712 



William F. Prickett, F. Dudley Bryant, and Paul Latimer 



280 m^. Also seen is a well defined scattering band at about 347 mp which is 

 apparently related to the DPNH band at 340 mji. Other structure is seen in 

 the scattering curve although it is not very much greater than randonn error. 



Although the experimental data extended from 240 - 460 mja, only a portion 

 of it is shown on the figure. At the shorter wavelengths, the scattering curve 

 simply rises sharply. At the longer wavelengths it agrees with a previously 

 reported curve. ' ' 



Fig. 1. Spinach chloroplast: 

 absolute absorbance and scat- 

 tering cross section. 



SCATTERING (Sp) 



ABSORBANCE (E^J 



X (rtvi) 



The only rigorous light scattering equations which have been used to 

 account for scattering spectra of biological particles are obtained from the 

 so called "large particle" postulates^^. 5)_ -pj^g particles are assumed to be 

 large compared with the wavelength of the incident light and without internal 

 scattering structures so that the simple laws of geometrical optics apply with- 

 in the particle. The difference between the refractive index of the particle 

 and that of the surrounding medium is assumed to be small, thus permitting 

 surface reflection and refractive bending to be neglected. In this case, scat- 

 tering can be explained in terms of destructive interference in the forward 

 direction, and constructive interference in the other directions, between light 

 passing through the particle and that passing around it. 



Several authors have used this type of equation to qualitatively account 

 for experinnental scattering and extinction curves of algae'"^' "' '> °' and red 

 blood cells' ' "'. In studies of oriented red cells which are relatively large 

 { '~^8 y. dianneter) and devoid of internal structure, the quantitative agreement 

 between this type of theory and experiment is reasonably good in the spectral 

 region near A = 0. 4 p (diameter/wavelength in water = 25). Such equations 

 nnay also be applicable to the chloroplasts (3 yL diameter) in the UV ( A =' . 3 p). 



