Chapter 22 



LIGHT ABSORPTION BY PIGMENTS IN THE LIVING CELL 



The determination of light absorption in sokitions or other homogeneous 

 media is a routine measurement, and the results permit a simple interpre- 

 tation (based on Beer's law) in terms of molecular absorption coefficients 

 (also called "extinction coefficients" — since attempts to discriminate be- 

 tween these two terms have not been successful in practice). The experi- 

 mental determination of the absorptive power of plants is less simple, and 

 often the exact meaning of the results is problematical. The measurement 

 of light energy absorbed by leaves, algal thalli or cell suspensions is com- 

 plicated by scattering, which is significant not only in multicellular tissues, 

 but even in suspensions of single cells (because the dimensions of the cells, 

 ~10~^ cm., are larger than the wave length of visible light, ~5 X 10~^ 

 cm.) . The interpretation of the results in terms of the absorption constants 

 of the pigments is complicated, not only by the light scattering on phase 

 boundaries, but also by inhomogcneous distribution of pigments in cells and 

 tissues, and by the shifting and deformation of the absorption bands caused 

 by adsorption and complexing. Let us assume, for example, that we have 

 measured the energy, /, of a beam of light falling on a vegetable object — 

 leaf, thallus or cell suspension — and the energy, /', emerging from this 

 object, taking care to integrate the latter over all directions so as to include 

 both the light transmitted forward (T) and the light reflected backward 

 (R), and thus to avoid the "gross" errors that may be caused by scattering. 

 If now we try to apply to the results Beer's law : 



(22.1) I' (= T + R) = 7 X 10-"^'' 



with the intention of calculating an absorption coefficient, a, we find, first 

 of all, that scattering has made the length of the path of the light in the ab- 

 sorbing medium — d in eq. 22.1 — indefinite (even its average — Mestre's "de- 

 tour factor" — is not constant, but depends on wavelength, cf. Kok 1948). 

 In the second place, we note that the local accumulation of pigments in the 

 chloroplasts has made the concentration of absorbing molecules in the path 

 of the individual light beams — c in equation (22.1) — variable: Some light 

 l)eams pass between the chloroplasts and encounter no pigment molecules 

 at all (a phenomenon to which we will refer later as the "sieve effect"). 



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