Absolute optical cross sections of cells and chloroplasts

Abstract

Absolute optical cross sections in square microns of Escherichia coli cells, spinach chloroplasts, and yeast cells were measured and compared with theoretical predictions for simple particle models. The area of these cross sections indicates the absolute amount of light per particle which is scattered at all angles or absorbed. The cross sections are obtained from transmissions of counted suspensions as measured with a special single-beam spectrophotometer. At a wavelength of 500 mμ, the average yeast cell is found to scatter 310% of the light striking its geometrical cross section. The spinach chloroplast scatters and absorbs a total of about 150% of the light that strikes it, while an E. coli cell scatters about 33%. Mie and anomalous diffraction equations were used to calculate theoretical cross sections from measured values of particle size, shape, refractive index, and gross internal structure. Absolute suspension transmissions were then calculated from these cross sections and the values of particle count. The resulting theoretical predictions quantitatively agree with observations. This agreement substantially establishes light-scattering theory as a reliable tool for studies of large biological particles. The same basic theory indicates that scattering by such particles is accomplished mainly by interference and diffraction, not by less potent mechanisms similar to surface reflection or Rayleigh scattering.