry, i.e., no variations in angle (image reproduced from [1] with permission of the authors).

Calvert, P. D., J. A. Peet, A. Bragin, W. E. Schiesser and E. N. Pugh Jr., Fluorescence Relaxation in 3D from Diffraction-limited Sources of PAGFP or Sinks of EGFP Created by Multiphoton Photoconversion, Journal of Microscopy, 225, No. 1, January, 2007, pp. 49-71.